It seems simple that plant-eaters have long guts and meat-eaters short ones, and there is truth in it. 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. 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.
Gorillas eat mostly plants such as leaves, stems and roots. 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. In a cow, fermentation in the rumen makes a lot of gas, 30 to 50 quarts per hour. Cows cannot digest grass on their own, and without microbes a ruminant could not use forages such as hay for energy as efficiently. 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.
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. 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, while wolves usually take the weak, old and young among their prey.
Scientists have several ways to read a menu. 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. Stable isotopes give a longer view of diet, because 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. Many owls hunt at night, and owl pellets help 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.
A menu can also change 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. They have taken more bison over time, and 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.
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. 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 hummingbird eats very often, because it uses so much energy. Ruby-throated hummingbirds feed on sugary nectar to fuel hovering, and on nights when their energy reserves are low they use an energy-saving state called torpor. In late summer they use torpor more often even at higher fat levels, which lets them build fat as nights get longer.
Words to know
- isotopes: traceable atom types
- torpor: deep energy-saving rest
- fermentation: microbe breakdown
- barrens: bare patches
- intestines: gut tubes
