Animals

What an animal is

The US Geological Survey (USGS) describes animals as organisms made of many cells and placed in the kingdom Animalia.^[1] Those cells are eukaryotic: each keeps its genetic material inside a distinct nucleus.^[1] Animals are also heterotrophic, which means they get their energy from organic substances that other living things produced.^[1]

The group is wider than it looks: NOAA, the US ocean and weather agency, counts corals as invertebrate animals.^[9]

Animals with a backbone

The US National Park Service (NPS) sorts vertebrates into five major classes, and its classroom summaries list the usual features of each:^[2]

• Fish: ectothermic, with scales, gills, fins and jelly-like eggs.^[2]
• Amphibians: ectothermic, with thin moist skin, lungs, no claws and jelly-like eggs.^[2]
• Reptiles: ectothermic, with dry scaly skin, clawed toes, lungs and leathery eggs.^[2]
• Birds: endothermic, with feathers, hollow bones, lungs and hard-shelled eggs.^[2]
• Mammals: endothermic, with hair or fur, a diaphragm and lungs, and they make milk.^[2]

An ectotherm cannot control its body heat from within, so it warms or cools itself through behaviour, such as lying in the sun or moving into shade.^[3] Reptiles lay amniotic eggs, which is what allows them to breed on dry land.^[3]

Animals without a backbone

Invertebrates are animals lacking the chain of interlocking vertebrae that forms a backbone.^[4] The NPS says invertebrates first evolved in the sea and then spread into almost every environment.^[4] One NPS page estimates, without giving a date, that invertebrates account for over 90% of living animal species.^[5]

Groups the NPS lists include cnidarians (corals, jellyfish, sea anemones), mollusks (bivalves, gastropods, cephalopods and others), arthropods (such as spiders and insects), sponges, and echinoderms such as crinoids.^[4]

• Insects have a hard outer covering, the exoskeleton, instead of a backbone, plus six legs, two antennae and a body in three parts: head, thorax and abdomen.^[6]
• Crustaceans, such as shrimp, krill, crabs and lobsters, are close relatives of insects and spiders and usually wear a hard shell.^[8]
• Mollusks are the next largest invertebrate group after arthropods; most have a soft body inside a shell, but slugs, squid and octopuses are mollusks too.^{[7, 8]}

Food and food chains

Every living thing is linked into food chains.^[12] Producers make their own food, while consumers get their energy by eating other organisms.^[12] Mussels are filter feeders that strain algae, bacteria and sediment from the water; one adult can process 20 gallons or more daily.^[7]

Removing one species can trigger a chain of effects on many others, and the US Fish and Wildlife Service (FWS) warns that nobody can fully predict them.^[27] Losing a keystone species can reshape a whole wildlife community.^[26]

Growing up

In incomplete metamorphosis, the stages are egg, nymph and adult, and nymphs of grasshoppers or aphids often resemble small adults.^[14] In complete metamorphosis, the stages are egg, larva, pupa and adult, as in butterflies, beetles, ants and flies, and the larva looks nothing like the adult.^[14]

The name amphibian comes from a Greek word for leading two lives, because the young and adult forms often differ greatly.^[3] Young amphibians live in water, breathing with gills and swimming with a tail, while adults move about on land and breathe with lungs.^[14] A bullfrog may need over a year for this change, a spadefoot toad under two weeks.^[14] Reptiles skip the change and hatch looking like small adults.^[3]

Getting through winter

Winter brings two problems: bitter cold and less to eat.^[15] Some animals leave: monarchs head for Mexico for the cold months, and caribou cover more than 2,000 miles on land.^[16] Others merely move downhill to less snowy ground, known as altitudinal migration.^[16]

Others stay and go dormant. Hibernation is one form of dormancy, but not every dormant animal hibernates.^[15] A true hibernator lowers its metabolism, heart rate and body temperature.^[16] A hibernating bat’s heart slows from 200–300 beats a minute to about 10, and the NPS puts its energy saving at roughly 98%.^[17]

Adaptations

An adaptation is a trait that helps an individual survive and reproduce where it lives.^[19] The NPS describes three kinds: physical ones such as body shape, skin and colour; behavioural ones; and physiological ones, which are changes in body chemistry.^[20]

• Physical: a bison’s shoulder hump lets it swing its head side to side to clear snow off food.^[13]
• Behavioural: beavers forage mostly at dawn and dusk, when predators are less of a threat.^[19]
• Physiological: a diving beaver slows its heart and metabolism, so it can stay under for up to 15 minutes.^[19]

Senses feed these responses: nerves carry signals from sense organs to the brain.^[19] The NPS says most birds of prey see at least ten times as far as people.^[19]

Habitats and ecosystems

A habitat is an organism’s address, the place where conditions suit it, while its niche is its way of life.^[21] An ecosystem is everything, living and non-living, in an area, and it can be on land or in water.^[24] Animals help build ecosystems: beaver dams create wetlands, and birds scatter seeds.^[24]

Hard corals live in colonies of many polyps joined by the calcium carbonate skeletons they produce, whereas soft corals build no reefs.^[9] NOAA estimates that reefs occupy under 1% of the seafloor yet support around a quarter of all marine creatures.^[9]

Why animals matter to people

People rely on the variety of life for food, medicines and materials.^[25] Farmers use insects and other animals that eat crop pests, a practice called biological control.^[26]

Endangered species and US law

Species have always gone extinct naturally, but the FWS says the evidence strongly points to a present rate well above the natural background, driven mainly by habitat loss.^[26] In the United States, the Endangered Species Act, made law in 1973, protects at-risk plants and animals however popular or obscure they are.^[27] It generally protects listed animals from people intentionally harming, harassing or killing them or destroying their habitat, acts grouped under the term take.^[27] Section 7 requires federal agencies to make sure their actions do not jeopardize the existence of listed species.^[28] The USGS stresses acting early: look after species while they are still plentiful, before any of them become threatened.^[29]

Common misconceptions

“Bears hibernate.” The NPS says bears are not true hibernators because their body temperature stays up in winter.^[15]

“Corals are plants or rocks.” Corals are invertebrate animals.^[9] What looks like one big tree or fan is often a colony of many polyps.^[9]

What this page does not cover

Its sources do not support an account of body plans, body systems such as digestion or breathing, reproduction and parental care in mammals, birds and fish, detailed diversity within those groups, animal evolution and the tree of life, intelligence and social behaviour, or domestication. It also gives no species counts, and the section on protection describes only the US system.

Going further

See Cells, Ecology, Plants, Microbes, Evolution, Genetics and Oceans for neighbouring parts of the map.