What an animal eats is a question about energy, and the answer begins with three words. An herbivore is an animal that eats plants, a carnivore eats other animals, and an omnivore eats both plants and animals. The line between them is not sharp: obligate carnivores rely entirely on animal flesh for nutrients, while facultative carnivores also eat non-animal food. Biologists sort meat-eaters by how much of the diet is animal food. A hypercarnivore gets at least 70 percent of its diet from animals, a mesocarnivore at least 50 percent, and a hypocarnivore less than 30 percent. Most bears are hypocarnivores.
Plants are producers: they make their own food from sunlight. Phytoplankton are the base of most marine food webs and they make almost half of Earth's oxygen. There are more than 20,000 species of them in eight major groups. They capture light energy with pigments such as chlorophyll, and turn water and carbon dioxide into sugars and oxygen, while zooplankton and shellfish graze on them. When there are too many nutrients in the water, phytoplankton can grow out of control and form harmful algal blooms.
Some animals cannot digest wood on their own. In a termite, cellulose is broken down in the hindgut by gut microbes, no one microbe can do the job, and the microbes give off gases such as methane. Beavers eat bark and the soft growing layer under it. They can digest cellulose, which most mammals cannot, because microorganisms in a sac called the cecum break it down. Koalas eat only 20 of the roughly 350 species of eucalyptus and prefer only 5, and their caecum is up to four times their body size. Eucalyptus leaves hold poisons, and koalas' stomach bacteria help them cope. Koalas sleep so much because those leaves give them very little energy. Koalas grind tough eucalyptus leaves with their cheek teeth and eat a little dirt now and then to help digest the leaves. Sloths eat leaves, and food stays in their gut for about a month, while hippos can store two days' worth of grass in their stomachs.
A cow shows how much work grass can take. A cow can eat about 70 kg (150 pounds) of grass in an 8-hour day, and its food takes 70 to 100 hours to pass through. You may have heard that a cow has four stomachs. Ruminants such as cattle have four stomach compartments, and the rumen is the biggest, but the four compartments are part of one stomach. Cows cannot digest grass on their own. Microbes in the rumen ferment the feed in a place with almost no oxygen, and without them a ruminant could not use forages such as hay for energy as efficiently. Fermentation makes a lot of gas: 30 to 50 quarts per hour in a cow. Inside the cow, every part has a job. Rumen microbes digest about 30 to 50 percent of the fiber units (cellulose and hemicellulose), and they also make B vitamins, vitamin K and amino acids that the cow can use. The last compartment, the abomasum, works like the stomach of a person. It is the only compartment lined with glands, and they release hydrochloric acid and digestive enzymes.
Giant pandas have a strict energy budget. Bamboo is over 99 percent of the diet, yet it is a poor food, and only about 17 percent of the nutrients in the leaves and stalks are extracted. A panda's digestive system is more like a carnivore's than a herbivore's, which is why it needs so much bamboo, 10 to 16 hours of eating a day. A panda's extra digit, an enlarged wrist bone, helps it grip and tear bamboo, and strong jaws and flat back teeth crush it. Pandas mostly eat bamboo but sometimes take small mammals, fish, insects, birds or rodents.
Hunters have to budget, too. Hunting costs energy, so a whale gains only if the meal returns more energy than the hunt used. A cheetah gets within about 100 yards of its prey and then sprints, and a full sprint lasts about 20 seconds. Lions hunt as a team because many of their prey animals are faster than they are, and lionesses are the main hunters in a lion pride. Wolves usually take the weak, old and young among their prey, and a wolf can eat up to about 20 pounds of meat at one meal.
Blue whales eat tiny animals called krill. Baleen whales feed by filtering: baleen plates are made of keratin and work as a sieve, and baleen grows continuously, like hair and nails. Different species of baleen whale have evolved their own styles of filter feeding. Some, such as the blue whale, find large swarms of krill and engulf them in one huge gulp. As of November 2026, the biggest blue whales may eat up to 6 tons of krill a day. Krill come in numbers that are hard to picture. Antarctic krill swarms can be huge: one measured about 400 metres long by 200 metres across, about 100 metres deep, with hundreds of millions of krill. Krill super-swarms can be seen from space. Whale sharks are filter feeders that eat krill, crab larvae, jellyfish, sardines, anchovies, small tunas and squid, and can sift out prey as small as 1 mm with their gill rakers.
How do scientists watch a whale eat? They attached suction-cup tags with sensors, including accelerometers, magnetometers, pressure and sound recorders, to whales off South Africa, Patagonia and the US coasts. The tags showed that whales feeding on krill followed a distinct pattern of activity. Humpbacks feeding on fish varied their timing, probably because fish can dodge more than krill. To gulp a mouthful of krill or fish, a rorqual whale has to time its lunge just right. The scientists found it opened its mouth at peak speed and closed it as it slowed back to normal speed.
Some animals store the energy to wait for the next meal. The fall feeding frenzy of bears has a name: hyperphagia. Bears eat nearly nonstop to gain weight, their bodies use the fat stored in summer and fall as energy through winter, and during hibernation their body temperature, pulse and breathing drop. A polar bear's main prey is the ringed seal, and one seal gives a bear enough energy for about 11 days. Polar bears typically eat every four to five days and can live on stored fat when food is scarce. A hummingbird works on a different budget. A hummingbird eats very often, because it uses so much energy. Ruby-throated hummingbirds feed on sugary nectar to fuel hovering. On nights when their energy reserves are low they use an energy-saving state called torpor, and in late summer they use torpor more often even at higher fat levels, which lets them build fat as nights get longer.
Diet is hard to see, so scientists use several clues. One way to learn what a fish eats is to open its stomach: scientists identify each prey item and sort it into the lowest possible taxonomic group. Stomach contents are only a snapshot. They show what a fish was eating when it was caught, but not whether something is eaten often or rarely, and fast-digesting prey may never be seen at all. Stable isotopes give a longer view of diet. They occur naturally and build into the tissues of all living things from their food, and because this happens gradually, they trace long-term diets. Owl pellets are a tool, too: because owl diets largely consist of small mammals, the pellets can be used to survey a local small-mammal community without trapping or disturbing the animals.
In a study of fur seals, scientists strained hard parts such as bones out of seal scat to identify prey. This only showed the last one to two days of eating, so they added stable isotopes from blood plasma for a longer-term view. Biologists can also let owls do the sampling: instead of spending days trapping and observing in the field, they identify the bones in the pellets to work out which small mammal species were present. Diet analysis of top predators matters in ecology because it helps show how energy moves through an ecosystem.
A predator's menu can change. As of November 2026, wolves in Yellowstone take mostly elk in winter, about 90 percent of winter prey, and also deer in summer, and they also take bison. Yellowstone wolves have taken more bison over time: elk is still the main prey, but bison has become an increasingly important food source. In the early years of wolf recovery, from 1995 to 2000, elk calves made up 50 percent of wolf prey in early winter and bull elk 25 percent. Sea otters alone do not clear urchin barrens. In Monterey Bay, otters skip starved urchins in the barrens because they are not worth the effort, and feed on well-fed urchins in the kelp forest instead.
Some animals bet everything on one food, and others eat almost anything. Snail kites are specialists. They eat apple snails, pluck them off plants, and use a sharp, curved beak to extract the snail from its shell, so their food supply is tied to the wetlands where snails live. A specialist diet carries a risk: it makes snail kites particularly vulnerable to changes in their environment, and the kite's fate is closely tied to that of its prey. Brown bears are generalists. They eat grasses, sedges, roots, moss and bulbs, and fruits, nuts and berries in summer and early autumn, and in places they feed on breeding salmon in summer.
A food chain shows who eats whom. Primary consumers are herbivores, secondary consumers eat the herbivores, and carnivores that eat other carnivores are called tertiary consumers. Many different food chains that overlap make up a food web. Energy flows one way through a food chain: producers turn sunlight into food energy, and a substantial part of the energy escapes as heat during respiration at each step, so only about 10 percent reaches the next level. That is why an ecosystem supports only a few trophic levels. Decomposers eat dead plants and animals and return nutrients to the soil, and detritivores eat dead plant and animal remains and turn waste into nutrient-rich soil.
Words to know
- hypocarnivore: under 30% meat
- detritivores: eat dead remains
- hyperphagia: nonstop eating
- isotopes: traceable atom types
- torpor: energy-saving rest
