Ask what an animal eats and the answer sounds simple: an herbivore is an animal that eats plants, a carnivore is an animal that eats other animals, and an omnivore is an animal that eats both plants and animals. Yet the labels blur. 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, and black bears eat mostly plants: nuts, berries and fruit. Obligate carnivores rely entirely on animal flesh for nutrients, while facultative carnivores also eat non-animal food, so the line between a carnivore and an omnivore is not sharp.

Bodies seem to follow diets, and the rule is useful even though it leaks. Plant-eaters usually have big, wide molars that grind leaves and grasses, while meat-eaters' teeth are built differently, and the two groups' guts differ as well. A study of mammal guts found that the more of a species' diet is animal food, the shorter its intestines are for all sections of the gut, and that diet mainly affects the large intestine, which is longer in herbivores. Animals that eat highly digestible food, such as meat and insects, do not need complex or long digestive tracts. But the idea that plant-eaters always have longer guts has exceptions, and the same study says body mass is more tightly linked to small intestine length than the large intestine, and that not all species follow the rule.

The giant panda is a good test of the rule, and it carries a strict energy budget. Bamboo is over 99 percent of the diet of a giant panda, yet it is a poor food, and only about 17 percent of the nutrients in the leaves and stalks are extracted. Pandas mostly eat bamboo but sometimes take small mammals, fish, insects, birds or rodents, and 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.

Many large herbivores solve the plant problem with partners. They keep helpful bacteria in their guts, and they have long, complex digestive tracts, which gives the microbes enough space and time to ferment tough plant fiber. Cows cannot digest grass on their own, and microbes in the rumen ferment the feed in a place with almost no oxygen, while without them a ruminant could not use forages such as hay for energy as efficiently. 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, but fermentation in the rumen also makes a lot of gas: 30 to 50 quarts per hour in a cow.

The partnership appears in wood-eaters, too, since 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, and 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, while eucalyptus leaves hold poisons that koalas' stomach bacteria help them cope with, and a little dirt now and then helps them digest the leaves.

Meat-eaters digest with different tools, and some of them contradict what people assume. A python is a constrictor, which grabs prey with its teeth, wraps coils around it, and squeezes, but it does not crush the prey's bones, and it then unfolds its jaw to swallow the prey whole and rests in a warm place while it digests. You may have heard that spiders only suck the juice out of their prey, but a spider actually bites its prey, wraps it in silk, waits, then vomits digestive fluid over the prey and takes the liquid in, and all spiders, as far as we know, digest some solid parts of their prey. Chimpanzees, by contrast, are broadly omnivorous, relying on ripe fruits and young leaves, with insects, small vertebrates and eggs added, and males typically eat more meat than females.

Nobody can follow a wild animal to every meal, so the evidence about diets is indirect, and each method has a gap. One way to learn what a fish eats is to open its stomach, where scientists identify each prey item and sort it into the lowest possible taxonomic group, but 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 (you are what you eat), and because this happens gradually, they trace long-term diets.

A study of fur seals shows how the methods can work together. Scientists strained hard parts such as bones out of seal scat to identify prey, but this only showed the last one to two days of eating, so they added stable isotopes from blood plasma for a longer-term view. The isotopes of carbon and nitrogen formed two clusters that separated inshore feeding from offshore feeding, which fits earlier work suggesting that a generalist feeder is often made up of subsets of animals with different diets. Diet research combines methods because each one has a gap: stomach contents and scat show only recent meals, scat hard parts can miss soft prey, and isotopes show long-term patterns but not which exact food was eaten.

Owls offer a way to sample a whole community without setting a single trap. 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, and biologists can let owls do the sampling, since 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. Owls differ in when and what they hunt: the great gray owl hunts by day and prefers voles, the tawny owl hunts voles at night, and the ural owl takes larger prey such as squirrels. An owl can hear a mouse stepping on a twig from 75 feet (23 meters) away, and barn owls are the most accurate birds at locating prey by sound.

Whales can be watched in the act. Scientists attached suction-cup tags with sensors, including accelerometers, magnetometers, pressure and sound recorders, to whales off South Africa, Patagonia and the US coasts to watch how they feed, and the tags showed that whales feeding on krill followed a distinct pattern of activity, while 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, and the scientists found it opened its mouth at peak speed and closed it as it slowed back to normal speed. Hunting costs energy, so a whale gains only if the meal returns more energy than the hunt used, and for that reason baleen whales take their time to assess a swarm before committing to a lunge.

The food of the blue whale is tiny. Blue whales eat tiny animals called krill, and in the Southern Ocean the main krill they eat is Euphausia superba (Antarctic krill), a planktonic crustacean less than 7 cm long, whose swarms can be huge: one measured about 400 metres long by 200 metres across, about 100 metres deep, with hundreds of millions of krill. As of November 2026, the biggest blue whales may eat up to 6 tons of krill a day. Baleen whales feed by filtering, since baleen plates are made of keratin and work as a sieve, and baleen grows continuously, like hair and nails, while different species of baleen whale have evolved their own styles of filter feeding.

A predator's menu is not fixed over time. 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, and they 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, while wolves usually take the weak, old and young among their prey. Wolves hunt in packs, which lets them bring down large animals such as elk and moose, and a wolf can eat up to about 20 pounds of meat at one meal.

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. A predator's role can differ from place to place: Southern California's kelp beds did not decline as much as those on the central and northern coasts.

Some animals bet their survival on a single food, while others eat almost anything. Snail kites are specialists, which 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, because 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, which 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, and what they eat changes with the season.

Bears live by the calendar. A black bear's diet changes through the year. Spring, after bears leave winter dens, is a time of scarce food, berries come in summer, and fall brings fruits, nuts and acorns that build fat. The fall feeding frenzy has a name, hyperphagia, and during it bears eat nearly nonstop to gain weight, and 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, while polar bears typically eat every four to five days and can live on stored fat when food is scarce.

Primary consumers are herbivores, secondary consumers eat the herbivores, and carnivores that eat other carnivores are called tertiary consumers, while detritivores eat dead plant and animal remains and turn waste into nutrient-rich soil. 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, which is why an ecosystem supports only a few trophic levels.

Words to know

• hypercarnivore: 70% meat eater
• isotopes: traceable atom types
• scat: animal droppings
• generalist: wide-diet eater
• hyperphagia: nonstop eating