Animal (Metazoa): overview, characteristics, evolution and ecological roles

Defining characteristics

Although animals are extremely diverse, they share a set of basic features. Most animals are multicellular and their cells are organized into tissues and organs. They typically ingest food and convert it into usable energy through processes of cellular respiration; introductory resources are available on cellular respiration and broader metabolism. Many animals respire using oxygen and release carbon dioxide as a by-product of metabolism (oxygen uptake, carbon dioxide release). Nervous and muscular systems, present in most major groups, enable coordination and movement (motility in animals).

Body plans and tissues

Animal bodies are organized according to recurring plans, including symmetry (bilateral or radial), segmentation, and the arrangement of body cavities. The fundamental tissue types are epithelial, connective, muscular and nervous tissue. Cell membranes and cellular structures in animals differ in composition and arrangement from those of plants and fungi; see comparative material on cell membranes and structure. Development from fertilized egg to adult involves characteristic embryonic stages studied in developmental biology.

Nutrition, feeding and lifestyles

Animals occupy many nutritional roles: most are consumers that feed on other organisms, some are detritivores that break down dead organic matter, and others live as parasites that derive resources from a host (parasites and consumers). Feeding mechanisms range from filter-feeding to predation, herbivory and complex hunting behaviors. Some animals host photosynthetic microorganisms that supplement their diet, a strategy common among certain marine invertebrates and corals.

Evolutionary origins and classification

Animals originated from single-celled ancestors and diversified into numerous lineages early in Earth’s history. The study of animals and their relationships is the domain of zoology (zoology), while the fossil record and the study of past life belong to palaeontology. Major animal groups include sponges, cnidarians, molluscs, annelids, arthropods, echinoderms and chordates; each group is defined by distinctive anatomical and developmental traits. Molecular and fossil evidence together help reconstruct these broad relationships, and protist evolution provides context for early animal ancestors (protist evolution).

Ecological roles and human interactions

Animals play essential ecological roles as consumers, pollinators, seed dispersers, ecosystem engineers and participants in complex food webs. They influence nutrient cycles, community structure and habitat formation. Humans rely on animals for food, clothing, labor, scientific research and companionship, while also managing their impacts on agriculture, health and conservation. Some species are pests or disease vectors, others are conservation priorities due to habitat loss or exploitation.

Comparisons with plants and fungi

Plants are multicellular eukaryotes that manufacture organic matter by photosynthesis using light, water and inorganic nutrients; for basic plant biology and structure see plant biology and plant tissues. Fungi are distinct in nutrition and cell structure. Animals differ from plants and fungi in typical modes of nutrition, mobility and certain cellular features, although exceptions and intermediate strategies occur.

Research, conservation and public understanding

Scientific study of animals ranges from anatomy and physiology to ecology, behavior, genetics and conservation science. Practical skills include field observation, taxonomy and laboratory techniques. Conservation efforts aim to protect biodiversity and maintain ecosystem services provided by animals. Public education, museum collections and academic resources provide accessible overviews and identification guides for those who wish to learn more about animal diversity and roles in nature.

• For introductory reading and educational resources see general portals in zoology and museum pages: zoology resources.
• For information on animal movement and behavior consult materials on motility and physiology references on metabolism.
• For contexts linking animals with microbes and protists, follow references on protist evolution, photosynthetic partners and symbiosis.
• Comparative cellular structure is summarized at cell membranes and structure and respiration basics at cellular respiration.
• General overviews on parasites, consumers and ecological roles are provided via parasites and consumers and resources on gas exchange at oxygen uptake and carbon dioxide release.
• For plant contrasts and foundational biology see plant biology and plant tissues.
• Basic cell and multicellularity primers are available at multicellularity and cells.

This entry summarizes broadly accepted features of animals and points to areas for further study; readers seeking detailed taxonomic or region-specific information should consult specialized references and current scientific literature.