Animals are multicellular organisms that obtain energy by consuming organic material, have cells without rigid cell walls, and develop from embryos that typically pass through a distinctive early stage called a blastula. Most animals can move at some point in their lives, and many have specialized tissues and organs that allow them to sense their surroundings, respond to stimuli, and coordinate complex activities.
These characteristics distinguish animals from plants, fungi, bacteria, and other forms of life. However, no single trait explains every animal. Some animals remain attached to a surface throughout adulthood, others lack nervous systems, and many live as parasites inside other organisms. To understand what makes an animal an animal, biologists consider its cellular structure, nutrition, development, anatomy, and evolutionary history together.
Animals are multicellular organisms with specialized cells
All animals belong to the biological kingdom Animalia. They are eukaryotes, meaning their cells contain a nucleus that houses DNA, along with other internal structures called organelles. This places animals in the same broad category of cellular life as plants, fungi, and many single-celled organisms.
What sets animals apart is how their cells are organized and function.
Unlike bacteria, which generally consist of a single cell without a nucleus, animals are made of many cells that cooperate. These cells can specialize for different tasks, allowing an animal to carry out functions that would be difficult or impossible for a single cell to perform alone.
Muscle cells contract to generate force. Nerve cells transmit signals in animals that possess nervous systems. Epithelial cells form protective coverings and line internal surfaces, while blood cells transport substances or contribute to immune defense in animals with blood-based circulatory systems.
This division of labor is a defining feature of animal biology. Rather than functioning as independent units, most animal cells operate as parts of an integrated organism.
The degree of specialization varies considerably. Sponges, among the simplest living animals in terms of body organization, lack true tissues and organs but still contain different cell types that perform distinct functions. In contrast, mammals possess highly specialized cells organized into complex systems, including the nervous, digestive, respiratory, and circulatory systems.
Animals also maintain relationships between their cells through chemical signals, physical connections, and interactions with the surrounding material known as the extracellular matrix. These relationships help coordinate growth, repair, and normal bodily functions.
Multicellularity alone does not make an organism an animal. Plants and fungi are also multicellular, and they have their own characteristic ways of organizing cells. The animal kingdom is distinguished by a particular combination of cellular features and developmental patterns inherited from a common evolutionary ancestor.
Animals obtain energy by consuming organic material
Animals are heterotrophs, meaning they obtain the carbon and energy needed for growth and maintenance from organic compounds produced by other organisms or derived from their remains.
Plants, by contrast, typically make energy-rich organic molecules through photosynthesis, using light energy to convert carbon dioxide and water into sugars. Animals generally cannot perform this process because they lack the photosynthetic machinery found in plants and algae.
Instead, animals acquire nutrients by eating plants, other animals, fungi, microorganisms, or organic material. These nutrients supply both chemical energy and the raw materials needed to build and maintain cells.
Most animals take food into their bodies and digest it internally. Enzymes break complex molecules such as proteins, fats, and carbohydrates into smaller substances that cells can absorb and use. Some animals have a complete digestive tract with separate openings for food intake and waste elimination. Others have simpler digestive arrangements.
Sponges illustrate a different approach. They draw water through their bodies and capture microscopic food particles, which their cells take up and digest. They do not have a conventional digestive tract, but they still obtain nutrients from organic material.
Fungi also depend on organic matter, yet their feeding method differs from that of most animals. Fungi typically release digestive enzymes into their surroundings and absorb the resulting small molecules. Animals generally digest food internally or within specialized cells.
This distinction helps explain why a mushroom is not an animal, even though both animals and fungi rely on organic matter for nutrition.
Animals also differ from plants in how they store and use certain energy reserves. Many animals store excess energy as glycogen, a carbohydrate that can be broken down when needed. Plants commonly store energy as starch. These are general patterns rather than absolute rules, but they reflect differences in metabolism and evolutionary history.
Not every animal eats in the familiar sense. Some obtain nutrients from symbiotic microorganisms living in or on their bodies, while others absorb nutrients from a host. Nevertheless, their nutrition ultimately depends on organic compounds rather than the direct production of food through photosynthesis.
Animal cells lack rigid cell walls
One of the most important cellular differences between animals and plants is the absence of rigid cell walls.
Plant cells generally have walls made primarily of cellulose, a strong structural carbohydrate. Fungal cell walls typically contain chitin and other materials. Animal cells, in contrast, are surrounded by flexible plasma membranes rather than rigid external walls.
This flexibility allows animal cells to change shape, move relative to neighboring cells, and participate in processes such as tissue formation, wound healing, and the engulfing of particles. It also helps make possible the diverse body structures found throughout the animal kingdom.
The absence of cell walls does not mean animal cells lack structural support. Many are reinforced by internal protein frameworks called the cytoskeleton, which helps maintain cell shape, move materials within cells, and organize cell division. Outside the cells, the extracellular matrix provides additional support and helps tissues retain their structure.
In animals with skeletons, structural support can operate at a much larger scale. Vertebrates have internal skeletons made primarily of bone and cartilage, while many arthropods, including insects and crabs, have external skeletons made largely of chitin. Other animals, such as earthworms, rely partly on fluid-filled body compartments and muscular walls for support.
These structures are not cell walls. They are tissues or body-level systems that support an organism while leaving its individual cells surrounded by flexible membranes.
The combination of flexible cells and specialized structural materials contributes to the wide range of forms animals can develop, from soft-bodied worms to armored insects and large mammals.
Animal development follows a distinctive pattern
The way animals develop from a fertilized egg provides some of the strongest evidence of their shared evolutionary ancestry.
In most sexually reproducing animals, development begins when a sperm cell and an egg cell unite to form a zygote. The zygote divides repeatedly, producing more cells without initially increasing the overall size of the developing embryo very much.
These early divisions are called cleavage. They produce a cluster of cells that develops into a stage known as a blastula. In many animals, the blastula is a hollow or partly hollow ball of cells, although its precise structure varies among groups.
The embryo then undergoes further changes, including gastrulation, a process in which cells move and reorganize to establish the primary tissue layers of the developing body. These layers give rise to different tissues and organs as development continues.
Many animals form two primary embryonic tissue layers, while others form three. In animals with three layers, the outer layer, called the ectoderm, contributes to structures such as the epidermis and nervous system. The inner layer, the endoderm, contributes to the lining of the digestive tract and associated organs. The middle layer, the mesoderm, gives rise to structures that may include muscles, connective tissues, and parts of the circulatory and reproductive systems.
These patterns are not identical across all animal groups. Sponges and certain other animals have simpler developmental organization than animals with well-defined germ layers and organs. Even among animals with broadly similar developmental stages, the timing and details can differ substantially.
Still, the formation of a blastula and the subsequent organization of embryonic cells are important characteristics of animal development. They help biologists distinguish animals from other multicellular organisms and reveal relationships among animal groups.
Not every animal reproduces sexually throughout its life cycle. Some reproduce asexually, producing offspring without the fusion of sperm and egg, and some alternate between sexual and asexual reproduction. These variations do not change their classification as animals because kingdom membership depends on evolutionary relationships and inherited biological characteristics, not on a single reproductive method.
Most animals can move, but movement is not universal
Movement is strongly associated with animals, yet it is not a requirement that every animal must be capable of moving freely as an adult.
Many animals actively move from place to place to find food, escape predators, locate mates, or reach suitable environments. Muscles generate force by contracting, while skeletons, hydrostatic support systems, or other body structures help direct that force into movement.
In vertebrates, muscles often pull on bones across joints, producing coordinated movements. In insects and other arthropods, muscles attach to an external skeleton. Earthworms use coordinated muscle contractions against a fluid-filled body compartment to move through soil.
Movement can also occur at microscopic scales. Cilia, which are tiny hairlike structures on the surfaces of certain cells, can move fluid or propel cells through their surroundings. In some animals, cilia help transport food or move larvae through water.
However, some adult animals are sessile, meaning they remain attached to a surface rather than moving from place to place. Adult barnacles, for example, attach themselves to hard surfaces and feed by capturing particles from the water. Many adult sponges remain fixed in place, filtering water to obtain food.
The life cycles of such animals often reveal why movement cannot serve as a universal defining characteristic. Barnacles have free-swimming larvae that eventually settle and develop into attached adults. Sponges can also have mobile larval stages even though their adult forms are generally stationary.
Other animals, including certain parasites, have greatly reduced movement because their life cycles depend on living within or on a host.
Movement is therefore a common and important animal trait, but its form and extent vary with an organism’s anatomy, life stage, and ecological role.
Animals respond to their environments in different ways
Animals must interact with changing conditions in their surroundings. They detect resources, avoid harmful conditions, and respond to threats through a range of sensory and physiological mechanisms.
In many animals, the nervous system coordinates these responses. Sensory cells detect stimuli such as light, chemicals, pressure, temperature, or sound. Signals are transmitted through nerve cells and processed by networks that can trigger appropriate actions.
The complexity of these systems varies widely. Mammals have highly developed brains and specialized sensory organs, while many invertebrates rely on simpler nerve networks or clusters of nerve cells called ganglia.
Sponges are an important exception because they lack neurons and a conventional nervous system. Nevertheless, their cells can coordinate activities such as water flow and responses to certain environmental disturbances. This demonstrates that coordination does not always require nerves.
Animal responses also extend beyond behavior. When conditions change, cells and tissues may adjust their activity to preserve internal stability. This regulation is called homeostasis. Depending on the animal, it may involve controlling body temperature, water balance, salt concentrations, blood chemistry, or energy availability.
Mammals and birds, for instance, regulate internal body temperature within a relatively narrow range under many environmental conditions. Most reptiles, amphibians, and fish rely more heavily on external heat sources, although they still regulate aspects of their internal environments.
Not every animal has the same regulatory abilities, and some depend on their surroundings to a considerable degree. What matters is that animals possess biological mechanisms that allow their cells and bodies to function under the conditions in which they live.
The ability to sense and respond to the environment is widespread among animals, but neither a brain nor a nervous system is universal across the kingdom.
Animal bodies are organized in many different ways
Although animals share fundamental characteristics, their body plans vary enormously. A body plan is the general arrangement of an organism’s structures, including its symmetry, tissues, internal spaces, and major organs.
Many animals have bilateral symmetry, meaning their bodies can be divided into roughly matching left and right halves along one plane. This arrangement is common in animals that move in a particular direction, such as insects, fish, and mammals. Bilateral symmetry often accompanies a concentration of sensory structures and nervous tissue toward the front of the body, a pattern known as cephalization.
Other animals have radial symmetry, in which body parts are arranged around a central axis. Adult sea anemones and many jellyfish display this general pattern, which can be useful for interacting with the environment from multiple directions.
Sponges often have irregular shapes rather than a consistent form of symmetry. Their bodies contain openings and channels that allow water to flow through them, supporting their filter-feeding lifestyle.
Animals also vary in the complexity of their internal organization. Some have specialized digestive, circulatory, respiratory, and excretory organs. Others exchange gases directly across body surfaces, transport materials over short distances, or use simpler internal structures.
A complex organ system is not a requirement for being an animal. Rather, these systems are evolutionary solutions to the demands of particular body sizes, habitats, and ways of life.
The same principle explains the diversity of skeletons. Vertebrates possess internal skeletons, many arthropods have external skeletons, and other animals rely on fluid pressure or flexible tissues for support. Each arrangement provides different advantages and imposes different constraints on movement, growth, and body size.
Animal diversity is therefore not a departure from a single ideal design. It is the result of evolutionary changes to a shared set of biological foundations.
Reproduction and life cycles vary across the animal kingdom
Animals reproduce through a wide range of strategies. Sexual reproduction is common and typically involves the union of sperm and egg cells, combining genetic material from two reproductive cells. Fertilization may occur outside the body, as in many aquatic animals, or internally, as in mammals, birds, and many reptiles.
Some animals also reproduce asexually. Certain hydras and sea anemones can produce offspring through budding, in which a new individual develops from an outgrowth of the parent. Some other animals can regenerate an entire individual from a body fragment under appropriate conditions.
Reproductive strategies are closely connected to an animal’s environment and life history. Aquatic species may release large numbers of eggs or larvae into the water, while many terrestrial animals invest more resources in protecting embryos or caring for young. These are broad tendencies rather than universal rules.
Animal life cycles can also include dramatic transformations. Metamorphosis is a developmental process in which an animal changes substantially in body form as it moves between life stages. Butterflies develop from caterpillars into adults, while many amphibians transform from aquatic larvae into forms adapted for life on land.
Other animals develop without such a pronounced transformation. In humans, for example, young individuals grow and mature while retaining the same general body plan.
These differences show that an animal’s identity is not determined by whether it lays eggs, gives birth to live young, undergoes metamorphosis, or reproduces in a particular way. Such traits help distinguish species and reveal adaptations, but the animal kingdom includes many variations on each pattern.
Evolution explains why these characteristics occur together
The defining features of animals make the most sense when considered as products of evolution.
Living animals descended from a common ancestral population that lived hundreds of millions of years ago. Over evolutionary time, inherited changes accumulated, populations diverged, and different lineages developed the forms and lifestyles seen today.
Many fundamental animal characteristics reflect features inherited from that shared ancestry. These include multicellularity, cells without rigid walls, particular patterns of embryonic development, and a mode of nutrition based on obtaining organic material from other sources.
Biologists use several kinds of evidence to reconstruct these relationships. Comparisons of DNA sequences reveal similarities and differences in genetic information. Developmental biology identifies shared patterns in how embryos form. Anatomy and the fossil record help show how body structures have changed over geological time.
No single characteristic is necessarily unique to animals when considered in isolation. Multicellularity occurs in plants and fungi. Heterotrophic nutrition occurs in fungi and many microorganisms. Cells without rigid walls are found in other groups of life, and movement is widespread among organisms that are not animals.
The distinction emerges from the combination of traits and, especially, from evolutionary relationships. A living organism belongs to the animal kingdom because of its ancestry and its place within the animal branch of the tree of life, not because it happens to display one familiar behavior.
This evolutionary perspective also explains why some animals seem to challenge common definitions. A sponge does not have a nervous system, an adult barnacle does not roam freely, and a parasitic animal may have a highly reduced body structure. Yet each shares fundamental biological and evolutionary characteristics with other animals.
How animals differ from plants, fungi, and other organisms
The differences among major groups of life are easiest to understand by comparing their overall biology rather than relying on a single feature.
Plants generally have cells with cellulose walls and obtain energy through photosynthesis, although some plants have evolved unusual nutritional strategies. Fungi typically have cell walls containing chitin and absorb nutrients after releasing digestive enzymes into their surroundings. Animals lack rigid cell walls and generally acquire nutrients by ingesting food or taking up organic material through specialized cells.
Bacteria differ from all three groups because they are prokaryotes: their cells do not contain a membrane-bound nucleus. They include organisms with a wide range of metabolic strategies, including photosynthesis, chemical energy use, and the breakdown of organic compounds.
Some organisms blur superficial distinctions. Euglena, for example, is a single-celled eukaryote that can use photosynthesis and can also obtain organic nutrients under certain conditions. It may move through water, but it is not an animal. Its cellular organization and evolutionary ancestry place it outside the animal kingdom.
Slime molds and other organisms can also resemble animals in particular stages of their life cycles without belonging to Animalia. Similar appearances or behaviors do not necessarily indicate close evolutionary relationships.
The central distinction is that animals form a particular evolutionary lineage characterized by a recognizable combination of cellular, nutritional, developmental, and organizational traits.
An animal does not need to walk, possess a brain, have a backbone, or eat in a way humans recognize. It may live in the ocean, burrow underground, remain attached to a rock, or spend most of its life inside a host. What makes it an animal is the underlying biological organization and evolutionary history that connect it to the rest of the animal kingdom.
