Two- and Three-toed Sloths main food source is leaves. These leaves provide very little energy or nutrition. The Sloths have adapted very large, specialized, slow-acting stomachs with multiple compartments in which symbiotic bacteria break down the tough leaves. As much as two-thirds of a well-fed sloth's body-weight consists of the contents of its stomach, and the digestive process can take as long as a month or more to complete. Sloths also have metabolic rates less than half of that expected for a creature of their size. To conserve energy Sloths move only when necessary and then very slowly. They can move at a marginally higher speed if they are in immediate danger from a predator (15 feet per minute), but they burn large amounts of energy doing so. On the ground their maximum speed is 5 feet per minute. They mostly move at 0.5 to 1 foot per minute.

Because sloths spend so much time hanging upside down, with their legs above their bodies, their hairs grow away from the extremities, the opposite of other mammals, in order to provide protection from the elements. In moist conditions, the fur hosts two species of symbiotic cyanobacteria, which serves two benefits for the Sloth. It gives the fur a green tinge which help camouflage the Sloth. The bacteria also provides nutrients to the sloth, who will lick them off the fur.

Dogs regulate their temperature through evaporation from the tongue. Brachycephalic (snub-nosed) dogs have a shorter than normal tongue. Because of this, dogs such as the pug or shih tzu have more trouble regulating their temperatures than other dogs do, and have a harder time tolerating high temperatures.

Based on fossils, blood, and DNA, the closest living relatives of whales and dolphins are the Hippopotamus. Evidence shows that their ancestors split approximately 60 million years ago, and they are the closest living descendants of that ancestor species.