The three diet words are simple. An herbivore eats plants, a carnivore eats other animals, and an omnivore eats both. Real animals are messier. 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, and a hypocarnivore gets less than 30 percent. Most bears are hypocarnivores. 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.
Plants are hard to digest, and bodies solve the problem in different ways. Plant-eaters usually have big, wide molars that grind leaves and grasses. 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. In fact the four compartments are part of one stomach. A cow cannot digest grass on its own. Microbes in the rumen ferment the feed in a place with almost no oxygen.
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 giant panda spends about half of each day eating bamboo, and strong jaws and flat back teeth crush it. Eucalyptus leaves hold poisons, and koalas' stomach bacteria help them cope. Koalas sleep so much because those leaves give them very little energy.
Some animals bet everything on one food. Snail kites are specialists. They eat apple snails 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.
Wolves usually take the weak, old and young among their prey. An owl can hear a mouse stepping on a twig from 75 feet (23 meters) away.
Baleen whales feed by filtering: baleen plates are made of keratin and work as a sieve, and baleen grows continuously, like hair and nails. 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.
Phytoplankton are the base of most marine food webs, and they make almost half of Earth's oxygen. On average only about 10 percent of the energy at one level of a food chain passes to the next level. In a food chain, carnivores that eat other carnivores are called tertiary consumers.
Sloths eat leaves, and food stays in their gut for about a month. Hippos can store two days' worth of grass in their stomachs. Vultures eat as much as they can when they find food, because the next meal may be up to two weeks away. A turkey vulture can smell its food, which is unusual for a bird.
How do scientists learn what a wild animal eats? Diet analysis of top predators matters in ecology because it helps show how energy moves through an ecosystem. 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. 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.
Words to know
- hypocarnivore: under 30% meat
- facultative: sometimes, not always
- specialists: narrow-diet eaters
- generalists: wide-diet eaters
- phytoplankton: floating algae
