What Are Animals?

Animals are a large and diverse group of living organisms. Humans, insects, fish, birds, worms, and whales may look very different, but they share several important characteristics.

All animals are:

• Multicellular – they are made of many cells.
• Eukaryotic – their cells contain a nucleus.
• Heterotrophic – they obtain energy by consuming other organisms or organic material.
• Made of cells that do not have cell walls.
• Able to respond to changes in their environment.
• Capable of movement during at least part of their life cycle.

Animals belong to the biological kingdom Animalia.

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Animal Cells

Animals are made from animal cells.

A typical animal cell contains structures such as:

• Cell membrane
• Cytoplasm
• Nucleus
• Mitochondria
• Ribosomes

Unlike plant cells, animal cells do not have:

• A cellulose cell wall
• Chloroplasts
• A large permanent central vacuole

The absence of a rigid cell wall allows many animal cells to have flexible and specialised shapes.

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Animals Are Multicellular

All animals are multicellular organisms, meaning they are made from many cells.

In most animals, different cells become specialised to perform particular functions.

Examples include:

• Muscle cells for movement
• Nerve cells for communication
• Red blood cells for transporting oxygen
• Sperm and egg cells for reproduction

Specialised cells can work together to form tissues, which form organs and organ systems.

For example:

Cells → Tissues → Organs → Organ systems​

This organisation allows animals to perform complex life processes.

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How Are Animals Different from Other Organisms?

Animals share some characteristics with plants, fungi, and other organisms, but there are also important differences.

One of the most important differences is how these organisms obtain nutrients and energy.

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How Animals Obtain Energy

Animals cannot produce glucose by photosynthesis because their cells do not contain chloroplasts.

Instead, animals are heterotrophs.

A heterotroph obtains organic nutrients by consuming other organisms or material produced by other organisms.

For example:

Herbivores eat plants.

Grass → Rabbit

Carnivores eat other animals.

Rabbit → Fox

Omnivores eat both plants and animals.

Humans, bears, pigs, and many other animals are omnivores.

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How Animals Use Energy

Food contains chemical energy.

Animals digest food into smaller molecules that can be absorbed and transported around the body.

Cells can then release usable energy from substances such as glucose through cellular respiration.

A simplified word equation for aerobic respiration is:

glucose + oxygen → carbon dioxide + water​

Energy released through respiration can be used for:

• Movement
• Growth
• Repair
• Reproduction
• Maintaining body temperature
• Active transport
• Producing new molecules
• Communication between cells

Animals therefore depend directly or indirectly on other organisms for their food supply.

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Movement in Animals

One important characteristic of animals is their ability to move.

Animals may move their entire body from one place to another. This is called locomotion.

Examples include:

• Fish swimming
• Birds flying
• Humans walking
• Snakes slithering
• Jellyfish swimming
• Insects crawling or flying

Movement can help animals find:

• Food
• Water
• Shelter
• Mates
• Suitable environmental conditions

It can also help animals escape from predators and other dangers.

Not every animal constantly moves from place to place. Some animals, such as adult corals, remain attached to a surface. However, movement usually occurs during some stage of their life cycle or within parts of their bodies.

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Responding to Stimuli

Animals can detect and respond to changes in their surroundings.

A change that causes a response is called a stimulus.

Examples of stimuli include:

• Light
• Sound
• Temperature
• Touch
• Chemicals
• Movement
• Pain

Animals have specialised receptors that detect these changes.

For example, your eyes contain receptors that detect light, while your ears contain receptors that respond to sound and movement.

A response then occurs.

Stimulus→Detection→Response​

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Why Are Responses Important?

Responding quickly to environmental changes can greatly improve an animal's chances of survival.

Imagine a deer hears a predator approaching.

Stimulus: Sound of the predator.

Detection: Receptors in the ears detect the sound.

Response: The deer runs away.

This response increases the animal's chance of surviving.

Responses can also help animals:

• Locate food.
• Avoid dangerous temperatures.
• Find mates.
• Protect offspring.
• Maintain suitable internal conditions.
• Communicate with other animals.

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Animal Diversity

Animals are one of the most diverse groups of organisms on Earth.

Scientists divide animals into many different groups based on characteristics such as their body structure, development, and evolutionary relationships.

Some major animal groups include:

• Mammals
• Birds
• Reptiles
• Amphibians
• Fish
• Insects
• Arachnids
• Crustaceans
• Molluscs
• Annelids
• Cnidarians
• Echinoderms

Some animals have a backbone and are called vertebrates.

Animals without a backbone are called invertebrates.

The majority of known animal species are invertebrates.

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Animals in Different Habitats

Animals live in almost every environment on Earth.

Different environments present different challenges, so animals have characteristics that help them survive in their particular habitat.

Desert Animals

Deserts have limited water and can experience extreme temperatures.

Animals such as camels, desert foxes, reptiles, and scorpions have adaptations that help them survive these conditions.

Some desert animals:

• Conserve water.
• Avoid activity during the hottest part of the day.
• Shelter underground.
• Have body structures that help release excess heat.

Polar Animals

Polar habitats are extremely cold.

Animals such as polar bears, seals, and penguins have adaptations that reduce heat loss.

These may include:

• Thick fur
• Dense feathers
• Layers of body fat
• Compact body shapes

Aquatic Animals

Aquatic animals live mainly or entirely in water.

Fish often have:

• Gills for gas exchange.
• Fins for movement.
• Streamlined bodies that reduce resistance.

Marine mammals such as whales and dolphins also have streamlined bodies, but they breathe air using lungs rather than gills.

Forest and Rainforest Animals

Forests provide complex habitats with many different sources of food and shelter.

Animals may live:

• On the forest floor.
• Underground.
• In trees.
• Near rivers and ponds.
• In the forest canopy.

This creates many different ecological niches, allowing a large variety of animals to live in the same ecosystem.

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Different Animals, Shared Characteristics

A whale, butterfly, earthworm, and human look extremely different.

However, they are all animals.

They share fundamental characteristics:

MulticellularEukaryoticHeterotrophicNo cell wallsRespond to stimuliMovement during the life cycle​​

The enormous differences between animal species are the result of adaptations to different environments and ways of life.

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Worked Example: Is It an Animal?

A scientist discovers a new multicellular organism.

The organism:

• Has cells containing nuclei.
• Has no cell walls or chloroplasts.
• Consumes other organisms for nutrients.
• Detects changes in light and moves toward food.

Is it likely to be an animal?

Yes.

The organism has several key characteristics associated with animals: it is multicellular and heterotrophic, lacks cell walls, and can respond to stimuli through movement.

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Key Vocabulary

Animal – A multicellular, eukaryotic organism belonging to the kingdom Animalia.

Multicellular – Made from many cells.

Eukaryotic – Having cells containing a nucleus and other membrane-bound organelles.

Heterotroph – An organism that obtains organic nutrients from other organisms.

Stimulus – A change in the internal or external environment that can cause a response.

Response – An action or change caused by a stimulus.

Locomotion – Movement of an organism from one place to another.

Habitat – The place where an organism lives.

Vertebrate – An animal with a backbone.

Invertebrate – An animal without a backbone.

Adaptation – A characteristic that helps an organism survive and reproduce in its environment.

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Key Takeaways

• Animals are multicellular, eukaryotic organisms.
• Animal cells have no cell walls or chloroplasts.
• Animals are heterotrophs and obtain organic nutrients from other organisms.
• Energy released through respiration supports movement, growth, repair, reproduction, and other life processes.
• Animals can detect and respond to stimuli.
• Movement helps animals find resources, escape danger, reproduce, and survive.
• Animals show enormous diversity and live in habitats ranging from deserts and oceans to rainforests and polar regions.
• Despite their differences, animals share a set of fundamental biological characteristics.