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. Diet mainly affects the large intestine, which is longer in herbivores. Plant-eaters usually have big, wide molars that grind leaves and grasses. Many large herbivores 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.

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. Ruminants such as cattle have four stomach compartments, and the rumen is the biggest; the common saying that a cow has four stomachs is not quite right, because the four compartments are part of one stomach. A cow's salivary glands can add 50 to 80 quarts of saliva to the rumen daily, which gives the microbes liquid and keeps the rumen between pH 6.2 and 6.8. Cows may spend 35 to 40 percent of each day chewing cud, which also re-mixes the feed with saliva before it is swallowed again.

Other animals manage in other ways. 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. 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. Pandas mostly eat bamboo but sometimes take small mammals, fish, insects, birds or rodents.

Meat-eaters have their own methods. Pythons are ambush hunters: they wait for food to come close. 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 then it 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. A spider bites its prey, wraps it in silk, waits, then vomits digestive fluid over the prey and takes the liquid in. All spiders, as far as we know, digest some solid parts of their prey.

Wolves in Yellowstone take different prey in winter and summer. 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. 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. Wolves hunt in packs, which lets them bring down large animals such as elk and moose. Lions hunt as a team because many of their prey animals are faster than they are.

Some animals eat both plants and animals. Chimpanzees are broadly omnivorous. They rely on ripe fruits and young leaves, with insects, small vertebrates and eggs added, and males typically eat more meat than females. 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.

Scientists use clues to learn what animals eat. One way to learn what a fish eats is to open its stomach, 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. Many owls hunt at night, and owl pellets are a tool for scientists. 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.

Phytoplankton 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. 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.

Words to know

• intestines: the gut tubes
• saliva: spit
• constrictor: squeezing snake
• hyperphagia: nonstop eating
• zooplankton: tiny floating animals