Mammals use food to keep themselves warm as well as to power their bodies and grow. As a result, they have to eat more frequently than cold-blooded animals, or ectotherms, which do not maintain a constant warm temperature. This is particularly true of the smallest mammals because, despite having fur, their small body size means that they lose heat at a rapid rate. To facilitate eating, specialized teeth help them to collect their food, and also to process it before it is swallowed. Mammals' teeth occlude, which means that they fit together in a precise way when the mouth is closed, allowing them to nibble, gnaw, slice, or chew.

The smallest predatory mammals feed on animals such as earthworms and insects, and often find their prey by touch. Bigger insect-eaters, such as pangolins and anteaters, usually locate their food by sight or smell. They gather food in much bigger quantities using their sticky tongues, which can sweep up thousands of ants or termites in a matter of minutes.

The majority of bats also feed on insects, but they have a very different technique for catching their flying prey. Using a system called echolocation, a bat sends out bursts of high-pitched sound toward objects and interprets the returning echoes as images that guide a bat toward its prey so that it can hunt even in total darkness. Dolphins and sperm whales use a similar system to locate food underwater.

True carnivores, which include dogs, cats, and their relatives, often spend a considerable time tracking down their prey. For most of these flesh-eating animals, the senses of smell and hearing are at least as important as vision. Carnivores normally hunt alone, but some species—notably wolves and lions—hunt in organized groups. By doing this they can tackle prey larger than themselves. Carnivores have pointed front teeth, called canines, which help them hold their prey. Most also have specialized rear teeth, called carnassials, which work like shears to slice through their food, and to crush bones.

The most successful of the plant-eating mammals are the ruminants—hoofed mammals that include cattle, sheep, goats, and their relatives. Ruminants have a highly specialized three- or four-part stomach that helps these animals digest the cellulose in plant cell walls, which no mammal can digest on its own. After eating their food, they regurgitate it and chew it a second time, before swallowing it once again. Microorganisms that reside in the stomach then break down the chewed-up mass, releasing nutrients that the mammal absorbs.

In water, few mammals apart from manatees and dugongs live purely on plants. Instead, most aquatic mammals eat animal food, but they catch it in two quite different ways. Pursuit hunters, such as otters, seals, and toothed whales, chase individual prey through the water, much like carnivores chase their prey on land. However, the largest whales feed on much smaller animals, scooping them up in vast quantities. These whales do not have teeth, and instead strain their food with a screen of fibrous plates called baleen. Using this system of filter feeding, a blue whale can consume over 4 metric tons of food in a day.

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