Animals are multicellular living organisms that obtain energy by consuming organic matter or absorbing nutrients from other organisms. They range from microscopic aquatic species to enormous whales, occupying nearly every major environment on Earth. Despite their extraordinary diversity, animals share several fundamental biological characteristics, including specialized cells, the ability to respond to their surroundings, and life cycles that typically begin with a fertilized egg or another early developmental stage.
Understanding animals involves examining how scientists classify them, what distinguishes them from other living things, how their bodies function, and how they survive in different habitats. These features are closely connected: an animal’s body structure, behavior, and physiological processes often reflect the conditions in which it lives.
What are animals?
Animals belong to the biological kingdom Animalia, one of the major groups of living organisms. Unlike plants, which generally produce their own food through photosynthesis, animals depend on organic materials for energy and nutrients. Most obtain these materials by eating plants, other animals, fungi, or organic matter.
Animal cells are eukaryotic, meaning their DNA is enclosed within a nucleus. They are also multicellular, with cells organized into tissues and, in many species, organs and organ systems. Unlike plant cells, animal cells lack rigid cell walls, a feature that contributes to the flexibility of animal bodies and allows many species to move through their environments.
Most animals can move during at least one stage of their lives. Movement may involve walking, swimming, flying, crawling, or contracting muscles to manipulate food and interact with the environment. However, not all adult animals move from place to place. Sponges and many adult corals, for example, remain attached to a surface, although their larvae can move freely.
Animals also respond to environmental changes through mechanisms that range from simple cellular responses to complex nervous systems. A worm may withdraw from a harmful stimulus, while a bird can use vision, memory, and learned behavior to locate food.
Animals play essential roles in ecosystems. They pollinate plants, disperse seeds, regulate populations of other organisms, recycle nutrients, and serve as prey or predators. Their relationships with other living things help maintain the structure and functioning of natural communities.
The main types of animals
Scientists classify animals according to shared characteristics, evolutionary relationships, and anatomical features. Two especially useful distinctions are whether an animal has a backbone and how its major body systems are organized.
The familiar division between vertebrates and invertebrates is useful for introductory biology, although it does not represent two equally ranked branches of the animal kingdom. Vertebrates are one group within the larger lineage of chordates, while invertebrates include numerous distinct animal lineages.
Vertebrates
Vertebrates are animals that possess a backbone, or vertebral column, during at least part of their development. Their skeletons may be made primarily of bone, cartilage, or a combination of both. Most have a skull that protects the brain and a well-developed nervous system.
Vertebrates include mammals, birds, reptiles, amphibians, and fishes. These groups differ in their body coverings, methods of reproduction, ways of regulating body temperature, and adaptations to their environments.
Mammals are characterized by hair or fur at some stage of life and mammary glands that produce milk for their young. Most mammals give birth to live young, although monotremes, such as the platypus, lay eggs. Mammals are endothermic, meaning they generate much of the heat that maintains their internal body temperature through metabolism. Examples include humans, dogs, elephants, bats, and dolphins.
Birds have feathers, beaks, and forelimbs modified into wings. All living birds are endothermic, and most have adaptations for flight, although some, such as ostriches and penguins, do not fly. Birds lay eggs with hard or relatively firm shells and possess lightweight, efficient respiratory systems. Examples include eagles, robins, hummingbirds, and albatrosses.
Reptiles include snakes, lizards, turtles, crocodilians, and the tuatara. Their skin is covered with scales or other protective structures, and they generally rely on environmental heat to regulate body temperature. Most lay eggs on land, although some species give birth to live young. Reptiles occupy habitats ranging from deserts and forests to freshwater wetlands and marine environments.
Amphibians include frogs, toads, salamanders, and caecilians. Many have life cycles that involve aquatic larvae and terrestrial or semiaquatic adults, although life histories vary considerably. Their skin is generally permeable and often contributes to gas exchange, making them sensitive to changes in moisture and environmental conditions. Many amphibians reproduce in water or moist environments.
Fishes are a diverse collection of aquatic vertebrates rather than a single formal evolutionary group. Most have gills for extracting dissolved oxygen from water and fins for movement and stability. Their bodies are adapted to life in freshwater or saltwater, and many have scales. Sharks, rays, salmon, tuna, and seahorses are examples. Some fishes have specialized respiratory structures or behaviors that allow them to tolerate conditions that many other aquatic animals cannot.
Invertebrates
Invertebrates are animals that lack a vertebral column. They make up the vast majority of known animal species and display an enormous range of body plans, from simple filter-feeding organisms to animals with sophisticated sensory systems and complex behavior.
Arthropods have segmented bodies, jointed appendages, and an external skeleton made largely of chitin. Their hard exoskeleton protects them and provides support, but it cannot expand continuously, so growth generally requires molting, the shedding of the old outer covering. Insects, spiders, scorpions, crabs, shrimp, and centipedes are arthropods. Insects, distinguished by having six legs as adults, are especially diverse and occupy terrestrial and freshwater habitats as well as some marine-associated environments.
Mollusks generally have soft bodies, often with a muscular foot, a mantle that covers or surrounds the internal organs, and sometimes a hard shell. Snails, clams, oysters, octopuses, and squids belong to this group. Their lifestyles vary widely: clams often filter food from water, snails may graze on plants or algae, and octopuses actively hunt prey.
Annelids, or segmented worms, have bodies divided into repeated sections. Earthworms move through soil by coordinating muscles and tiny bristles, while many marine annelids live in sediments or construct tubes. Leeches are another familiar example. Annelids contribute to nutrient cycling and form important parts of aquatic and terrestrial food webs.
Cnidarians include jellyfish, sea anemones, corals, and hydras. They possess specialized stinging cells used to capture prey or defend themselves. Many have radial symmetry, meaning their body parts are arranged around a central axis. Reef-building corals are especially important because their colonies can create complex marine habitats that support many other species.
Echinoderms are marine animals that include sea stars, sea urchins, sand dollars, and sea cucumbers. Adults typically exhibit a form of radial symmetry, often arranged in five-part patterns, while their larvae are bilaterally symmetrical. Many use a water vascular system, a network of fluid-filled canals that helps operate their tube feet for movement, feeding, or attachment.
Sponges are among the simplest animals in terms of body organization. They lack true organs and most of the specialized tissues found in many other animal groups. Water flows through their bodies, allowing them to filter out food particles and exchange gases. Sponges are primarily aquatic and are especially common in marine environments.
Other invertebrates include flatworms, roundworms, rotifers, and many microscopic animal groups. Their diversity demonstrates that a backbone is only one possible body plan among the many evolutionary solutions found in the animal kingdom.
Common characteristics of animals
Although animals differ greatly in appearance and behavior, several biological traits help distinguish them from other organisms. These characteristics are broad patterns rather than rules without exceptions.
Multicellular organization
Animals consist of many cells that perform different functions. In complex species, cells form tissues such as muscle and nervous tissue, which combine into organs and organ systems.
The digestive system breaks down food and absorbs nutrients. The respiratory system exchanges oxygen and carbon dioxide, while the circulatory system transports gases, nutrients, hormones, and waste products. The nervous and endocrine systems coordinate activities throughout the body.
Not every animal has all these systems in a recognizable form. Sponges, for instance, lack true organs, and many small aquatic animals exchange gases directly with the surrounding water through their body surfaces.
Heterotrophic nutrition
Animals are heterotrophs, meaning they obtain energy and building materials from organic substances rather than producing their own food through photosynthesis.
Many animals ingest food and digest it internally. Digestive enzymes break complex molecules into smaller substances that cells can absorb and use. These nutrients support growth, tissue repair, movement, reproduction, and the maintenance of essential physiological functions.
Animal feeding strategies vary considerably. Predators capture other animals, herbivores consume plants or algae, and detritivores feed on dead organic material and waste. Some animals filter suspended particles from water, while others obtain nutrients through close associations with different organisms.
Movement and sensory responses
Movement allows animals to search for food, escape predators, find mates, and reach suitable habitats. Muscles generate force, while skeletal structures or other supporting tissues help transmit that force.
Animals use a variety of sensory systems to detect their surroundings. Eyes detect light, ears or other sound-sensitive structures respond to vibrations, and chemical receptors detect substances in the air, water, or on surfaces. Some species can sense electric fields, magnetic fields, or infrared radiation.
Sensory abilities reflect ecological needs. Bats use echolocation, producing sounds and interpreting returning echoes to navigate and locate prey. Many fishes detect water movements through a lateral-line system, a specialized sensory structure that helps them perceive nearby motion and pressure changes.
Movement and sensation are not equally developed in all animals. Some rely on relatively simple responses, while others possess nervous systems capable of learning, memory, and complex decision-making.
Reproduction and development
Most animals reproduce sexually, with sperm and egg cells combining during fertilization to produce a new organism. Sexual reproduction creates genetic variation among offspring, which can influence how populations respond to changing environments.
Some animals reproduce asexually, producing offspring without the fusion of sperm and egg cells. This occurs in certain sponges, cnidarians, and other groups. Depending on the species and reproductive mechanism, offspring may be genetically very similar to the parent.
Animal development varies widely. Some species develop directly into smaller versions of their adult form, while others undergo metamorphosis, a major change in body structure during development.
A butterfly, for example, passes through egg, caterpillar, pupa, and adult stages. The caterpillar specializes in feeding and growth, whereas the adult butterfly is adapted primarily for reproduction and, in many species, dispersal. Frogs also undergo metamorphosis, with many species developing from aquatic tadpoles into adults with limbs and lungs.
These different life cycles allow animals to use distinct food sources or habitats at different stages of life, sometimes reducing competition between young and adults.
Homeostasis and body temperature
Homeostasis is the regulation of internal conditions within ranges that allow cells and organs to function. Animals regulate factors such as water balance, salt concentration, acidity, and internal temperature through physiological and behavioral mechanisms.
Endothermic animals, including birds and mammals, produce substantial metabolic heat and can maintain relatively stable internal temperatures across a range of environmental conditions. They may use insulation, sweating, panting, shivering, or changes in blood flow to regulate heat.
Ectothermic animals, including most fishes, amphibians, and reptiles, depend more heavily on external heat sources. Many bask in sunlight, seek shade, burrow underground, or move between warmer and cooler areas to control body temperature.
The distinction is not simply that one group is warm and the other is cold. Ectothermic animals can be warm when their surroundings are warm, and endothermic animals can become dangerously cold or overheated if their regulatory mechanisms are overwhelmed. Each strategy has advantages and energetic costs.
Animal habitats and where they live
A habitat is the natural environment in which an organism lives and obtains the resources needed to survive and reproduce. A habitat includes physical conditions such as temperature, moisture, light, and salinity, as well as living resources such as food, mates, and shelter.
Animals occupy habitats ranging from polar ice and mountain peaks to tropical forests, deserts, grasslands, rivers, lakes, and the deep ocean. Their distribution depends on physiological tolerances, access to resources, interactions with other species, and their ability to disperse into suitable areas.
Terrestrial habitats
Terrestrial habitats occur on land and include forests, grasslands, deserts, tundra, and mountain environments. Each presents distinct challenges related to water availability, temperature, shelter, and food.
Forests provide layered vegetation, nesting sites, and numerous food sources. Tropical rainforests support animals such as monkeys, toucans, sloths, and many insects. Temperate forests are home to deer, foxes, woodpeckers, and salamanders, while boreal forests support animals adapted to long winters, including moose, lynx, and snowshoe hares.
Grasslands are dominated by grasses and other low-growing plants. They often support large grazing mammals, burrowing animals, and predators that hunt in relatively open terrain. Bison and prairie dogs are characteristic of North American grasslands, while zebras and many antelope species occupy grasslands elsewhere. Seasonal rainfall and recurring disturbances such as fire help shape these ecosystems.
Deserts receive relatively little precipitation, although temperatures vary greatly among desert regions. Animals must often conserve water, avoid extreme heat or cold, and take advantage of brief periods of food availability. Kangaroo rats can obtain much of their water from food and conserve it through highly efficient kidneys. Desert tortoises may shelter underground during unfavorable conditions, while many desert mammals and reptiles are active mainly at night.
Tundra and polar environments impose challenges associated with cold temperatures, seasonal darkness, and limited plant growth. Arctic foxes, caribou, snowy owls, and polar bears are adapted to life in the Arctic, though they occupy different ecological niches. Antarctica supports a different animal community, including penguins and seals, many of which depend directly or indirectly on marine food webs.
Mountains contain habitats that change with elevation. Temperature, oxygen availability, vegetation, and exposure to wind vary as altitude increases. Mountain goats use specialized hooves to move across steep rocky terrain, while pikas gather vegetation for food in cold alpine environments.
Aquatic habitats
Aquatic habitats include freshwater systems, estuaries, coastal waters, coral reefs, and the open ocean. Water supports buoyancy but also creates challenges involving dissolved oxygen, salinity, water movement, and pressure.
Freshwater habitats include rivers, streams, lakes, and ponds. Rivers and streams often have flowing water that influences oxygen levels and the movement of food. Lakes and ponds may contain distinct layers of temperature and oxygen, especially when water circulation is limited. Trout, frogs, freshwater mussels, beavers, and many aquatic insects depend on freshwater environments.
Marine habitats range from shallow coastal waters to the open ocean and deep seafloor. Coastal areas often receive nutrients from land and support productive communities. Coral reefs provide shelter and feeding opportunities for many fishes and invertebrates, while kelp forests create underwater structures used by sea otters, fish, sea urchins, and other species.
In the open ocean, animals face different conditions depending on depth. Sunlight supports photosynthesis near the surface, but light declines rapidly with depth. Many deep-sea animals rely on sinking organic material or other food sources, and some have adaptations for high pressure, scarce food, and darkness.
Estuaries form where rivers meet the sea. Their water is often brackish, meaning it contains a mixture of freshwater and saltwater. Salinity can change with tides, rainfall, and river flow. Estuaries serve as feeding grounds, nursery areas, and migration routes for many fishes, crustaceans, and birds.
Animals in the air and underground
Some habitats are defined by how animals use particular physical spaces rather than by a single ecosystem category.
The atmosphere serves as a medium for flight and travel, but most flying animals still depend on land or water for food, shelter, and reproduction. Birds, bats, and insects use wings to generate lift and control movement. Different wing shapes and flight styles reflect differences in body size, feeding behavior, and the distances animals travel.
Underground habitats offer shelter from temperature extremes and predators. Earthworms live within soil, while moles excavate tunnels in search of invertebrate prey. Prairie dogs construct extensive burrow systems, and some amphibians spend long periods underground to avoid drying out. Burrowing animals can also influence soil structure, water infiltration, and nutrient cycling.
How animals adapt to their environments
An adaptation is an inherited characteristic that improves an organism’s ability to survive or reproduce in a particular environment. Adaptations arise through evolutionary processes, especially natural selection, which can increase the frequency of heritable traits that contribute to reproductive success.
Adaptations may be structural, physiological, or behavioral. These categories overlap, and the same environmental challenge can produce several complementary responses.
Structural adaptations
Structural adaptations involve physical features of an animal’s body.
The thick fur and insulating body fat of polar bears reduce heat loss in cold environments. Their large paws distribute body weight across snow and provide traction on ice. These traits contribute to survival in Arctic conditions, although they do not make the animals immune to environmental change.
A giraffe’s long neck helps it reach vegetation high in trees, while its specialized cardiovascular system supports blood circulation despite the vertical distance between its heart and brain. The neck also plays a role in interactions between individuals.
Many aquatic animals have streamlined bodies that reduce resistance as they move through water. The torpedo-shaped bodies of tuna, for example, support efficient swimming, while the flattened bodies of rays help them move close to the seafloor.
Camouflage is another structural or color-related adaptation. A leaf-tailed gecko blends into its surroundings through its body shape and coloration, making it harder for predators or prey to detect. Camouflage can also be produced by changes in color, as seen in some cephalopods.
Physiological adaptations
Physiological adaptations involve internal processes that help animals function under particular conditions.
Marine fishes face the challenge of regulating water and salt. Many bony marine fishes lose water to their salty surroundings and gain excess salts, which they remove through specialized cells in their gills and other mechanisms. Freshwater fishes face the opposite challenge: water tends to enter their bodies, while salts can be lost, so they use different strategies to maintain internal balance.
Some mammals enter torpor, a temporary reduction in metabolic activity that conserves energy. Hibernation involves prolonged periods of reduced activity and altered physiology in certain species, although the timing and depth of these changes differ among animals.
Deep-diving marine mammals can tolerate conditions that would be dangerous for humans. Their bodies manage oxygen stores and undergo physiological changes that help support dives while limiting some of the effects of pressure and reduced oxygen availability.
Venom is another physiological adaptation. Certain snakes, spiders, and other animals produce complex chemical mixtures that help capture prey or defend against threats. Venom differs from poison: venom is generally delivered through a bite, sting, or similar mechanism, while poisons cause harm when absorbed, ingested, or otherwise taken into the body.
Behavioral adaptations
Behavioral adaptations involve actions that increase survival or reproductive success.
Migration allows animals to move between areas as food availability, weather, or breeding opportunities change. Many birds travel between seasonal breeding and wintering grounds, while some mammals and fishes migrate across extensive distances. Migration can improve access to resources, but it also exposes animals to hazards along their routes.
Nocturnal activity helps some animals avoid daytime heat or reduce exposure to predators. Many desert mammals forage at night, when temperatures are lower and water loss may be reduced. Other species are crepuscular, meaning they are most active around dawn and dusk.
Social behavior can also improve survival. Wolves cooperate during some forms of hunting, meerkats take turns watching for danger, and many primates live in social groups that influence access to food and mates. However, group living can also increase competition, spread disease, or make animals more noticeable to predators.
Adaptations do not arise because individual animals consciously develop the traits they need. Instead, heritable variation exists within populations, and natural selection changes the prevalence of traits across generations. A characteristic that is beneficial in one environment may be less useful or even harmful in another.
What animals eat and how they obtain food
Animal diets reflect body structure, digestive physiology, behavior, and the resources available in a habitat. Feeding relationships connect organisms into food webs, which describe how energy and nutrients move through ecosystems.
Herbivores primarily consume plants or algae. Deer browse on leaves and shoots, while rabbits eat grasses and other vegetation. Many herbivores have specialized digestive systems that help them obtain nutrients from plant material. Some rely on microorganisms that break down cellulose, a structural carbohydrate that animals generally cannot digest using their own enzymes alone.
Carnivores primarily consume other animals. Lions hunt large mammals, owls capture small vertebrates, and many spiders feed on insects. Predation can influence prey behavior and population sizes, shaping the structure of ecological communities.
Omnivores consume both plant and animal material. Raccoons, black bears, and humans are familiar examples. An omnivorous diet can provide flexibility when the availability of particular foods changes.
Other feeding strategies are equally important. Filter feeders, including many clams and some whales, capture food particles from water. Scavengers consume animals that have died, while detritivores feed on decomposing organic material. Parasites obtain resources from living hosts, often causing harm without immediately killing them.
These categories are not always exclusive. An animal’s diet can vary with age, season, habitat, and opportunity. A bear may eat fruit, insects, fish, and mammals at different times of year. Feeding classifications describe broad patterns rather than rigid rules.
The role of animals in ecosystems
Animals contribute to ecosystem processes through their interactions with plants, microorganisms, and other animals. Their effects can influence biodiversity, nutrient availability, vegetation patterns, and the movement of energy through food webs.
Pollinators such as bees, butterflies, moths, and some bats transfer pollen between flowers, helping many plants reproduce. Seed-dispersing animals carry seeds away from parent plants, sometimes transporting them over substantial distances. These processes can influence plant distribution and forest regeneration.
Predators help regulate prey populations and can alter how prey use their habitats. The effects of predation extend beyond the animals directly involved. When predators influence the abundance or behavior of herbivores, they may indirectly affect plant communities and the organisms that depend on them. Such indirect effects are called trophic cascades, although their strength varies among ecosystems.
Animals also help recycle nutrients. Earthworms mix organic material into soil, while scavengers and detritivores consume dead organisms and contribute to decomposition. Microorganisms perform much of the chemical breakdown, but animals can accelerate the process by fragmenting material and moving nutrients between locations.
Some animals create habitats for other species. Beavers build dams that alter water flow and create wetlands. Coral colonies form reef structures that provide shelter, feeding grounds, and breeding sites for many marine organisms. Burrowing animals create spaces used by other species and can modify soil conditions.
Ecological roles depend on context. A species may provide important benefits in its native habitat but become invasive when introduced to a new region without the predators, parasites, or competitors that normally limit its population.
How animals communicate and behave
Animal communication involves signals that influence the behavior of other individuals. Signals may use sound, movement, color, touch, or chemical substances. Communication can help animals attract mates, warn others of danger, establish territories, coordinate group activities, or maintain social relationships.
Birdsong is often associated with territorial defense and mate attraction. Some birds also use alarm calls that alert nearby individuals to predators. Whales and dolphins produce sounds that function in communication and, in some species, echolocation.
Chemical communication is especially important among insects and many mammals. Ants use pheromones, chemicals that affect the behavior of other members of their species, to mark trails or coordinate activities. Many mammals use scent marks to communicate information about territory or reproductive condition.
Visual displays can communicate an animal’s identity, condition, or intentions. The bright plumage of some birds contributes to courtship, while the color changes of certain cephalopods can serve in camouflage or signaling.
Animal behavior can be instinctive, learned, or a combination of both. Instinctive behaviors are strongly influenced by inherited mechanisms, while learning changes behavior through experience. Some animals can solve complex problems, recognize individuals, or learn from others. The cognitive abilities of different species vary, and similar-looking behaviors do not necessarily involve identical mental processes.
Threats to animals and their habitats
Animal populations are influenced by natural processes, but human activities have intensified several pressures on wildlife. Habitat loss and fragmentation, pollution, overexploitation, invasive species, and climate change can reduce population sizes or alter where species can survive.
Habitat loss occurs when natural environments are converted to other uses, such as agriculture, roads, mining, or urban development. Fragmentation divides remaining habitat into smaller, isolated areas. This can restrict movement, reduce access to mates, and make populations more vulnerable to local extinction.
Pollution can harm animals directly or change the conditions they depend on. Pesticides may affect nontarget organisms, plastic debris can injure or entangle wildlife, and excess nutrients entering waterways can contribute to oxygen depletion. Certain pollutants accumulate in organisms and may become more concentrated at higher levels of a food web.
Overexploitation occurs when animals are hunted, fished, or collected faster than populations can replace themselves. Species with slow growth, late maturity, or few offspring may be particularly vulnerable because recovery can take many years.
Invasive species can compete with native animals, prey on them, transmit diseases, or alter habitats. Their effects depend on the species involved and the ecological conditions of the invaded area.
Climate change affects animals by altering temperature, rainfall, snow cover, ocean conditions, and the timing of seasonal events. Some species shift their ranges or change migration and breeding patterns, while others face shrinking suitable habitats. Animals adapted to narrow environmental conditions may have fewer options when those conditions change rapidly.
Conservation efforts aim to maintain viable populations and functioning ecosystems. Common approaches include protecting and restoring habitats, establishing wildlife corridors, regulating hunting and fishing, controlling invasive species, reducing pollution, and supporting sustainable land and water management. Captive breeding and reintroduction can help some threatened species, but they are most effective when the underlying causes of decline are also addressed.
Why animal diversity matters
Animal diversity reflects millions of years of evolution, during which different lineages developed distinct ways of obtaining energy, moving, reproducing, sensing their surroundings, and surviving environmental challenges. Comparing these organisms helps scientists understand how biological systems function and how species are related.
Animals also support human well-being through ecological processes, food systems, scientific research, cultural traditions, and recreation. Their importance extends beyond species that are large, familiar, or immediately useful to people. Small invertebrates, deep-sea animals, soil-dwelling organisms, and other less visible species can contribute to essential ecological processes.
Understanding animals requires looking beyond their outward appearance. Their characteristics emerge from the interaction of anatomy, physiology, behavior, evolutionary history, and habitat. These relationships explain why different animals thrive in different environments and why protecting the conditions that sustain them is essential to maintaining the diversity of life on Earth.