Reproduction and Survival Strategies
| Site: | Young Education |
| Cours: | Animal Physiology |
| Livre: | Reproduction and Survival Strategies |
| Imprimé par: | 访客用户 |
| Date: | lundi 5 octobre 2026, 04:59 |
1. Reproductive Strategies
Learning outcomes
- I can compare sexual and asexual reproduction.
- I can explain the advantages and disadvantages of different reproductive strategies.
- I can identify reproductive adaptations in animals.
- I can explain how reproductive strategies affect survival.
- I can analyze reproductive success in different environments.
Why Do Animals Reproduce?
Every individual animal eventually dies, but populations and species can continue because organisms reproduce.
Reproduction produces new individuals and passes genetic information from one generation to the next.
Animals use a remarkable variety of reproductive strategies. These strategies influence:
- How many offspring are produced.
- How much energy parents invest in reproduction.
- How much genetic variation exists among offspring.
- How offspring develop.
- How likely offspring are to survive.
- How quickly populations can increase.
- How populations respond to environmental change.
Two fundamental forms of reproduction are:
- Sexual reproduction
- Asexual reproduction
Neither strategy is automatically better. Its success depends on the organism and the environment in which it lives.
Sexual Reproduction
Sexual reproduction involves the combination of genetic material from two gametes.
In animals, these gametes are usually:
- Sperm
- Egg cells
When their nuclei combine during fertilization, they produce a zygote.
A simplified sequence is:
Sperm + egg → fertilization → zygote → embryo → offspring
The offspring receives genetic information from both parents.
Gametes
Gametes are specialised reproductive cells.
In most animals:
- Sperm are relatively small and mobile.
- Eggs are relatively large and contain resources that support early development.
Gametes contain only one set of chromosomes.
When sperm and egg nuclei combine, the normal chromosome number is restored in the zygote.
Genetic Variation in Sexual Reproduction
One major feature of sexual reproduction is that offspring are genetically different from one another and from their parents.
Variation arises partly because:
- Meiosis produces genetically different gametes.
- Gametes from different individuals combine.
- Fertilization is generally a random process.
As a result, each offspring usually receives a unique combination of alleles.
This genetic variation can be extremely important when environments change.
Advantages of Sexual Reproduction
Sexual reproduction can provide several advantages.
Genetic Variation
Offspring are genetically varied.
Adaptation to Changing Environments
If environmental conditions change, some individuals may possess characteristics that increase their chances of survival and reproduction.
Resistance to Disease
Genetic variation can reduce the likelihood that every individual in a population will respond identically to a particular pathogen.
Evolution
Variation provides the raw material upon which natural selection can act.
Over many generations, this can contribute to evolutionary change.
Disadvantages of Sexual Reproduction
Sexual reproduction can also involve substantial costs.
Animals may need to:
- Find a mate.
- Compete for mates.
- Produce specialised reproductive structures.
- Perform courtship behaviours.
- Spend time and energy mating.
- Risk injury or predation while reproducing.
In many species, only part of the adult population directly produces offspring.
Sexual reproduction can therefore be slower than some forms of asexual reproduction.
Asexual Reproduction
Asexual reproduction involves reproduction from a single parent without the fusion of sperm and egg nuclei.
Offspring produced asexually are usually genetically very similar to the parent, although mutations can still introduce variation.
Asexual reproduction occurs in several groups of animals.
Examples can be found among:
- Hydra.
- Some flatworms.
- Sea stars.
- Some insects.
- Some crustaceans.
- Some reptiles.
Budding
In budding, a new individual develops as an outgrowth from the parent's body.
Hydra can reproduce in this way.
A small bud forms through cell division.
The bud grows and develops structures such as tentacles.
Eventually, it may separate from the parent and live independently.
One parent can therefore produce offspring without finding a mate.
Fragmentation
Some animals can reproduce when part of the body separates and develops into another individual.
This is called fragmentation when it functions as a reproductive strategy.
It is often associated with regeneration, the ability to regrow missing structures.
Some flatworms and other invertebrates can reproduce through forms of fragmentation.
It is important to distinguish regeneration from reproduction: repairing a damaged body part does not automatically mean that a new organism has been produced.
Parthenogenesis
Parthenogenesis is reproduction in which an embryo develops from an unfertilized egg.
It occurs in some:
- Insects.
- Crustaceans.
- Reptiles.
- Other animals.
Parthenogenesis can allow rapid reproduction when mates are unavailable.
The genetics of parthenogenesis varies between species, so the offspring are not always genetically identical to their mother.
Advantages of Asexual Reproduction
Asexual reproduction can provide several important advantages.
Only One Parent Is Required
An isolated individual may still be able to reproduce.
Rapid Population Growth
Under suitable conditions, populations can sometimes increase quickly.
Less Energy Spent Finding Mates
Animals do not need to invest as much energy in courtship or mate searching.
Successful Genetic Combinations Can Be Preserved
If an organism is well adapted to a stable environment, asexual reproduction can produce offspring with similar characteristics.
Disadvantages of Asexual Reproduction
The major limitation is generally lower genetic variation.
If many individuals are genetically similar, they may also have similar vulnerabilities.
For example, a major environmental change or new disease could affect a large proportion of the population.
Lower variation can therefore reduce the range of characteristics available for natural selection.
This can become especially important in rapidly changing environments.
Sexual and Asexual Reproduction Compared
| Sexual Reproduction | Asexual Reproduction |
|---|---|
| Usually involves gametes from two parents | Requires one parent |
| Fertilization occurs | No fertilization |
| Usually produces high genetic variation | Usually produces less genetic variation |
| Mate finding may be required | Mate finding unnecessary |
| Can require substantial time and energy | Can sometimes be rapid |
| Particularly useful for generating variation | Can be effective in stable, favourable conditions |
Some organisms use both strategies, switching according to environmental conditions.
Reproductive Success
From an evolutionary perspective, reproduction is not simply about surviving.
Reproductive success refers to an organism's success in passing its genetic information to future generations through surviving, reproducing descendants.
An animal might live for many years but leave no offspring.
Another might have a shorter life but produce offspring that survive and reproduce.
The second individual may therefore have greater reproductive success.
Producing Many Offspring
Some animals produce enormous numbers of offspring.
Examples include many:
- Fish.
- Marine invertebrates.
- Amphibians.
- Insects.
In these strategies:
- Each offspring may receive relatively little parental investment.
- Mortality may be high.
- Producing many offspring increases the chance that at least some survive.
This strategy can be particularly effective where juvenile survival is unpredictable.
Producing Fewer Offspring
Other animals produce relatively few offspring but invest heavily in each one.
Examples include many:
- Mammals.
- Birds.
Investment can include:
- Protection.
- Feeding.
- Teaching.
- Transport.
- Grooming.
- Defending young from predators.
Producing fewer offspring can be successful when parental investment greatly increases each offspring's probability of survival.
A Reproductive Trade-Off
Organisms have limited resources.
Energy invested in one activity cannot simultaneously be used for everything else.
Animals must effectively divide resources among:
- Growth.
- Maintenance.
- Defence.
- Movement.
- Reproduction.
Within reproduction itself, there can be a trade-off between:
Number of offspring ↔ investment in each offspring
Producing thousands of offspring usually prevents extremely high investment in every individual.
Producing only one or two offspring can allow much greater investment in each.
Parental Care
Parental care includes behaviours that increase offspring survival after eggs are produced or young are born.
Examples include:
- Guarding eggs.
- Building nests.
- Feeding offspring.
- Carrying young.
- Protecting young from predators.
- Teaching feeding or hunting behaviour.
Parental care can substantially increase offspring survival, but it also requires time and energy.
External Fertilization
In external fertilization, sperm and eggs meet outside the parents' bodies.
This is common in many aquatic animals.
Examples include many:
- Fish.
- Amphibians.
- Marine invertebrates.
Aquatic environments are particularly suitable because water prevents gametes from rapidly drying out and allows sperm to move.
Advantages and Limitations of External Fertilization
External fertilization can allow many gametes to be released at once.
However, there are challenges:
- Many gametes never meet.
- Eggs may be eaten.
- Environmental conditions can damage developing embryos.
- Gametes can be dispersed.
Animals using this strategy often produce large numbers of gametes.
Timing can also be important.
If many individuals release gametes at approximately the same time, the probability of fertilization increases.
Internal Fertilization
In internal fertilization, sperm is transferred into the reproductive tract of the female, and fertilization occurs inside the body.
This occurs in groups including:
- Reptiles.
- Birds.
- Mammals.
- Many insects.
Internal fertilization can increase the probability that sperm and egg meet and protects gametes from drying out.
It is particularly important for reproduction in terrestrial environments.
Eggs and Reproductive Adaptations
Animals have evolved many adaptations that protect developing embryos.
Bird and reptile eggs, for example, possess structures that allow development on land.
The amniotic egg includes protective membranes and usually a shell.
These structures help:
- Protect the embryo.
- Reduce water loss.
- Provide nutrients.
- Allow gas exchange.
- Store metabolic wastes.
The evolution of the amniotic egg was an important adaptation for reproduction away from water.
Courtship Behaviour
Animals often use courtship behaviours before mating.
These can involve:
- Sounds.
- Colours.
- Movement.
- Chemical signals.
- Gifts.
- Displays.
- Physical competition.
Courtship can help animals:
- Identify members of the same species.
- Determine whether a potential mate is receptive.
- Assess potential mates.
- Coordinate mating behaviour.
Courtship may be energetically expensive, but it can increase reproductive success.
Sexual Selection
Some characteristics increase an individual's chances of obtaining mates.
This can produce a form of natural selection called sexual selection.
Examples can include:
- Bright feathers.
- Large antlers.
- Complex songs.
- Courtship dances.
- Large body size.
These characteristics may increase mating success even when they require substantial energy or create other costs.
Competition for Mates
Members of the same sex may compete for access to mates.
Competition can involve:
- Physical contests.
- Territorial defence.
- Displays.
- Calls.
- Establishing dominance.
For example, antlers can be used in contests between male deer.
Winning a contest can increase access to mates, but fighting also consumes energy and creates a risk of injury.
Mate Choice
In many species, individuals select among potential mates.
Characteristics influencing mate choice can include:
- Appearance.
- Courtship behaviour.
- Territory quality.
- Ability to provide food.
- Health.
- Signals of physical condition.
Mate choice can influence which characteristics become more common over generations.
Reproductive Timing
Animals must often reproduce at the appropriate time.
Environmental cues can include:
- Day length.
- Temperature.
- Rainfall.
- Food availability.
For example, producing offspring when food is abundant can increase juvenile survival.
Reproductive timing therefore links an animal's life cycle to environmental conditions.
Reproductive Synchronization
In some species, many individuals reproduce during a relatively short period.
This is called synchronous reproduction.
Potential advantages include:
- Increased probability of fertilization.
- Large numbers of offspring appearing simultaneously.
- Matching reproduction with favourable environmental conditions.
For species using external fertilization, synchronization can be especially important because eggs and sperm must encounter one another outside the body.
Reproduction in Stable Environments
Imagine an environment that remains relatively stable for many generations.
An organism is already very well adapted to those conditions.
Asexual reproduction can sometimes be advantageous because:
- Reproduction can occur rapidly.
- No mate is necessary.
- Successful genetic combinations can be maintained.
However, environmental stability is rarely permanent.
Reproduction in Changing Environments
Now imagine that:
- Temperature changes.
- A new predator arrives.
- A new disease appears.
- Food availability changes.
A genetically varied population may contain some individuals better able to tolerate the new conditions.
Sexual reproduction continually generates new combinations of alleles.
This can increase the range of characteristics present within the population.
Worked Example: Disease Outbreak
Imagine two populations of the same type of animal.
Population A reproduces mainly asexually and has relatively low genetic variation.
Population B reproduces sexually and has higher genetic variation.
A new infectious disease appears.
If nearly all individuals in Population A have similar susceptibility, the disease could spread very effectively.
In Population B, genetic differences may mean that some individuals are more resistant than others.
Those individuals may be more likely to survive and reproduce.
This illustrates one possible advantage of genetic variation.
Reproduction in Unpredictable Environments
Environments can change unpredictably.
Conditions might vary in:
- Temperature.
- Rainfall.
- Food availability.
- Predator abundance.
- Disease exposure.
Strategies producing greater genetic diversity can become particularly important under these circumstances.
However, reproductive success still depends on many factors. Sexual reproduction does not guarantee survival, and asexual reproduction can remain highly successful in many environments.
Switching Reproductive Strategies
Some animals can reproduce both sexually and asexually.
For example, some populations of aphids can use parthenogenesis during favourable conditions.
This allows populations to increase rapidly.
Sexual reproduction can occur at other stages of the life cycle.
Using different strategies at different times can combine some advantages of both reproductive modes.
Reproduction and Population Growth
Reproductive strategy can strongly influence how quickly a population changes.
Consider an organism that:
- Matures rapidly.
- Produces many offspring.
- Reproduces frequently.
Its population may increase rapidly when conditions are favourable.
Another organism that:
- Matures slowly.
- Produces few offspring.
- Provides extensive parental care.
will generally have a lower maximum rate of population increase.
However, each offspring may have a greater probability of surviving.
Offspring Survival
Producing offspring does not automatically produce reproductive success.
Offspring must survive long enough to contribute to future generations.
Factors affecting survival include:
- Food availability.
- Predation.
- Disease.
- Competition.
- Parental care.
- Environmental conditions.
- Genetic characteristics.
Therefore, reproductive strategy involves more than simply maximizing the number of offspring produced.
Reproductive Adaptations
Animals possess many adaptations related to reproduction.
Structural Adaptations
Examples include:
- Reproductive organs.
- Protective eggshells.
- Structures used in courtship.
- Structures used in competition.
Physiological Adaptations
Examples include:
- Hormonal control of reproduction.
- Production of gametes.
- Internal fertilization.
- Pregnancy.
Behavioural Adaptations
Examples include:
- Courtship.
- Nest building.
- Territorial behaviour.
- Parental care.
- Migration to breeding sites.
Reproductive success often depends on several types of adaptation working together.
Worked Example: Sea Turtle
A sea turtle may lay a large number of eggs on a beach.
The mother does not remain to provide extensive parental care after nesting.
Young turtles face many dangers, including:
- Predators.
- Heat.
- Dehydration.
- Difficulty reaching the ocean.
Mortality is high, but producing many offspring increases the chance that some will survive to adulthood.
Worked Example: Elephant
Elephants use a very different strategy.
They:
- Produce relatively few offspring.
- Have long pregnancies.
- Provide extensive parental care.
- Protect their young.
- Have long developmental periods.
Considerable resources are invested in each offspring.
This can greatly increase the survival probability of individual young.
Worked Example: Frog
Many frogs reproduce using external fertilization.
Adults may release large numbers of gametes into water.
This strategy is suited to aquatic reproduction because:
- Sperm can move through water.
- Eggs remain moist.
- Many offspring can be produced.
However, eggs and tadpoles may experience high mortality.
Some frog species compensate with parental care, demonstrating that even related species can evolve very different reproductive strategies.
Reproductive Strategy and Natural Selection
Reproductive strategies evolve through natural selection.
Characteristics that increase successful reproduction can become more common over generations.
The important outcome is not simply:
Who produces the most offspring?
Instead, evolutionary success depends more closely on:
Which individuals leave surviving descendants that themselves reproduce?
This explains why very different reproductive strategies can persist.
Producing thousands of poorly protected offspring can work.
Producing one highly protected offspring can also work.
Success depends on the environmental and ecological context.
Trade-Offs in Reproductive Strategies
There is rarely a reproductive strategy with every possible advantage.
Animals face trade-offs such as:
Many offspring ↔ high investment per offspring
Rapid reproduction ↔ extensive parental care
Mate competition ↔ risk and energy expenditure
Large reproductive investment ↔ resources available for survival
Genetic similarity ↔ genetic diversity
Evolution acts on these trade-offs over many generations.
Common Mistakes
Saying Sexual Reproduction Always Requires Two Separate Animals
Sexual reproduction involves the fusion of gametes. Some organisms can produce both types of gametes, although many animal species have separate male and female individuals.
Saying Asexual Offspring Are Always Completely Identical
They are generally genetically very similar, but mutations and some forms of asexual reproduction can produce differences.
Saying Asexual Reproduction Has No Advantages
It can be extremely effective because it requires only one parent and can permit rapid reproduction.
Saying Sexual Reproduction Is Always Better
Sexual reproduction provides important genetic variation but also carries costs. The effectiveness of either strategy depends on environmental conditions.
Confusing Fertilization With Mating
Fertilization specifically means the fusion of gamete nuclei. Mating refers to reproductive behaviour that may lead to fertilization.
Assuming More Offspring Always Means Greater Reproductive Success
Producing offspring is useful only if enough survive and ultimately contribute to future generations.
Thinking All Animals Provide Parental Care
Many animals provide little or no care after eggs are produced.
Saying External Fertilization Means Reproduction Is Uncontrolled
Animals can coordinate spawning through behaviour and environmental cues, greatly increasing the chance of fertilization.
Check Your Understanding
1. Distinguish between sexual and asexual reproduction.
2. Explain why sexual reproduction usually produces genetically varied offspring.
3. Give two advantages and two disadvantages of sexual reproduction.
4. Give two advantages and two disadvantages of asexual reproduction.
5. Describe budding and give an example of an animal that reproduces this way.
6. What is parthenogenesis?
7. Compare internal and external fertilization.
8. Explain why external fertilization is particularly common in aquatic environments.
9. Why do many animals using external fertilization produce large numbers of gametes?
10. Explain the trade-off between offspring number and parental investment.
11. Give three examples of reproductive adaptations.
12. Explain how parental care can increase reproductive success.
13. Why might genetic variation be particularly valuable in a changing environment?
14. Compare the reproductive strategies of a sea turtle and an elephant.
15. A population lives in an environment that suddenly becomes much more variable. Explain why genetic variation among offspring could influence the population's long-term survival.
Key Terms
- Reproduction – process by which organisms produce new individuals.
- Sexual reproduction – reproduction involving the fusion of gametes.
- Asexual reproduction – reproduction without fusion of gametes.
- Gamete – reproductive cell containing one set of chromosomes.
- Sperm – typically small, mobile male gamete.
- Egg – typically larger female gamete containing resources for early development.
- Fertilization – fusion of gamete nuclei.
- Zygote – cell produced by fertilization.
- Genetic variation – genetic differences among individuals.
- Budding – asexual reproduction in which a new organism develops as an outgrowth of the parent.
- Fragmentation – reproductive process in which a body fragment can develop into another individual.
- Regeneration – regrowth or replacement of missing or damaged structures.
- Parthenogenesis – development of offspring from an unfertilized egg.
- External fertilization – fertilization occurring outside the parent's body.
- Internal fertilization – fertilization occurring within the reproductive tract.
- Parental care – parental behaviour that increases offspring survival.
- Courtship – behaviour associated with attracting and selecting mates.
- Sexual selection – selection associated with differences in mating and reproductive success.
- Reproductive success – success in passing genetic information into future generations through surviving descendants.
- Reproductive strategy – combination of characteristics and behaviours involved in reproduction and offspring survival.
- Reproductive investment – time, energy, and resources devoted to reproduction and offspring.
- Natural selection – process in which heritable characteristics affecting survival or reproduction influence which traits become more common over generations.
Key Takeaways
- Animals use a wide range of reproductive strategies.
- Sexual reproduction involves the fusion of gametes and usually produces genetically varied offspring.
- Asexual reproduction requires only one parent and generally produces less genetic variation.
- Genetic variation can increase the range of responses available to a population when environmental conditions change.
- Asexual reproduction can allow rapid reproduction without the need to find a mate.
- Budding, fragmentation, and parthenogenesis are forms of asexual reproduction found in animals.
- Fertilization can occur internally or externally.
- External fertilization is especially common in aquatic environments.
- Internal fertilization protects gametes from drying out and increases opportunities for fertilization on land.
- Animals vary greatly in the number of offspring produced and the investment made in each offspring.
- Producing many offspring can compensate for high juvenile mortality.
- Producing fewer offspring allows greater parental investment in each one.
- Courtship, mate competition, parental care, reproductive timing, and specialised reproductive structures can all increase reproductive success.
- Reproductive strategies involve trade-offs because animals have limited energy and resources.
- Different environments favour different combinations of reproductive characteristics.
- Reproductive success depends not simply on producing offspring, but on producing descendants that survive and reproduce.
- There is no single reproductive strategy that is optimal in every environment.
2. Fertilization and Development
Learning outcomes
- I can distinguish between internal and external fertilization.
- I can describe major stages of animal development.
- I can explain how embryos obtain nutrients and protection.
- I can compare development in different animal groups.
- I can relate developmental strategies to survival.
From Gametes to a New Animal
Reproduction is only the beginning of producing a new animal. After sperm and egg are produced, they must come together, and the resulting cell must develop into a functioning organism.
In sexually reproducing animals, the general sequence is:
Gametes → fertilization → zygote → embryo → growth and development → juvenile → adult
Different animal groups accomplish these stages in very different ways.
Some release millions of gametes into water. Others fertilize eggs internally. Some embryos develop inside eggs, while others develop inside the parent's body.
These differences are closely related to the animal's environment, lifestyle, and reproductive strategy.
Fertilization
Fertilization is the fusion of the nuclei of male and female gametes.
In most animals:
- The male gamete is the sperm.
- The female gamete is the egg.
Each gamete contains one set of chromosomes.
When their nuclei fuse, they form a zygote containing genetic information from both parents.
Sperm + egg → zygote
The zygote is the first cell of the new individual.
What Happens During Fertilization?
Although the details vary between animal groups, fertilization generally involves several important events.
A sperm reaches an egg.
The sperm interacts with and penetrates the egg's outer layers.
The cell membranes of the gametes fuse.
The sperm nucleus enters the egg.
The genetic material from the two gametes combines.
Mechanisms then normally prevent additional sperm from fertilizing the same egg.
The resulting zygote can then begin development.
External Fertilization
External fertilization occurs when sperm and eggs meet outside the body.
It is particularly common among aquatic animals, including many:
- Fish.
- Amphibians.
- Marine invertebrates.
During a process called spawning, adults may release sperm and eggs into the surrounding water.
Water provides a suitable environment because it prevents gametes from drying out and allows sperm to move toward eggs.
Challenges of External Fertilization
External fertilization can be inefficient.
Once released, gametes may:
- Be dispersed by currents.
- Be eaten.
- Fail to encounter another gamete.
- Be exposed to unsuitable temperatures.
- Be damaged by changing environmental conditions.
Animals using external fertilization often compensate by producing large numbers of gametes.
If thousands or millions of eggs and sperm are released, the probability that at least some eggs are fertilized increases.
Timing External Fertilization
Timing is extremely important.
If one animal releases eggs several hours or days before another releases sperm, fertilization may be unlikely.
Many species therefore synchronize spawning.
Environmental signals can include:
- Temperature.
- Day length.
- Tides.
- Rainfall.
- Lunar cycles.
- Chemical signals from other animals.
Large numbers of animals releasing gametes at approximately the same time can greatly increase the probability of successful fertilization.
Internal Fertilization
Internal fertilization occurs when sperm and egg meet inside the reproductive tract.
It occurs in many:
- Insects.
- Reptiles.
- Birds.
- Mammals.
- Sharks and rays.
- Other animals.
Internal fertilization is particularly useful in terrestrial environments because gametes are protected from drying out.
Advantages of Internal Fertilization
Internal fertilization provides several potential advantages.
Gametes are:
- Protected from dehydration.
- Less exposed to predators.
- Less likely to be dispersed.
- More likely to encounter one another.
As a result, animals using internal fertilization often do not need to produce as many gametes as animals that release them freely into the environment.
However, internal fertilization usually requires mating and specialised reproductive structures or behaviours.
Internal and External Fertilization Compared
| Feature | External Fertilization | Internal Fertilization |
|---|---|---|
| Where gametes meet | Outside body | Inside reproductive tract |
| Common environment | Often aquatic | Aquatic or terrestrial |
| Number of gametes | Often very large | Often fewer |
| Gamete protection | Relatively low | Relatively high |
| Chance of individual gametes meeting | Often lower | Generally higher |
| Mating required | Not always direct mating | Usually involves sperm transfer |
| Examples | Many fish and amphibians | Reptiles, birds, mammals |
Neither method guarantees reproductive success. Each is associated with different advantages, costs, and adaptations.
Development Begins
After fertilization, the zygote begins to divide.
This occurs through mitosis.
One cell becomes two.
Two become four.
Four become eight.
Cell division continues, producing an increasing number of cells.
During these early divisions, the embryo begins the complex process of becoming a multicellular organism.
From Zygote to Embryo
The early stages of animal development can be simplified as:
Zygote → repeated cell division → multicellular embryo → cell differentiation → tissues and organs
The exact developmental stages and terminology vary among animal groups.
A crucial point is that producing more cells is not enough.
Those cells must also become specialized.
Cell Differentiation
Differentiation is the process by which cells become specialised for particular functions.
Early embryonic cells eventually give rise to many cell types, including:
- Muscle cells.
- Neurons.
- Blood cells.
- Skin cells.
- Bone cells.
Nearly all these cells contain the same basic genetic information.
They become different because different genes are active in different cells.
Formation of Tissues and Organs
Specialised cells become organised into tissues.
Tissues form organs.
Organs become integrated into organ systems.
For example:
Muscle cells → muscle tissue → muscular structures → movement
and
Neurons → nervous tissue → brain, spinal cord and nerves → coordination
Development therefore involves increasing levels of biological organisation.
Embryos Need Resources
An embryo is alive and developing rapidly.
It needs:
- Nutrients.
- Water.
- Oxygen.
- Suitable temperature.
- Protection from physical damage.
- A way of dealing with metabolic wastes.
Different animal groups have evolved different methods of meeting these needs.
Development in Aquatic Eggs
Many fish and amphibians lay eggs in water.
The developing embryo may obtain nutrients from yolk stored in the egg.
Oxygen can diffuse from the surrounding water.
Waste products can diffuse away.
The surrounding water helps prevent dehydration, but the eggs may remain vulnerable to predators and environmental changes.
Yolk
Yolk is a nutrient-rich material stored within many animal eggs.
It can provide developing embryos with substances needed for:
- Respiration.
- Growth.
- Formation of new tissues.
Species whose embryos develop for longer periods without receiving nutrients directly from a parent often require substantial stored resources.
The amount and distribution of yolk vary greatly among animal groups.
Development on Land
Reproduction on land creates a major problem:
How can an embryo remain protected without drying out?
Reptiles, birds, and other amniotes evolved an important solution — the amniotic egg.
An amniotic egg creates a protected environment in which an embryo can develop away from open water.
The Amniotic Egg
Important structures of a typical amniotic egg include:
- Shell – provides protection while allowing gas exchange.
- Amnion – surrounds the embryo with fluid.
- Yolk sac – contains stored nutrients.
- Chorion – contributes to gas exchange.
- Allantois – involved in waste storage and gas exchange.
These structures help solve several problems associated with development on land.
Why the Amnion Matters
The amnion forms a fluid-filled environment around the embryo.
This:
- Cushions the embryo.
- Helps protect it from physical shocks.
- Prevents the embryo itself from drying out.
- Provides an aquatic-like environment inside the egg.
This was a major evolutionary development because reproduction no longer had to depend directly on an external body of water.
The Eggshell
A bird's eggshell must solve two apparently conflicting problems.
It must:
- Protect the embryo.
- Allow gases to move between the embryo and the environment.
The shell contains microscopic pores.
These allow:
- Oxygen to enter.
- Carbon dioxide to leave.
The shell also reduces water loss while still allowing sufficient gas exchange.
Development Inside the Parent
In many mammals, the embryo develops inside the female reproductive system.
After fertilization and early cell division, the embryo eventually becomes associated with the wall of the uterus.
A specialised organ called the placenta develops.
The placenta allows substances to be exchanged between the maternal and fetal circulatory systems without their blood normally mixing directly.
The Placenta
The placenta provides an exchange surface.
Useful substances can move toward the developing embryo or fetus, including:
- Oxygen.
- Glucose.
- Amino acids.
- Other nutrients.
Waste products can move in the opposite direction, including:
- Carbon dioxide.
- Urea and other metabolic wastes.
The developing offspring is connected to the placenta through the umbilical cord.
Adaptations of the Placenta
An effective exchange surface requires efficient transfer of substances.
The placenta has features that support exchange, including:
- Large surface area.
- Thin barriers between maternal and fetal blood supplies.
- Good blood supply.
- Maintenance of concentration gradients.
These features resemble adaptations found at other biological exchange surfaces, such as:
- Alveoli.
- Intestinal villi.
- Fish gills.
Different organs can therefore use similar structural principles to solve similar exchange problems.
Embryo and Fetus
In mammals, the terms embryo and fetus describe different stages of prenatal development.
During the embryonic stage, major body structures and organ systems begin forming.
During the fetal stage, growth and further development of these structures become increasingly important.
The precise timing differs among species.
Oviparous Animals
Animals that lay eggs from which offspring later hatch are described as oviparous.
Examples include most:
- Birds.
- Reptiles.
- Amphibians.
- Fish.
- Insects.
Embryos generally rely heavily on nutrients stored within the egg.
Viviparous Animals
In viviparous animals, offspring develop inside the parent and are born alive.
Most mammals are viviparous.
Some reptiles, fish, and other animals also give birth to live young.
Internal development can provide:
- Protection from predators.
- More stable temperature.
- Protection from dehydration.
- In some species, continuous nutrient supply from the parent.
However, carrying developing offspring requires considerable parental energy.
Ovoviviparity and Other Strategies
Animal development does not always fit neatly into simple categories.
In some species, eggs are retained inside the parent's body until they hatch or nearly hatch.
The embryo may depend mainly on yolk rather than receiving nutrients through a placenta.
This strategy has traditionally been called ovoviviparity, although modern zoology often uses more precise descriptions because reproductive systems vary considerably among species.
This illustrates an important biological principle:
Reproductive strategies form a continuum rather than a few perfectly separate categories.
Direct Development
In direct development, young animals emerge looking broadly similar to smaller versions of adults.
They still grow and mature, but there is no dramatically different larval stage.
Examples occur in:
- Mammals.
- Birds.
- Many reptiles.
Indirect Development
In indirect development, the young pass through a distinct larval stage that differs substantially from the adult.
Examples include:
- Frogs.
- Butterflies.
- Many marine invertebrates.
A major transformation called metamorphosis may occur.
Metamorphosis
Metamorphosis is a major change in body form during development.
A frog provides a familiar example.
Egg → tadpole → developing frog → adult frog
A tadpole differs considerably from an adult frog.
Tadpoles generally:
- Live entirely in water.
- Swim using a tail.
- Use gills early in development.
- Often feed differently from adults.
During metamorphosis:
- Legs develop.
- Lungs become increasingly important.
- The tail is reduced.
- Feeding structures change.
Complete Metamorphosis in Insects
Many insects undergo particularly dramatic development.
For example, butterflies undergo:
Egg → larva → pupa → adult
The larva and adult can have very different:
- Body structures.
- Diets.
- Behaviours.
- Habitats.
This can reduce competition between juveniles and adults for the same resources.
Incomplete Metamorphosis
Other insects undergo incomplete metamorphosis.
A typical sequence is:
Egg → nymph → adult
Examples include grasshoppers and many other insects.
A nymph resembles a smaller version of the adult but lacks some adult structures, such as fully developed wings and reproductive organs.
Comparing Development in Major Animal Groups
| Animal Group | Typical Fertilization | Development | Embryo Nutrition | Protection |
|---|---|---|---|---|
| Many fish | External | Aquatic eggs | Yolk | Egg coverings/water |
| Many amphibians | External | Egg and larval stages | Yolk | Jelly-like egg coverings/water |
| Reptiles | Internal | Usually eggs or internal development | Often yolk | Amniotic structures |
| Birds | Internal | Eggs | Large yolk supply | Amniotic egg and shell |
| Most mammals | Internal | Inside uterus | Maternal supply through placenta | Parent's body |
These are general patterns. Exceptions occur within several animal groups.
Worked Example: Fish
Many fish release eggs and sperm into water.
Fertilization occurs externally.
The embryo develops within an egg.
Stored yolk supplies nutrients.
Oxygen enters from the surrounding water.
After hatching, the young may receive little or no parental care.
Producing many eggs can compensate for high mortality.
Worked Example: Frog
Many frogs also use external fertilization.
Their eggs are generally laid in water.
The embryo develops into a larval stage called a tadpole.
The tadpole later undergoes metamorphosis.
This strategy allows different life stages to exploit different ecological conditions.
Worked Example: Bird
Birds use internal fertilization.
The fertilized egg receives nutrients before it is laid.
The embryo develops inside an amniotic egg.
The yolk supplies nutrients.
The shell and membranes provide protection while permitting gas exchange.
Parents commonly incubate the eggs, helping maintain suitable developmental temperatures.
After hatching, many bird species provide extensive parental care.
Worked Example: Placental Mammal
In a placental mammal:
- Fertilization occurs internally.
- The zygote begins dividing.
- The early embryo travels toward the uterus.
- Implantation occurs.
- The placenta develops.
- Nutrients and oxygen are transferred from the mother.
- Wastes are transferred away from the developing offspring.
- Development continues until birth.
This strategy provides considerable protection but requires substantial investment by the mother.
Development and Parental Investment
Developmental strategy is closely connected to parental investment.
Consider two extremes.
Strategy A
An animal produces thousands of eggs.
Each embryo receives relatively little parental protection.
Most offspring die before reaching adulthood.
A small number survive.
Strategy B
An animal produces one offspring.
The offspring develops internally.
The parent protects and feeds it for a long period.
Its probability of survival may be much higher.
Both strategies can successfully maintain a population.
Protection Versus Number of Offspring
Animals have limited energy and resources.
A major reproductive trade-off therefore exists:
Many offspring + relatively little investment in each
or
Few offspring + relatively high investment in each
Neither strategy is automatically superior.
Success depends on environmental conditions and the organism's ecology.
Development in Unpredictable Environments
In environments where juvenile survival is unpredictable, producing many offspring can increase the probability that at least some survive.
For example, aquatic eggs may face:
- Predators.
- Changing temperatures.
- Currents.
- Changing oxygen levels.
- Disease.
Producing many offspring spreads reproductive investment across many individuals.
Development in Protected Environments
Internal development provides a relatively protected environment.
The developing offspring may experience:
- More stable temperature.
- Reduced dehydration.
- Physical protection.
- Reduced exposure to predators.
- Reliable access to nutrients in placental species.
However, internal development can limit how many offspring can be supported at one time.
It also increases energetic costs for the parent.
Developmental Timing
The length of development varies greatly among animals.
Factors influencing developmental time can include:
- Body size.
- Temperature.
- Nutrient availability.
- Metabolic rate.
- Developmental strategy.
- Degree of development required before hatching or birth.
Some offspring emerge relatively independent.
Others require prolonged parental care.
Precocial Young
Precocial young are relatively well developed when they hatch or are born.
They may be able to:
- Walk.
- Follow parents.
- Feed relatively independently.
- Respond quickly to threats.
Examples include young of many ground-nesting birds.
Producing well-developed young requires considerable energy before hatching or birth.
Altricial Young
Altricial young are relatively underdeveloped at hatching or birth and require substantial parental care.
Many songbirds, for example, hatch:
- Small.
- Relatively helpless.
- Dependent on parents for food.
- Unable to maintain body temperature effectively.
Parents must invest heavily after hatching.
This represents another developmental trade-off:
Greater development before birth/hatching ↔ greater development afterward
Embryonic Survival
For development to succeed, an embryo must survive several challenges.
It must obtain:
- Energy.
- Raw materials.
- Oxygen.
It must also:
- Remove metabolic wastes.
- Avoid dehydration.
- Avoid physical damage.
- Remain within a suitable temperature range.
- Avoid infection and predation.
Different animal groups have evolved very different structures to solve these same fundamental problems.
Development and Natural Selection
Developmental strategies are shaped by natural selection.
Characteristics that increase the probability that offspring survive and eventually reproduce can become more common over generations.
For example:
- Protective eggshells can improve survival on land.
- Internal fertilization can prevent gamete dehydration.
- Metamorphosis can reduce competition between life stages.
- Parental care can increase juvenile survival.
- Large numbers of eggs can compensate for high juvenile mortality.
Development is therefore closely connected to an animal's ecological environment.
Comparing Three Strategies
Consider three animals.
Frog
External fertilization.
Many eggs.
Aquatic embryo.
Aquatic larval stage.
Metamorphosis.
Bird
Internal fertilization.
Relatively few eggs.
Amniotic development.
Protected shell.
Often considerable parental care.
Placental Mammal
Internal fertilization.
Usually relatively few offspring.
Internal development.
Placental nutrient supply.
Often extensive parental care.
Each strategy solves the same fundamental problem — producing surviving offspring — in a different way.
Worked Example: Why Are Frog Eggs Usually Laid in Water?
Frog eggs generally lack the protective shell found around bird eggs.
If exposed to dry terrestrial conditions, they can lose water rapidly.
Water:
- Prevents dehydration.
- Supports external fertilization.
- Allows sperm to move.
- Provides an environment for early development.
The aquatic environment is therefore closely connected to both fertilization and development.
Worked Example: Why Can Birds Reproduce Away from Water?
Birds use internal fertilization.
Their embryos develop within amniotic eggs.
The egg contains:
- Stored nutrients.
- Protective membranes.
- Fluid surrounding the embryo.
- A shell that reduces water loss while allowing gas exchange.
The developing embryo therefore carries its own protected environment.
This greatly reduces dependence on external water for reproduction.
Worked Example: Why Does a Placenta Improve Embryonic Survival?
A mammalian embryo developing inside the uterus does not need to rely entirely on a fixed amount of stored yolk.
The placenta allows continued exchange with the mother.
As the embryo grows:
Oxygen and nutrients → toward developing offspring
Carbon dioxide and metabolic wastes → away from developing offspring
This allows prolonged internal development and can produce relatively well-developed young at birth.
Common Mistakes
Confusing Fertilization With Development
Fertilization produces a zygote. Development includes the subsequent processes that transform the zygote into a multicellular organism.
Saying External Fertilization Means External Development
The two ideas are related but different. Fertilization describes where sperm and egg meet; development describes where and how the embryo grows.
Saying All Fish Use External Fertilization
Many do, but some fish use internal fertilization.
Saying All Mammals Have Placentas Like Humans
Placental mammals use highly developed placentas, but mammalian reproductive strategies include important variations, including marsupials and egg-laying monotremes.
Saying the Placenta Mixes Maternal and Fetal Blood
The two blood supplies normally remain separate. Substances are exchanged across specialised tissues.
Thinking an Eggshell Is Completely Sealed
Bird eggshells contain microscopic pores that permit gas exchange.
Saying Yolk Is the Embryo
The yolk is primarily a nutrient supply. The embryo is the developing animal.
Confusing Growth and Differentiation
Growth increases size and cell number. Differentiation produces specialised cell types.
Assuming More Parental Care Is Always Better
Parental care can increase offspring survival but requires considerable time and energy and may reduce the number of offspring that can be produced.
Check Your Understanding
1. Define fertilization.
2. Distinguish between internal and external fertilization.
3. Explain why external fertilization is especially common in aquatic environments.
4. Why do many animals using external fertilization release large numbers of gametes?
5. Describe the sequence from fertilization to the formation of a multicellular embryo.
6. Explain the importance of cell differentiation during development.
7. What resources does a developing embryo require?
8. Explain the role of yolk.
9. Describe three ways an amniotic egg protects or supports a developing embryo.
10. Explain how gases can be exchanged through a bird's egg.
11. Describe the role of the placenta.
12. Why do maternal and fetal circulatory systems need to be positioned close together in the placenta?
13. Distinguish between direct and indirect development.
14. Explain one potential advantage of metamorphosis.
15. Compare the developmental strategies of a frog, a bird, and a placental mammal.
16. Explain how developmental strategy can influence the probability that an offspring survives to adulthood.
Key Terms
- Fertilization – fusion of male and female gamete nuclei.
- Internal fertilization – fertilization occurring inside the reproductive tract.
- External fertilization – fertilization occurring outside the body.
- Gamete – specialised reproductive cell.
- Sperm – male gamete.
- Egg – female gamete.
- Zygote – first cell formed following fertilization.
- Embryo – early developing multicellular organism.
- Fetus – later stage of prenatal development in mammals.
- Mitosis – cell division producing genetically similar daughter cells.
- Differentiation – process by which cells become specialised.
- Yolk – nutrient-rich material that supplies a developing embryo.
- Amnion – membrane enclosing a fluid-filled environment around an embryo.
- Amniotic egg – egg containing specialised membranes that support development on land.
- Placenta – organ allowing exchange between maternal and developing offspring's systems.
- Umbilical cord – structure connecting a developing mammal to the placenta.
- Oviparous – producing offspring by laying eggs.
- Viviparous – producing live young after internal development.
- Metamorphosis – major change in body form during development.
- Larva – immature developmental stage that differs substantially from the adult.
- Pupa – developmental stage between larva and adult in insects undergoing complete metamorphosis.
- Nymph – immature stage of an insect undergoing incomplete metamorphosis.
- Precocial – relatively well developed at hatching or birth.
- Altricial – relatively underdeveloped and dependent at hatching or birth.
- Parental investment – resources, time, and energy devoted to producing and supporting offspring.
Key Takeaways
- Fertilization occurs when male and female gamete nuclei combine to produce a zygote.
- Fertilization can occur internally or externally.
- External fertilization is especially common in aquatic environments and often involves the production of many gametes.
- Internal fertilization protects gametes and is particularly important for reproduction on land.
- After fertilization, repeated mitosis produces a multicellular embryo.
- Differentiation produces specialised cells that eventually form tissues, organs, and organ systems.
- Every developing embryo requires nutrients, oxygen, suitable conditions, waste removal, and protection.
- Yolk provides stored nutrients for embryos developing in many types of eggs.
- The amniotic egg provides a protected environment for development on land.
- Bird eggs combine protection, nutrient storage, reduced water loss, and gas exchange.
- In placental mammals, the placenta allows exchange of oxygen, nutrients, and metabolic wastes.
- Direct development produces young broadly similar in body form to adults.
- Indirect development involves a distinct larval stage and often metamorphosis.
- Different life stages can occupy different ecological niches, reducing competition between juveniles and adults.
- Some animals produce many relatively unprotected offspring, while others produce fewer offspring with greater protection and parental investment.
- Developmental strategies involve trade-offs between offspring number, protection, energy investment, and survival probability.
- Fertilization and development are closely adapted to an animal's environment and way of life.
3. Parental Care
Learning outcomes
- I can define parental care and explain its importance.
- I can compare different parental care strategies.
- I can explain the relationship between parental care and offspring survival.
- I can identify examples of parental care in different animals.
- I can evaluate the costs and benefits of parental investment.
What Is Parental Care?
Producing offspring is only part of reproductive success. In many animal species, parents also perform behaviours that increase the probability that their offspring will survive.
Parental care is any behaviour by a parent that increases the survival or future reproductive success of its offspring, usually at some cost to the parent.
Parental care can include:
- Protecting eggs.
- Building nests.
- Keeping eggs warm.
- Providing food.
- Carrying offspring.
- Defending young from predators.
- Cleaning or grooming offspring.
- Teaching or assisting young in finding food.
- Helping young learn important behaviours.
However, parental care requires time, energy, and resources. Different species therefore show very different levels of parental investment.
Why Does Parental Care Matter?
Young animals are often particularly vulnerable.
They may:
- Be unable to find enough food.
- Have difficulty regulating body temperature.
- Be unable to defend themselves.
- Have limited mobility.
- Lack experience.
- Be vulnerable to predators.
- Be susceptible to environmental changes.
Parental care can reduce these risks.
In species where young depend heavily on their parents, successful parental care can have a major effect on the number of offspring that survive to adulthood.
Parental Investment
Parental investment refers to the time, energy, and resources that a parent devotes to an offspring in ways that can improve that offspring's chances of survival and reproduction.
Investment can begin before birth or hatching.
Examples include:
- Producing nutrient-rich eggs.
- Pregnancy.
- Building a nest.
- Selecting a safe nesting site.
- Incubating eggs.
It can continue after birth or hatching through:
- Feeding.
- Protection.
- Transport.
- Teaching.
- Grooming.
Parental Care Before Hatching
Parental care does not necessarily begin when offspring emerge from an egg.
Many animals protect their developing embryos.
Birds commonly:
- Build nests.
- Incubate eggs.
- Defend nests.
- Turn eggs.
- Maintain suitable temperatures.
Some fish guard nests containing eggs.
Some reptiles guard nests or remain near eggs.
These behaviours can increase the probability that embryos survive until hatching.
Nest Building
A nest provides a controlled location for eggs or young.
Nests can provide:
- Protection from predators.
- Protection from wind and rain.
- Insulation.
- A suitable temperature.
- A place where parents can locate and care for offspring.
Nests can range from simple depressions in the ground to elaborate structures requiring considerable time and energy to construct.
Incubation
Bird embryos require suitable temperatures for normal development.
Many birds therefore incubate their eggs.
During incubation, a parent sits on or otherwise warms the eggs.
This helps maintain a suitable developmental temperature.
However, incubation has costs.
A parent spending long periods incubating eggs may have:
- Less time to find food.
- Increased energy requirements.
- Greater exposure to predators.
- Fewer opportunities to reproduce again.
Parental care therefore involves trade-offs.
Feeding Offspring
Many young animals cannot obtain enough food independently.
Parents may provide food directly.
Examples include:
- Birds bringing insects to chicks.
- Mammals producing milk.
- Predators bringing prey to their young.
- Some insects supplying food to larvae.
Providing food can increase:
- Growth rate.
- Development.
- Survival.
- Ability to resist disease.
- Probability of eventually reaching reproductive age.
Protection from Predators
Predation is a major cause of mortality among young animals.
Parents may protect offspring by:
- Guarding them.
- Hiding them.
- Attacking predators.
- Giving alarm calls.
- Moving offspring to safer locations.
- Keeping young together.
Protection can greatly improve offspring survival, but it may also place the parent in danger.
Carrying Offspring
Some animals physically transport their offspring.
Examples include:
- Kangaroos carrying young in pouches.
- Primates carrying infants.
- Crocodilians carrying young in their mouths.
- Some frogs carrying tadpoles.
- Some spiders carrying egg sacs or young.
Carrying offspring can provide protection and help young reach suitable feeding or developmental areas.
Parental Care in Mammals
Mammals typically show substantial parental investment.
One defining mammalian characteristic is the production of milk by mammary glands.
Milk provides young mammals with:
- Water.
- Energy-rich nutrients.
- Proteins.
- Minerals.
- Other substances that support development.
Many mammals also provide:
- Protection.
- Warmth.
- Grooming.
- Social interaction.
- Opportunities for learning.
In some species, parental care continues for years.
Learning from Parents
In animals with complex behaviour, parental care may involve more than feeding and protection.
Young animals may learn important behaviours by observing or interacting with adults.
These can include:
- Hunting techniques.
- Food selection.
- Migration routes.
- Predator recognition.
- Social behaviour.
- Communication.
- Use of tools.
This can be particularly important in animals with long developmental periods and complex nervous systems.
Parental Care in Birds
Birds display an enormous range of parental behaviours.
These can include:
- Nest construction.
- Egg incubation.
- Egg turning.
- Nest defence.
- Feeding chicks.
- Removing waste from nests.
- Teaching young to locate food.
In many bird species, both parents contribute to care.
Sharing parental duties can increase the amount of food and protection available to offspring.
Altricial Young
Some birds produce altricial young.
Altricial offspring hatch in a relatively underdeveloped condition.
They may be:
- Nearly helpless.
- Unable to fly.
- Poorly insulated.
- Unable to obtain food independently.
These offspring require extensive parental care.
Many songbirds follow this strategy.
Precocial Young
Other birds produce precocial young.
Precocial offspring are relatively well developed at hatching.
They may:
- Have feathers or down.
- Walk soon after hatching.
- Follow their parents.
- Feed more independently.
Ducklings are a familiar example.
Precocial offspring may require less direct feeding, although parents can still provide protection, guidance, and warmth.
Parental Care in Fish
Although many fish provide little care after spawning, others show sophisticated parental behaviour.
Some fish:
- Build nests.
- Guard eggs.
- Fan water across eggs.
- Defend young.
- Carry eggs or young in their mouths.
Mouthbrooding occurs when a parent carries eggs or young inside its mouth for protection.
This can dramatically reduce predation but may interfere with the parent's ability to feed.
Parental Care in Amphibians
Amphibian parental care varies enormously.
Many frogs lay large numbers of eggs and provide little additional care.
Other species may:
- Guard eggs.
- Keep eggs moist.
- Carry tadpoles.
- Transport tadpoles to small pools.
- Provide unfertilized eggs as food.
This demonstrates that closely related animals can evolve very different reproductive strategies.
Parental Care in Reptiles
Parental care among reptiles also varies.
Many species provide relatively little care after laying eggs.
Others show extensive parental behaviour.
Crocodilians, for example, may:
- Guard nests.
- Assist hatchlings leaving the nest.
- Carry young toward water.
- Defend young after hatching.
Parental care is therefore not restricted to mammals and birds.
Parental Care in Invertebrates
Some invertebrates also provide remarkable parental care.
Examples include:
- Spiders carrying egg sacs.
- Insects guarding eggs.
- Beetles preparing food for larvae.
- Octopuses guarding eggs.
A female octopus may guard and ventilate her eggs for an extended period, often eating little or not at all during this time.
This represents an extremely high reproductive investment.
Maternal Care
When the female provides most or all parental care, this is called maternal care.
It is common in mammals because pregnancy and milk production already create a strong biological connection between mothers and offspring.
Maternal care can include:
- Nursing.
- Protection.
- Carrying.
- Grooming.
- Teaching.
- Maintaining warmth.
However, parental-care patterns vary greatly among mammal species.
Paternal Care
In some species, males provide substantial care.
This is called paternal care.
Male care can include:
- Guarding eggs.
- Incubating eggs.
- Feeding young.
- Carrying offspring.
- Defending offspring.
Male emperor penguins provide a famous example. The male incubates the egg on his feet beneath a fold of skin while experiencing extremely harsh Antarctic conditions.
Biparental Care
In biparental care, both parents contribute to raising offspring.
This occurs in many birds and some mammals, fish, and other animals.
Potential advantages include:
- More food for offspring.
- Greater nest defence.
- Shared incubation.
- More effective protection.
- Reduced workload for each parent.
However, both parents must devote resources to the current offspring.
Cooperative Care
Parental care does not always involve only the biological parents.
In some social species, other members of the group help care for young.
This is sometimes called cooperative breeding or cooperative care.
Helpers may:
- Feed offspring.
- Guard them.
- Carry them.
- Protect them.
- Teach them.
This occurs in animals such as some birds, meerkats, and certain primates.
Little or No Parental Care
Not all animals care for their offspring.
Many species produce eggs or young and then leave them.
Examples occur among many:
- Fish.
- Marine invertebrates.
- Amphibians.
- Insects.
These animals often compensate by producing large numbers of offspring.
If hundreds or thousands are produced, some may survive even if mortality is high.
Number of Offspring and Parental Care
A broad reproductive trade-off exists between:
Number of offspring ↔ investment in each offspring
Animals producing many offspring generally cannot provide intensive care to every one.
Animals producing relatively few offspring can potentially invest much more in each.
This is not an absolute rule, but it is an important general pattern in animal reproductive biology.
Worked Example: Cod and Elephant
Imagine comparing two very different reproductive strategies.
A large fish may produce enormous numbers of eggs.
An elephant normally produces one calf at a time and provides years of care.
The fish strategy depends partly on:
Very many offspring × low probability of survival for each
The elephant strategy depends more heavily on:
Few offspring × high parental investment × relatively high survival probability
Both strategies can result in successful reproduction.
Benefits of Parental Care
Parental care can provide many benefits to offspring.
These include:
- Increased protection from predators.
- More reliable food supply.
- Improved temperature regulation.
- Reduced exposure to environmental extremes.
- Protection from competitors.
- Opportunities to learn.
- Increased growth.
- Increased probability of reaching adulthood.
The ultimate evolutionary benefit is increased probability that offspring survive and eventually reproduce.
The Costs of Parental Care
Parental care is not free.
Parents have limited amounts of:
- Energy.
- Food.
- Time.
- Body reserves.
Resources invested in current offspring cannot be used for other purposes.
Potential costs include:
- Increased energy expenditure.
- Reduced feeding time.
- Increased exposure to predators.
- Increased risk of injury.
- Delayed future reproduction.
- Reduced number of additional offspring.
- Reduced parental survival.
This creates an important biological trade-off.
Current Versus Future Reproduction
Imagine an animal has limited energy.
It could invest heavily in its current offspring.
This may increase their probability of survival.
However, extremely high investment could reduce the parent's condition and decrease its ability to reproduce again.
Alternatively, the parent could provide less care now and preserve resources for future offspring.
Natural selection therefore acts on a balance between:
Current reproductive success ↔ future reproductive opportunities
Worked Example: Feeding Chicks
Suppose two parent birds are feeding four chicks.
Every feeding trip requires energy.
It may also expose the adults to predators.
If the parents make more trips:
- Chicks receive more food.
- Chicks may grow faster.
- Chick survival may increase.
But:
- Parents use more energy.
- Parents may experience greater predation risk.
- Parents may lose body condition.
The optimal amount of parental care therefore involves both benefits and costs.
Offspring Competition
When several offspring are raised together, they may compete for parental resources.
Competition can involve:
- Food.
- Space.
- Warmth.
- Parental attention.
In a bird nest, chicks may beg loudly or position themselves closer to the parent.
Parents may respond to these signals when distributing food.
Parent-Offspring Conflict
Parents and offspring do not always have identical interests.
An individual offspring can benefit from receiving more resources.
However, a parent must divide resources among:
- That offspring.
- Its siblings.
- Future offspring.
- The parent's own survival.
This can produce parent-offspring conflict.
For example, offspring may continue begging for food even when reducing feeding would help preserve the parent's condition.
Care and Offspring Development
The amount of parental care required often depends on how developed offspring are at birth or hatching.
Altricial offspring generally require more intensive care.
Precocial offspring can often perform more behaviours independently.
However, even highly developed young may still benefit from:
- Protection.
- Guidance.
- Social learning.
- Access to food.
Developmental strategy and parental-care strategy are therefore closely connected.
Care and Learning
Long periods of parental care can create opportunities for learning.
Young predators may need time to learn how to hunt.
Young primates may learn:
- Food selection.
- Social relationships.
- Communication.
- Tool use.
Young elephants may learn:
- Migration routes.
- Water locations.
- Social behaviours.
In animals with complex behaviours, learning can be a major benefit of prolonged care.
Parental Care and Survival
The effect of parental care can be understood as a probability.
Suppose two hypothetical species each produce 100 offspring.
Species A provides no care, and 5 offspring survive.
Species B provides extensive care, and 60 offspring survive.
Species B does not necessarily need to produce as many offspring to maintain its population.
This helps explain why species with extensive parental care often produce fewer offspring.
Environmental Conditions Matter
The value of parental care depends partly on the environment.
Parental care may be especially valuable where:
- Predation risk is high.
- Food is difficult for young to obtain.
- Temperatures are challenging.
- Suitable nesting sites are limited.
- Young require complex learned behaviours.
In other environments, producing many offspring with relatively little care may be successful.
Parental Care and Predation
Parental behaviour can affect predation in different ways.
Guarding can reduce the probability that offspring are eaten.
However, parents moving repeatedly to and from a nest might attract attention.
Parents therefore face a balance between:
Providing resources ↔ avoiding detection
This demonstrates why parental behaviour is often highly adapted to local ecological conditions.
Parental Care and Natural Selection
Parental-care behaviours can evolve when they increase overall reproductive success.
Imagine a genetic or behavioural characteristic that causes parents to protect their offspring slightly more effectively.
If those offspring are more likely to survive and reproduce, the characteristics associated with effective care may become more common over generations.
However, care that is excessively costly could reduce parental survival or future reproduction.
Natural selection therefore acts on both:
- Benefits to offspring.
- Costs to parents.
Evaluating Parental Investment
When evaluating a parental-care strategy, it is useful to ask:
- How many offspring are produced?
- How vulnerable are they?
- How much care does each receive?
- How much does care increase survival?
- How much energy does care require?
- Does care expose parents to danger?
- Does it reduce future reproduction?
- Do offspring need to learn complex behaviours?
- How predictable is the environment?
There is rarely a simple "best" strategy.
The effectiveness of a strategy depends on the animal's biology and environment.
Worked Example: Sea Turtle
A sea turtle lays many eggs.
The mother invests considerable energy in:
- Producing eggs.
- Migrating to nesting areas.
- Digging a nest.
- Depositing and burying the eggs.
However, after nesting, there is generally no prolonged parental care.
Many hatchlings die, but producing numerous offspring increases the probability that some survive.
Worked Example: Emperor Penguin
Emperor penguins use a very different strategy.
Only a small number of offspring are produced at one time.
The egg and chick receive substantial parental investment.
Parents:
- Incubate the egg.
- Protect it from extreme cold.
- Provide food.
- Protect the chick.
Each offspring represents a large investment, so protecting that offspring can have a major effect on reproductive success.
Worked Example: Poison Dart Frog
Some poison dart frogs show surprisingly extensive parental care.
Depending on the species, parents may:
- Guard eggs.
- Keep eggs moist.
- Carry tadpoles on their backs.
- Transport tadpoles to suitable pools.
This increases offspring survival but requires considerable parental time and energy.
Worked Example: Octopus
Some female octopuses invest heavily in a single reproductive event.
A female may:
- Guard her eggs.
- Clean them.
- Move water over them to provide oxygen.
- Defend them from predators.
During this period, she may eat very little.
This represents a dramatic example of the cost of parental investment.
Comparing Parental Care Strategies
| Strategy | Typical Offspring Number | Care per Offspring | Possible Survival of Each Offspring |
|---|---|---|---|
| Little or no care | Often high | Low | Often relatively low |
| Maternal or paternal care | Variable | Moderate to high | Often increased |
| Biparental care | Often lower | High | Often relatively high |
| Cooperative care | Often lower | Potentially very high | Can be high |
These are broad patterns rather than universal rules.
Animal reproductive strategies are highly diverse.
Parental Care and Reproductive Success
The goal of parental care, in evolutionary terms, is not simply to keep offspring comfortable.
Parental care can increase the probability that offspring:
Survive → mature → reproduce
If parental behaviour increases the number of surviving descendants, it can increase reproductive success.
However, the benefits must be considered alongside the costs to the parent.
Common Mistakes
Saying All Animals Care for Their Young
Many animals provide little or no care after eggs or offspring are produced.
Thinking Parental Care Begins at Birth
Nest building, egg guarding, incubation, pregnancy, and provisioning eggs can all represent investment before birth or hatching.
Saying More Parental Care Is Always Better
Care requires energy and time. Excessive investment in one reproductive event could reduce parental survival or future reproduction.
Assuming Only Mothers Provide Care
Maternal, paternal, biparental, and cooperative care all occur in animals.
Assuming Animals With Little Care Are Unsuccessful
Producing many offspring with little care can be a highly successful reproductive strategy.
Confusing Parental Care With Parental Investment
Parental care usually refers to behaviours that benefit offspring. Parental investment is broader and includes resources devoted to producing and supporting offspring.
Assuming More Offspring Means Greater Reproductive Success
What ultimately matters is how successfully genetic information is passed into future generations through surviving, reproducing descendants.
Check Your Understanding
1. Define parental care.
2. Give four examples of parental-care behaviour.
3. Explain how parental care can increase offspring survival.
4. What is parental investment?
5. Compare maternal, paternal, and biparental care.
6. Explain one advantage of biparental care.
7. Why do altricial offspring generally require extensive parental care?
8. Explain the relationship between offspring number and parental investment.
9. Give one example of parental care in a fish, amphibian, reptile, bird, or mammal.
10. Explain why guarding offspring can create both a benefit and a cost.
11. Why might feeding offspring reduce a parent's future reproductive success?
12. Explain why learning can make prolonged parental care especially useful in animals with complex behaviour.
13. Compare the parental-care strategies of a sea turtle and an emperor penguin.
14. Why might environmental conditions affect how useful parental care is?
15. A bird can either spend additional energy feeding its current chicks or preserve that energy for future reproduction. Explain the biological trade-off involved.
Key Terms
- Parental care – parental behaviour that increases offspring survival or future reproductive success.
- Parental investment – time, energy, and resources devoted to offspring that can improve their chances of survival and reproduction.
- Maternal care – care provided primarily by the mother.
- Paternal care – care provided primarily by the father.
- Biparental care – care provided by both parents.
- Cooperative care – care in which additional group members help parents raise offspring.
- Incubation – maintaining suitable conditions for developing eggs, particularly temperature.
- Altricial – relatively underdeveloped and dependent at hatching or birth.
- Precocial – relatively well developed and mobile at hatching or birth.
- Mouthbrooding – carrying eggs or young in the mouth for protection.
- Reproductive success – success in passing genetic information to future generations through surviving descendants.
- Trade-off – situation in which increasing investment in one biological function reduces resources available for another.
- Parent-offspring conflict – differences between the amount of parental investment that benefits an individual offspring and the amount that best balances the parent's overall reproductive interests.
Key Takeaways
- Parental care includes behaviours that increase offspring survival or future reproductive success.
- Care can occur before or after birth or hatching.
- Nest building, incubation, feeding, guarding, carrying, grooming, and teaching are forms of parental care.
- Animals show enormous variation in the amount and type of care they provide.
- Care may be maternal, paternal, biparental, or cooperative.
- Parental care can protect offspring from predators, starvation, environmental extremes, and other dangers.
- Extended care can provide opportunities for young animals to learn complex behaviours.
- Species producing relatively few offspring often invest more resources in each offspring.
- Species providing little care often compensate by producing larger numbers of offspring.
- Parental investment requires time, energy, and resources.
- Caring for current offspring can reduce resources available for parental survival or future reproduction.
- Parental-care strategies therefore involve important cost-benefit trade-offs.
- Extensive care is not automatically superior to limited care; different strategies can succeed under different ecological conditions.
- Natural selection can favour parental behaviours when their benefits to reproductive success outweigh their costs.
- Ultimately, parental investment is successful when it contributes to offspring surviving, reaching reproductive age, and producing descendants of their own.
4. Behavioral Adaptations
Learning outcomes
- I can define behavioral adaptations and explain their importance.
- I can identify examples of innate and learned behaviors.
- I can explain how behavior improves survival and reproduction.
- I can describe behaviors such as migration, communication, and social interactions.
- I can analyze how environmental conditions influence behavior.
What Is a Behavioral Adaptation?
Animals do more than possess physical structures that help them survive. They also behave in ways that increase their chances of surviving and reproducing.
A behavior is an action or response carried out by an organism.
A behavioral adaptation is a behavior that improves an organism's ability to survive or reproduce in its environment.
Examples include:
- Birds migrating between seasonal habitats.
- Wolves hunting cooperatively.
- Meerkats giving alarm calls.
- Nocturnal animals becoming active after dark.
- Penguins huddling together for warmth.
- Animals defending territories.
- Young predators learning hunting techniques.
- Animals performing courtship displays.
Behavior can be just as important to survival as structures such as claws, wings, camouflage, or thick fur.
Why Is Behavior Important?
Animals constantly interact with changing environments.
They must respond to:
- Food availability.
- Predators.
- Competitors.
- Potential mates.
- Temperature.
- Water availability.
- Changes in seasons.
- Other members of their species.
Appropriate behavior allows animals to respond to these conditions rather than simply experiencing them.
For example, an animal may not be able to change the outside temperature, but it can move into shade, enter water, burrow underground, or become active at a cooler time of day.
Behavior therefore provides animals with a powerful way of responding to environmental change.
Behavioral and Structural Adaptations
It is important to distinguish between behavioral adaptations and structural adaptations.
A structural adaptation is a physical feature.
A behavioral adaptation is something an animal does.
For example:
Thick fur → structural adaptation
Huddling together → behavioral adaptation
Webbed feet → structural adaptation
Seasonal migration → behavioral adaptation
Sharp claws → structural adaptation
Cooperative hunting → behavioral adaptation
The two often work together.
A bird may have wings that make long-distance flight possible, but migration itself is a behavior.
Innate Behavior
An innate behavior is a behavior that develops largely without being learned through experience.
Animals inherit biological mechanisms that make these behaviors possible.
Examples can include:
- Suckling by newborn mammals.
- Web-building in many spiders.
- Certain escape responses.
- Courtship displays.
- Some migration behaviors.
- Reflexes.
Innate behaviors can be especially useful when an animal must perform an important action correctly without having time to learn it.
Innate Does Not Mean Simple
Some innate behaviors can be extremely complex.
A spider can construct an organised web without being taught by another spider.
Young animals of some migratory species can travel toward appropriate destinations despite having limited previous experience.
Complex behavior therefore does not necessarily have to be learned.
Genetic information can influence:
- Responses to stimuli.
- Motivation.
- Timing.
- Orientation.
- Patterns of movement.
However, many behaviors contain both innate and learned components.
Learned Behavior
A learned behavior develops or changes through experience.
Learning allows an animal to modify its behavior according to information gathered during its lifetime.
Examples include:
- Learning where food is located.
- Learning to avoid dangerous areas.
- Young predators improving hunting skills.
- Recognizing individuals.
- Learning migration routes.
- Learning communication signals.
- Learning how to use tools.
Learning is especially valuable when environmental conditions are variable or unpredictable.
Innate and Learned Behavior Compared
| Innate Behavior | Learned Behavior |
|---|---|
| Develops largely without training | Develops or changes through experience |
| Strong biological basis | Influenced strongly by environment and experience |
| Often present early in life | Often develops during life |
| Can produce rapid appropriate responses | Allows flexible responses |
| Example: reflex | Example: learning a food location |
These categories are useful, but animal behavior is often influenced by both inheritance and experience.
Learning Through Habituation
One simple form of learning is habituation.
Habituation occurs when an animal gradually reduces its response to a repeated stimulus that is not dangerous.
Imagine birds feeding near a road.
At first, every passing vehicle might cause them to fly away.
After repeated exposure without harm, they may respond less strongly.
This prevents animals from wasting time and energy responding to harmless stimuli.
Learning Through Association
Animals can also learn that one event predicts another.
For example, an animal may learn that:
- A particular sound indicates food.
- A certain location contains danger.
- A particular smell is associated with a predator.
- A specific behavior produces a reward.
This is called associative learning.
Associative learning allows animals to use previous experience to make future behavior more effective.
Learning by Observation
Some animals can learn by observing others.
Young animals may watch adults:
- Hunt.
- Find food.
- Use tools.
- Avoid predators.
- Interact socially.
This is sometimes called social learning.
Social learning can allow useful information to spread through a group without every individual having to discover it independently.
Behavior and Survival
Behavior can improve survival in many ways.
Animals can behave in ways that help them:
- Find food.
- Avoid predators.
- Maintain suitable body temperature.
- Find water.
- Avoid dangerous environments.
- Defend resources.
- Cooperate with other individuals.
A behavior does not have to guarantee survival to be adaptive.
It needs to increase survival or reproductive success, on average, under the conditions in which it evolved.
Predator Avoidance
Animals use many behaviors to avoid predators.
These include:
- Freezing.
- Hiding.
- Fleeing.
- Grouping together.
- Giving alarm calls.
- Remaining active at safer times.
- Entering shelters or burrows.
A meerkat acting as a lookout, for example, can detect danger and give an alarm call.
Other group members can then seek shelter.
Feeding Behavior
Animals must obtain enough energy and nutrients to survive.
Feeding behaviors can include:
- Hunting.
- Grazing.
- Filter feeding.
- Scavenging.
- Ambushing prey.
- Cooperative hunting.
- Food storage.
Behavior can greatly influence how effectively an animal obtains food.
For example, predators may learn where prey is most likely to occur or when it is most active.
Cooperative Hunting
Some animals hunt in groups.
Examples include:
- Wolves.
- Lions.
- Dolphins.
- Some birds.
Cooperative hunting can allow predators to:
- Capture larger prey.
- Surround prey.
- Reduce escape routes.
- Share the energetic cost of hunting.
However, captured food may then have to be shared among group members.
This demonstrates another behavioral trade-off.
Migration
Migration is regular movement between different geographical areas, often associated with seasonal changes.
Animals migrate for reasons including:
- Finding food.
- Reaching breeding areas.
- Avoiding severe weather.
- Finding suitable temperatures.
- Reaching nursery areas.
Migration occurs in many groups, including:
- Birds.
- Mammals.
- Fish.
- Insects.
- Marine animals.
Why Migrate?
Imagine a bird living in a region where winters are extremely cold and food becomes scarce.
Remaining in the same location could reduce survival.
Migration allows the bird to move to an area where:
- Temperatures are more suitable.
- Food is more available.
- Conditions for survival are improved.
It may later return when breeding conditions become favourable.
Migration therefore allows animals to exploit different environments at different times of year.
The Costs of Migration
Migration can provide major benefits, but it is also costly.
Long-distance movement requires:
- Large amounts of energy.
- Time.
- Navigation.
- Suitable resting locations.
Migrating animals may also face:
- Predators.
- Storms.
- Food shortages.
- Physical exhaustion.
- Human-made barriers.
Migration is therefore worthwhile only when its benefits outweigh its costs over evolutionary time.
How Do Animals Navigate?
Migrating animals may use several types of information.
These can include:
- The position of the Sun.
- Stars.
- Earth's magnetic field.
- Landmarks.
- Smells.
- Ocean currents.
- Learned routes.
Navigation may involve a combination of innate mechanisms and learned information.
Young animals of some species inherit a general migration direction, while experience can improve navigation over time.
Communication
Communication occurs when information passes from one animal to another and influences behavior.
Animals communicate using several types of signals.
These include:
- Sound.
- Visual signals.
- Chemicals.
- Touch.
- Vibrations.
- Electrical signals in some species.
Communication can improve both survival and reproductive success.
Sound Communication
Sound can travel over considerable distances and can work when animals cannot see one another.
Animals use sound to:
- Attract mates.
- Warn of predators.
- Defend territories.
- Maintain contact with group members.
- Identify individuals.
Examples include:
- Bird songs.
- Frog calls.
- Wolf howls.
- Primate alarm calls.
- Whale songs.
Visual Communication
Animals can communicate using:
- Body posture.
- Colour.
- Movement.
- Facial expressions.
- Courtship displays.
A peacock displaying its feathers is a familiar example of visual communication associated with reproduction.
Visual signals can be very effective when animals are close enough to see one another.
Chemical Communication
Chemical signals used for communication are often called pheromones.
Animals can use chemical signals to communicate information about:
- Reproductive condition.
- Territory.
- Food.
- Trails.
- Identity.
- Danger.
Ants provide a familiar example.
Some ants deposit chemical trails that other members of the colony can follow toward food.
Worked Example: Ant Trail
Suppose an ant discovers food.
It returns toward the colony while depositing a chemical trail.
Other ants detect the chemical.
They follow the trail toward the food.
If they also find food, they may strengthen the trail.
This can produce an organised group response without any individual ant directing the entire colony.
Courtship Behavior
Courtship includes behaviors involved in attracting and selecting mates.
Examples include:
- Songs.
- Dances.
- Displays.
- Calls.
- Gift giving.
- Nest construction.
Courtship can provide information about:
- Species identity.
- Health.
- Physical condition.
- Territory quality.
- Ability to obtain resources.
Courtship behavior therefore contributes directly to reproductive success.
Territorial Behavior
A territory is an area that an animal actively defends against others.
Animals may defend territories containing:
- Food.
- Nesting sites.
- Shelter.
- Mates.
Territorial behavior can reduce competition for resources.
However, defending territory also consumes energy and may lead to injury.
Social Behavior
Many animals live in groups.
Examples include:
- Wolf packs.
- Primate troops.
- Elephant herds.
- Fish schools.
- Bird flocks.
- Insect colonies.
Group living can influence:
- Feeding.
- Protection.
- Reproduction.
- Communication.
- Learning.
- Care of offspring.
Benefits of Group Living
Living in a group can provide several advantages.
Predator Detection
More individuals can watch for predators.
Defence
Groups may collectively defend themselves.
Finding Food
Individuals can share information about food sources.
Cooperative Hunting
Groups may capture prey that individuals could not.
Care of Young
Other group members may help protect offspring.
Learning
Young animals can learn from experienced individuals.
Costs of Group Living
Group living also has disadvantages.
These can include:
- Greater competition for food.
- Competition for mates.
- Increased spread of disease.
- Increased visibility to predators.
- Conflict between group members.
Therefore:
Group living provides benefits, but also creates costs.
Whether it is advantageous depends on the environment and the species.
Schooling and Flocking
Fish often form schools, while birds may form flocks.
Group movement can provide protection through several mechanisms.
A predator may find it difficult to focus on one individual among many moving animals.
Groups may also detect predators sooner because many individuals are watching.
Some group formations can even reduce the energy required for movement.
Huddling
Huddling is a behavioral adaptation used by some animals to reduce heat loss.
Emperor penguins provide a well-known example.
Individuals gather closely together.
This:
- Reduces exposed surface area.
- Reduces heat loss.
- Provides protection from wind.
Animals may change positions within the huddle so that the same individuals are not always exposed to the coldest conditions.
Behavioral Thermoregulation
Animals can use behavior to control body temperature.
Examples include:
- Lizards basking in sunlight.
- Animals moving into shade.
- Mammals entering water.
- Desert animals becoming nocturnal.
- Penguins huddling.
- Animals entering burrows.
Behavior can therefore contribute to homeostasis.
Worked Example: Lizard Thermoregulation
A lizard's body temperature is strongly influenced by environmental conditions.
If the lizard is too cool, it may:
- Move into sunlight.
- Position its body to absorb more radiation.
If it becomes too warm, it may:
- Move into shade.
- Enter a burrow.
- Change body orientation.
The animal changes its behavior in response to environmental temperature.
This helps keep its body within a suitable temperature range.
Nocturnal Behavior
A nocturnal animal is primarily active at night.
In hot environments, nocturnal activity can reduce:
- Exposure to high daytime temperatures.
- Water loss.
- Heat stress.
Night activity may also affect predator avoidance and access to food.
Behavioral timing is therefore closely related to environmental conditions.
Hibernation
Some animals greatly reduce their activity and metabolism during prolonged periods of difficult environmental conditions.
Hibernation is a prolonged state involving major reductions in metabolic activity and body functions.
It can help animals survive periods when:
- Temperatures are low.
- Food is scarce.
- Maintaining normal activity would require too much energy.
Hibernation is more than simply sleeping.
It involves major physiological changes as well as behavioral changes.
Estivation
Estivation is a period of reduced activity associated particularly with hot or dry conditions.
Some animals estivate when:
- Temperatures become extremely high.
- Water becomes scarce.
- Food availability decreases.
Animals may shelter underground or in protected locations until conditions improve.
Both hibernation and estivation demonstrate how behavior can respond to predictable environmental challenges.
Environmental Conditions Influence Behavior
Animal behavior is not fixed under all circumstances.
Animals may change their behavior in response to:
- Temperature.
- Light.
- Rainfall.
- Food availability.
- Predator abundance.
- Population density.
- Availability of mates.
- Seasonal changes.
A change in the environment can therefore cause a change in behavior.
Worked Example: Drought
Imagine a population of grazing animals experiencing a severe drought.
As water and vegetation become scarce, animals might:
- Travel greater distances.
- Change feeding locations.
- Gather near remaining water sources.
- Migrate.
- Change daily activity patterns.
These behavioral changes may increase survival during difficult conditions.
However, gathering around limited water sources may also increase:
- Competition.
- Disease transmission.
- Predator encounters.
A behavioral response can therefore have both benefits and costs.
Behavior Can Be Flexible
One major advantage of behavior is that it can often change relatively quickly.
An animal cannot evolve thicker fur during a single cold afternoon.
But it might immediately:
- Enter shelter.
- Curl into a compact position.
- Huddle with others.
- Move to a warmer location.
Behavioral flexibility therefore allows animals to respond rapidly to changing conditions.
Behavior and Reproduction
Behavior is also essential for reproductive success.
Reproductive behaviors can include:
- Finding mates.
- Courtship.
- Competing for mates.
- Defending territories.
- Nest building.
- Mating.
- Parental care.
An animal may survive very successfully but leave no descendants if it cannot reproduce.
Behavioral adaptations therefore affect both survival and reproduction.
Behavior and Natural Selection
Behavioral adaptations can evolve through natural selection.
Suppose individuals differ in how they respond to predators.
Individuals that respond more effectively may:
- Survive more often.
- Produce more offspring.
- Pass on genetic influences associated with those behaviors.
Over many generations, behaviors that improve reproductive success can become more common.
However, learned behavior can also change within an individual's lifetime.
This creates an important distinction:
Evolution changes populations over generations.
Learning changes the behavior of an individual during its lifetime.
Behavior Has Costs
Behaviors that provide benefits often also have costs.
Migration requires large amounts of energy.
Territorial defence can cause injury.
Courtship displays can attract predators.
Group living can increase disease transmission.
Parental care uses time and energy.
When analyzing a behavioral adaptation, it is therefore useful to consider both its benefits and costs.
Worked Example: Alarm Calling
Imagine an animal spots a predator and gives a loud alarm call.
Potential Benefits
- Relatives may escape.
- Group members can seek shelter.
- The predator may lose the advantage of surprise.
Potential Costs
- Calling requires energy.
- The predator may detect the caller.
- Feeding time may be lost.
Whether alarm calling is advantageous depends on how these costs and benefits affect reproductive success.
Worked Example: Migration
Consider a bird living in an area with warm summers but extremely cold winters.
Remaining throughout winter could mean:
- Very little food.
- Severe temperatures.
- Increased mortality.
Migration allows access to a more suitable winter habitat.
However, migration also involves:
- High energy expenditure.
- Risk of storms.
- Predation.
- Navigation challenges.
The behavior persists when the long-term reproductive benefits outweigh these costs.
Worked Example: Social Hunting
A pack of predators encounters prey that would be difficult for one individual to capture.
By cooperating, individuals can:
- Surround the prey.
- Coordinate attacks.
- Reduce escape routes.
- Capture larger animals.
However, the food must then be shared.
Cooperation is therefore beneficial when the increased probability of obtaining food compensates for the cost of sharing it.
Environmental Change and Behavior
Environmental conditions can change naturally or because of human activity.
Animals may encounter:
- New roads.
- Artificial lighting.
- Urban environments.
- Habitat fragmentation.
- New food sources.
- Changing temperatures.
- Changes in seasonal timing.
Some species adjust their behavior successfully.
Others may struggle if environmental changes occur faster than their behavioral responses can adapt.
Behavioral Adaptation Versus Individual Response
Not every behavior performed by an animal should automatically be called an adaptation.
An adaptation is a heritable characteristic shaped over generations because it contributed to reproductive success.
An individual animal can also modify its behavior through learning.
For example:
A species may possess an inherited tendency to avoid predators.
An individual may then learn that a particular location contains a predator.
Both inherited and learned influences can contribute to the final behavior.
This is why animal behavior is often best understood as an interaction between:
Genes + development + experience + environment
Common Mistakes
Thinking All Behavior Is Learned
Many behaviors have strong innate components and can occur without previous experience.
Thinking Innate Means Simple
Innate behaviors can be extremely complex, such as web construction or migration.
Thinking Learned Behaviors Are Inherited
An individual does not genetically inherit a specific learned experience. However, the biological capacity to learn can itself be influenced by genes.
Confusing Structural and Behavioral Adaptations
A wing is a structure. Migration is a behavior.
Thinking Migration Is Random Movement
Migration is generally regular, directed movement associated with particular biological or environmental conditions.
Assuming Group Living Is Always Beneficial
Groups can improve protection and feeding but can also increase competition and disease transmission.
Thinking Every Animal Behavior Is an Adaptation
Some behaviors are individual responses or learned actions. An evolutionary adaptation specifically refers to a characteristic shaped through natural selection.
Saying Animals Change Behavior Because They "Want to Evolve"
Evolution occurs across populations and generations. Individual animals respond to stimuli or learn during their lifetime.
Check Your Understanding
1. Define a behavioral adaptation.
2. Give three examples of behavioral adaptations.
3. Distinguish between structural and behavioral adaptations.
4. Distinguish between innate and learned behavior.
5. Give two examples of innate behavior.
6. Explain one advantage of learning.
7. What is habituation?
8. Explain why animals migrate.
9. Give two benefits and two costs of migration.
10. Describe three different methods animals use to communicate.
11. Explain how group living can improve survival.
12. Give two disadvantages of group living.
13. Explain how huddling helps penguins survive cold conditions.
14. Describe how an animal could use behavior to regulate body temperature.
15. Explain how environmental conditions can influence animal behavior.
16. A group of grazing animals changes its feeding location during a drought. Explain how this behavior could improve survival and identify one possible disadvantage.
17. Explain how a behavioral adaptation can affect reproductive success.
18. Why should both costs and benefits be considered when evaluating an animal behavior?
Key Terms
- Behavior – an action or response carried out by an organism.
- Behavioral adaptation – behavior that contributes to survival or reproductive success and has been shaped by natural selection.
- Innate behavior – behavior that develops largely without learning or previous experience.
- Learned behavior – behavior developed or modified through experience.
- Habituation – reduced response to a repeated stimulus that proves unimportant.
- Associative learning – learning that one event or stimulus is connected with another.
- Social learning – learning by observing or interacting with other individuals.
- Migration – regular movement between geographical areas, often associated with seasonal conditions.
- Communication – transfer of information from one animal to another that influences behavior.
- Pheromone – chemical signal used for communication between members of the same species.
- Courtship – behavior associated with attracting or selecting mates.
- Territory – area defended by an animal against others.
- Social behavior – interactions between members of the same species.
- Nocturnal – primarily active at night.
- Hibernation – prolonged state involving greatly reduced activity and metabolism, typically associated with cold conditions or food scarcity.
- Estivation – period of reduced activity associated particularly with hot or dry conditions.
- Natural selection – process through which heritable characteristics associated with reproductive success can become more common over generations.
Key Takeaways
- Behavioral adaptations can improve an animal's survival and reproductive success.
- Behavior allows animals to respond rapidly to changing environmental conditions.
- Innate behaviors develop largely without learning, while learned behaviors are modified through experience.
- Many behaviors contain both innate and learned components.
- Learning allows animals to use previous experience to respond more effectively to future situations.
- Migration allows animals to move between areas as environmental conditions and resource availability change.
- Animal communication can involve sound, visual signals, chemicals, touch, and other signals.
- Social behavior can improve predator detection, defence, feeding, learning, and care of offspring.
- Group living also creates costs such as competition and increased disease transmission.
- Animals can regulate body temperature behaviorally by basking, seeking shade, changing activity times, burrowing, or huddling.
- Hibernation and estivation can help animals survive periods of difficult environmental conditions.
- Courtship, territoriality, communication, and parental care can directly affect reproductive success.
- Behavioral adaptations usually involve trade-offs between benefits and costs.
- Environmental factors such as temperature, rainfall, food availability, predators, and seasonal changes can alter animal behavior.
- Learning changes individuals during their lifetimes, while evolution changes populations over generations.
- Animal behavior often results from an interaction between genetic influences, development, experience, and environmental conditions.
5. Life Histories and Survival
Learning outcomes
- I can explain what is meant by a life history strategy.
- I can compare species that produce many offspring with little care and those that produce few offspring with extensive care.
- I can describe how growth, reproduction, and lifespan are related.
- I can explain trade-offs involved in survival and reproduction.
- I can analyze how life history strategies are adapted to different environments.
What Is a Life History Strategy?
Every animal has a life history — the pattern of growth, development, reproduction, and survival that occurs during its lifetime.
A life history strategy describes how an organism allocates its limited time and energy among:
- Growth.
- Body maintenance.
- Survival.
- Reproduction.
- Parental care.
Animals cannot maximize all of these at the same time.
For example, energy used to produce hundreds of offspring cannot also be used to provide intensive care to each offspring.
Life history strategies therefore involve trade-offs.
Different strategies can be successful in different environments.
Limited Energy and Resources
Every organism has a limited supply of energy and resources.
Energy comes ultimately from food and must be divided among different biological processes.
An animal may use energy for:
- Growth.
- Movement.
- Repair.
- Immune function.
- Maintaining body temperature.
- Finding food.
- Avoiding predators.
- Producing gametes.
- Pregnancy or egg production.
- Parental care.
Energy invested in one process is unavailable for another.
This creates one of the central ideas in life history biology:
Organisms must allocate limited resources among competing needs.
The Main Life History Characteristics
Scientists studying life histories often examine characteristics such as:
- Age at maturity.
- Size at maturity.
- Number of offspring.
- Size of offspring.
- Frequency of reproduction.
- Amount of parental care.
- Growth rate.
- Lifespan.
- Probability of survival at different ages.
These characteristics tend to be connected rather than independent.
For example, producing larger offspring often means that fewer can be produced with the same amount of energy.
Many Offspring with Little Care
Some animals produce very large numbers of offspring but provide relatively little parental care.
Examples occur among many:
- Fish.
- Marine invertebrates.
- Insects.
- Amphibians.
A female fish, for example, may release thousands or even millions of eggs.
Only a small proportion may survive to adulthood.
However, producing so many offspring increases the probability that at least some survive.
Characteristics of a Many-Offspring Strategy
Species using this general strategy often show some combination of:
- Large numbers of offspring.
- Small offspring.
- Relatively low investment per offspring.
- Limited parental care.
- Relatively high juvenile mortality.
- Rapid development.
- Early reproduction.
- Shorter generation times.
These are broad patterns rather than strict rules.
Not every species with many offspring possesses all of these characteristics.
Why Produce So Many Offspring?
Imagine an animal whose young face extremely high mortality.
Each offspring may have only a small probability of reaching adulthood.
Producing only one offspring would therefore be risky.
Producing hundreds or thousands spreads reproductive investment across many offspring.
For example:
If an animal produces 2,000 offspring and only 0.5% survive:
2,000 × 0.005 = 10 surviving offspring
A very low individual survival rate can therefore still result in several surviving descendants.
Few Offspring with Extensive Care
Other animals produce relatively few offspring but invest heavily in each one.
Examples include many:
- Mammals.
- Birds.
- Primates.
- Large-bodied animals.
An elephant, for example, usually produces one calf at a time.
The calf receives extensive care and protection for years.
Each offspring represents a very large investment.
Characteristics of a High-Investment Strategy
Species following this general pattern may show:
- Fewer offspring.
- Larger offspring.
- Greater investment per offspring.
- Extensive parental care.
- Higher juvenile survival.
- Slower development.
- Later reproductive maturity.
- Longer generation times.
- Longer lifespans.
Again, these are general tendencies rather than fixed categories.
Comparing the Strategies
| Characteristic | Many Offspring / Lower Investment | Fewer Offspring / Higher Investment |
|---|---|---|
| Number of offspring | Usually high | Usually low |
| Investment per offspring | Lower | Higher |
| Parental care | Often limited | Often extensive |
| Offspring size | Often smaller | Often larger |
| Juvenile mortality | Often higher | Often lower |
| Development | Often faster | Often slower |
| Age at maturity | Often earlier | Often later |
| Lifespan | Often shorter | Often longer |
Animals occur across a continuum between these patterns.
They should not be treated as two perfectly separate groups.
The Number-Size Trade-Off
One important life history trade-off involves the number and size of offspring.
Suppose an animal has a limited amount of energy available for reproduction.
It could produce:
Many small offspring
or
Fewer large offspring
Producing larger offspring can provide advantages.
Larger young may:
- Have more stored energy.
- Be better able to compete.
- Escape predators more effectively.
- Tolerate difficult conditions.
But larger offspring require more resources to produce.
Therefore:
Increasing investment per offspring usually limits the number that can be produced.
Worked Example: Sea Turtle
Sea turtles produce many eggs during a nesting season.
After laying and burying the eggs, the mother generally provides no prolonged care.
Young turtles face many dangers:
- Predators.
- Dehydration.
- Extreme temperatures.
- Difficulty reaching the ocean.
- Marine predators after entering the water.
Only a small proportion survive to adulthood.
Producing many offspring compensates partly for this high mortality.
Worked Example: Elephant
An elephant uses a very different strategy.
A female typically:
- Produces one calf at a time.
- Has a long pregnancy.
- Produces milk.
- Protects the calf.
- Lives in a social group that can help protect young.
- Invests in the calf for years.
Producing offspring is slow and energetically expensive, but each calf has substantial parental and social support.
Growth Requires Energy
Before an animal can reproduce, it often must grow.
Growth requires:
- Energy.
- Proteins and other nutrients.
- Time.
Energy invested in growth cannot simultaneously be invested in reproduction.
This creates another trade-off:
Grow now ↔ reproduce now
Delaying reproduction may allow an animal to become larger and stronger.
However, waiting also creates a risk that the animal could die before reproducing.
Early Reproduction
Reproducing early can provide important advantages.
An animal begins passing its genes to the next generation sooner.
This may be useful in environments where:
- Adult mortality is high.
- Conditions are unpredictable.
- Lifespan is relatively short.
However, early reproduction can have costs.
A young animal may be:
- Smaller.
- Less experienced.
- Able to produce fewer offspring.
- Less capable of providing parental care.
Delayed Reproduction
Other animals delay reproduction until they are older.
This allows additional time for:
- Growth.
- Development.
- Learning.
- Establishing social position.
- Accumulating resources.
Delayed reproduction can be advantageous if larger or more experienced adults have greater reproductive success.
However, delayed maturity also means an individual must survive longer before reproducing.
Growth and Final Body Size
Life history strategy can influence adult body size.
Long periods of growth may allow animals to reach larger sizes.
Large body size can provide advantages such as:
- Reduced vulnerability to some predators.
- Greater competitive ability.
- Larger energy reserves.
- Ability to produce larger offspring.
However, becoming large requires substantial resources and time.
There is therefore no universally ideal body size.
Lifespan
Lifespan is the length of time an organism lives.
Species vary enormously in lifespan.
Some animals live for:
- Days or weeks.
- Several years.
- Several decades.
- More than a century.
Lifespan is connected with many other life history characteristics, including growth rate, age at maturity, reproductive rate, body size, and mortality risk.
Fast and Slow Life Histories
Biologists sometimes describe life histories along a broad fast-slow continuum.
A relatively fast life history may involve:
- Rapid growth.
- Early maturity.
- Frequent reproduction.
- Many offspring.
- Lower investment per offspring.
- Shorter lifespan.
A relatively slow life history may involve:
- Slower development.
- Later maturity.
- Fewer offspring.
- Greater investment per offspring.
- Longer lifespan.
This framework is more accurate than treating animals as belonging to two rigid categories.
A Note About r- and K-Selection
Older biology materials sometimes divide organisms into r-selected and K-selected species.
In this model:
- r-selected species are described as producing many offspring with relatively little investment.
- K-selected species are described as producing fewer offspring with greater investment.
This can be useful as an introductory comparison, but modern life history biology generally recognises that real species show much more complex combinations of characteristics.
It is usually better to think of life histories as a continuum of strategies and trade-offs.
Reproduction Has Costs
Reproduction requires energy.
Animals may need energy to:
- Produce sperm or eggs.
- Find mates.
- Compete for mates.
- Perform courtship.
- Build nests.
- Carry embryos.
- Produce milk.
- Protect offspring.
Energy invested in reproduction cannot be used for everything else.
Therefore:
Greater reproductive investment can reduce resources available for growth, maintenance, or survival.
Survival Versus Reproduction
Imagine an animal has a limited amount of stored energy.
It could use this energy to:
- Repair tissues.
- Maintain immune function.
- Store fat.
- Escape predators.
Or it could use it to:
- Produce eggs.
- Find a mate.
- Feed offspring.
Investing heavily in reproduction may increase current reproductive success but reduce the animal's probability of surviving to reproduce again.
This is a major life history trade-off:
Current reproduction ↔ future survival and reproduction
Worked Example: Breeding Bird
Imagine a bird feeding chicks.
Making more feeding trips could increase chick survival.
However, every trip requires energy.
Frequent trips may also:
- Increase exposure to predators.
- Reduce the parent's body reserves.
- Reduce time available for the parent to feed itself.
If parental condition becomes poor, the bird may be less likely to survive or reproduce successfully the following year.
The parent must therefore balance current and future reproductive investment.
Reproducing Once or Many Times
Some animals reproduce repeatedly during their lives.
Others make an extremely large reproductive investment and reproduce only once.
These strategies are known as:
- Iteroparity – reproduction occurs during multiple reproductive events.
- Semelparity – reproduction occurs in a single major reproductive episode.
Pacific salmon provide a well-known example of semelparity.
They invest enormous resources in migration and reproduction and generally die after spawning.
Why Reproduce Only Once?
Semelparity may appear wasteful, but it can be successful when concentrating resources into one reproductive event increases reproductive success.
An organism may invest heavily in:
- Producing many offspring.
- Reaching a breeding site.
- Courtship.
- Spawning.
If future survival is unlikely, saving resources for another reproductive season may provide little advantage.
Repeated Reproduction
Many mammals and birds are iteroparous.
They reproduce during multiple breeding seasons.
This strategy spreads reproduction across time.
If one breeding attempt fails because of:
- Bad weather.
- Predation.
- Food shortage.
the animal may reproduce successfully in another year.
However, individuals must survive between reproductive events.
Mortality Risk Influences Strategy
The probability of dying at different stages of life can strongly influence life history evolution.
If adult mortality is extremely high, delaying reproduction may be risky.
If adults have high survival probabilities, delayed maturity and repeated reproduction may be more successful.
Similarly, if juvenile mortality is high, selection may favour strategies such as:
- Producing many young.
- Increasing parental protection.
- Producing larger offspring.
Different species solve the same survival problem in different ways.
Environmental Stability
Life history strategies can be influenced by how predictable an environment is.
In highly variable environments:
- Food availability may change rapidly.
- Habitats may disappear temporarily.
- Mortality may be unpredictable.
Strategies involving rapid development and early reproduction can sometimes be advantageous.
In more stable environments, longer development and greater investment in individual offspring may sometimes be successful.
These relationships are not absolute rules.
Worked Example: Temporary Pond
Imagine an aquatic animal living in a pond that dries up every year.
Individuals that develop slowly may fail to reproduce before the pond disappears.
Natural selection could favour:
- Rapid growth.
- Early maturity.
- Rapid reproduction.
A short-lived environment can therefore favour a relatively fast life history.
Worked Example: Large Stable Habitat
Now consider a long-lived animal in a relatively stable habitat.
Adults may have a good chance of surviving from one year to the next.
Under these conditions, a strategy involving:
- Slow development.
- Learning.
- Repeated reproduction.
- High parental investment.
may be successful.
Long-lived primates provide examples of relatively slow life histories.
Predation and Life History
Predation can influence life history strategy.
If young animals experience very high predation, species may evolve strategies involving:
- More offspring.
- Better hiding.
- Parental defence.
- Faster growth.
- Different breeding locations.
If adults experience high mortality, reproducing earlier may become advantageous.
The effect depends on which life stage experiences the greatest mortality.
Competition and Life History
Competition can also influence life history strategies.
When resources are limited, individuals may benefit from:
- Larger body size.
- Greater competitive ability.
- Territorial behavior.
- Greater parental investment.
- Producing offspring capable of competing effectively.
But these characteristics require resources.
Again, the result is a trade-off rather than a perfect solution.
Life History and Parental Care
Life history strategy is closely connected with parental care.
Species producing many offspring may provide relatively little care to each one.
Species producing fewer offspring can often provide more care per offspring.
However, there are many exceptions.
The important question is not simply how many offspring are produced.
It is how many ultimately survive and contribute to future generations.
Life History and Reproductive Success
From an evolutionary perspective, the effectiveness of a life history strategy depends on reproductive success.
Producing 10,000 eggs does not automatically mean greater success than producing one offspring.
Imagine:
Species A produces 10,000 offspring, but only 2 eventually reproduce.
Species B produces 2 offspring, and both eventually reproduce.
Both parents have successfully contributed descendants to the next generation.
The number produced is therefore only one part of reproductive success.
Life History Strategies Can Change with Conditions
Even within the same species, individuals may adjust reproductive investment depending on conditions.
Factors can include:
- Food availability.
- Population density.
- Age.
- Body condition.
- Temperature.
- Predation risk.
For example, an animal in poor condition may delay reproduction because it lacks the resources required to produce viable offspring.
Life history patterns therefore involve interactions between inherited biology and environmental conditions.
Life History and Climate
Seasonal conditions can strongly influence reproduction.
Animals may time reproduction so that offspring appear when:
- Food is abundant.
- Temperatures are favourable.
- Water is available.
- Predator pressure is lower.
If environmental conditions change, the timing that was previously successful may become less effective.
This is one reason environmental change can influence reproductive success.
Survivorship Curves
Scientists can study life histories by examining survivorship — the proportion of individuals that remain alive at different ages.
Three idealised patterns are often described.
Type I
Most individuals survive through early and middle life, followed by increased mortality at older ages.
This pattern is associated with some large mammals.
Type II
The probability of dying remains relatively constant across much of the lifespan.
Some birds and small mammals approximate this pattern.
Type III
Very high mortality occurs early in life, but individuals that survive the early period may live considerably longer.
This pattern occurs in many organisms that produce large numbers of offspring.
Real species do not always match these idealised curves perfectly.
Interpreting Survivorship Curves
Consider a species with a Type III survivorship pattern.
The steep decrease early in life indicates:
High juvenile mortality
If the curve becomes flatter later, this indicates that individuals surviving the dangerous juvenile period have a better probability of surviving each additional period.
A Type I pattern shows the opposite general pattern:
High survival through much of life → mortality rises strongly at older ages
Survivorship curves therefore provide a useful way to visualize differences in life history.
Worked Example: Mouse and Elephant
A mouse and an elephant have very different life histories.
A mouse generally:
- Matures relatively quickly.
- Produces several offspring at a time.
- Can reproduce repeatedly over relatively short intervals.
- Has a relatively short lifespan.
An elephant generally:
- Develops slowly.
- Matures much later.
- Usually produces one calf at a time.
- Provides extensive parental care.
- Has a much longer lifespan.
Neither strategy is inherently superior.
Each has evolved under different biological and environmental conditions.
Worked Example: Environmental Disturbance
Imagine a habitat that experiences frequent unpredictable disturbances.
A species that:
- Develops rapidly.
- Reproduces early.
- Produces many offspring.
may be able to recolonize quickly after population losses.
Now imagine a stable habitat where adults survive for many years.
A species may instead benefit from:
- Slow development.
- Greater learning.
- High parental investment.
- Repeated reproduction over many years.
Life history strategy therefore reflects the conditions under which ancestors survived and reproduced.
Evaluating a Life History Strategy
When analyzing an unfamiliar species, consider several questions:
- How many offspring does it produce?
- How large are the offspring?
- How much parental care is provided?
- How quickly does it grow?
- When does it first reproduce?
- How frequently does it reproduce?
- How long does it live?
- What is juvenile mortality like?
- What is adult mortality like?
- How predictable is the environment?
- What major threats does it face?
The answers can reveal how growth, survival, and reproduction are balanced.
There Is No Perfect Strategy
A crucial idea in life history biology is that there is no universally perfect life history strategy.
A strategy that is successful in one environment may be unsuccessful in another.
For example:
Producing thousands of offspring can be successful when juvenile mortality is extremely high.
Producing one highly protected offspring can be successful when intensive parental investment greatly increases survival.
Natural selection does not create organisms that maximize every characteristic.
Instead, evolution produces combinations of traits that reflect biological and environmental trade-offs.
Common Mistakes
Thinking "Strategy" Means a Conscious Decision
A life history strategy is an evolved pattern. Animals do not consciously design their evolutionary life history.
Saying More Offspring Always Means Greater Reproductive Success
Many offspring may die before reproducing. Reproductive success depends on surviving descendants.
Assuming Fewer Offspring Is Always Better
High parental investment can increase survival, but it limits the number of offspring that can be produced.
Assuming Many-Offspring Species Never Provide Parental Care
There are many exceptions. Some species produce numerous offspring while still providing substantial care.
Treating Life Histories as Two Fixed Categories
Real species occur across a continuum and can show combinations of characteristics.
Assuming Long Lifespan Automatically Means Greater Reproductive Success
Lifespan matters only as part of the complete relationship between survival and reproduction.
Confusing Individual Adjustment with Evolution
An individual can adjust reproductive behavior during its lifetime, but evolutionary changes in life history occur across generations.
Check Your Understanding
1. Define a life history strategy.
2. List five characteristics scientists might examine when studying an animal's life history.
3. Explain why organisms face trade-offs when allocating energy.
4. Compare a species producing many small offspring with one producing a few large offspring.
5. Explain why high juvenile mortality can favour the production of many offspring.
6. Why might delaying reproduction provide an advantage?
7. What is one potential disadvantage of delaying reproduction?
8. Explain the trade-off between current reproduction and future reproduction.
9. Distinguish between semelparity and iteroparity.
10. Explain why high adult mortality might favour earlier reproduction.
11. Describe the general characteristics of a relatively fast life history.
12. Describe the general characteristics of a relatively slow life history.
13. Explain how parental care is connected to life history strategy.
14. Compare Type I and Type III survivorship patterns.
15. Why should life history strategies be viewed as a continuum rather than two fixed categories?
16. An animal lives in temporary pools that frequently dry up. Predict two life history characteristics that might be advantageous and explain why.
17. A large mammal takes many years to mature, produces one offspring at a time, and provides several years of parental care. Explain the trade-offs involved in this strategy.
Key Terms
- Life history – pattern of growth, reproduction, survival, and development during an organism's lifetime.
- Life history strategy – evolved pattern of allocating resources among growth, survival, and reproduction.
- Trade-off – situation in which investment in one biological function reduces resources available for another.
- Parental investment – time and resources devoted to offspring.
- Reproductive success – success in passing genetic information into future generations through surviving descendants.
- Age at maturity – age at which an organism becomes capable of reproduction.
- Lifespan – length of time an organism lives.
- Generation time – average time between one generation and the next.
- Juvenile mortality – death occurring before reproductive maturity.
- Semelparity – reproduction concentrated into one major reproductive episode.
- Iteroparity – reproduction occurring during multiple reproductive episodes.
- Survivorship – proportion of individuals surviving to particular ages.
- Survivorship curve – graph showing survival across different ages.
- Fast life history – general pattern involving relatively rapid development and early reproduction.
- Slow life history – general pattern involving slower development, later reproduction, and often greater investment per offspring.
Key Takeaways
- A life history strategy describes how an organism allocates limited resources among growth, survival, and reproduction.
- Organisms cannot maximize every aspect of growth, survival, and reproduction simultaneously.
- Producing many offspring usually limits the resources available for each offspring.
- Producing fewer offspring can allow greater parental investment.
- Species with high juvenile mortality may compensate by producing many offspring.
- Extensive parental care can increase the probability that individual offspring survive.
- Growth requires resources and can delay reproduction.
- Early reproduction provides reproductive opportunities sooner but may occur before an animal reaches its maximum size or condition.
- Delayed reproduction can allow additional growth and learning but increases the risk of dying before reproducing.
- Reproduction itself has energetic and survival costs.
- Animals face a trade-off between current reproduction and future survival or reproduction.
- Some organisms reproduce once, while others reproduce repeatedly throughout their lives.
- Life histories can be described along a broad fast-slow continuum rather than as two rigid categories.
- Lifespan, age at maturity, offspring number, offspring size, parental care, and mortality are interconnected.
- Survivorship curves help show how mortality differs across different stages of life.
- Environmental stability, predation, competition, and resource availability can influence which life history characteristics are successful.
- A successful life history strategy is one that contributes to reproductive success under the environmental conditions experienced by the population.