Seal, fin-footed mammal of the order Pinnipedia, which includes both the sea lions (family Otariidae) and the true seals (Phocidae). True seals have no external ears and have a thick coat of strong guard hairs. Seals are animals of the colder seas of both hemispheres. Northern species (subfamily Phocidae) include the bearded seal, the gray seal, and the common, or harbor, seal. Southern species (subfamily Monachinae) include the monk seals, elephant seals, crabeater, and Weddell seals. Most seals are gregarious; all live on the open seas and many go ashore only to breed. A single, light-colored pup is born, and further mating takes place immediately afterward. Males form harems of females on the breeding grounds. Many species are now endangered, having been extensively hunted for their skins and meat.