Pinnipeds are found in the Otariidae and Phocidae families. All pinnipeds are large, perhaps because water conducts heat well and large animals have a low surface area to body mass ratio, which minimizes heat loss due to conduction. Their bodies are insulated by a thick layer of fat called blubber. In all species, the external ears are small or absent, the external genitalia and nipples are hidden in slits or depressions in the body, and the tail is very small. The forelimbs and hindlimbs are transformed into paddles.
Most species have a relatively short rostrum, and the orbits are large.
It is easier to maintain small pinnipeds in the lab than cetaceans, and consequently their physiological adaptations for diving have been studied extensively. Currently, the most strongly supported scheme places pinnipeds as a monophyletic group within the caniform lineage, most closely related to bears, red pandas, and raccoons. The sister group to this clade is the family Mustelidae, and the canids appear as an early offshoot of the caniforms. We recognize 11 families and around 270 species among the living Carnivora. L. C. Drickamer, S. H. Vessey, and J. F. Merritt.