Reproduction and Survival Strategies
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.