A Seal on the beach.

There are 33 species of pinnipeds alive today, most of which are known as seals. Pinnipedia is made up of three main groups: The walrus, which is the only living member of the family Odobenidae; the eared seals of Otariidae, including numerous kinds of fur seal and sea lion; and the earless seals, known as true seals or Phocidae. Despite the name, earless seals have ears—they’re just hidden beneath the surface of their skin.

Pinnipeds can be found on every continent on Earth, though most species occur in cold-water environments. Thick layers of fat, also known as blubber, keep the animals warm, in addition to dense fur. Walruses are the exception to the rule, as these large, tusked pinnipeds have nearly hairless bodies.

Seals range greatly in size, from the gargantuan southern elephant seal, which can weigh more than a pickup truck, to the relatively slender, 100-pound Baikal seal.

While there are many differences among the species, all seals have feet shaped like fins. In fact, the word pinniped means "fin-footed" in Latin. Those fin-shaped feet make them supreme swimmers, and all pinnipeds are considered semi-aquatic marine mammals. This means they must spend some part of their lives on land or sea ice, usually during the mating and birthing seasons.

With so much time spent in water, some species like the elephant seals have evolved the ability to hold their breath for up to two hours and dive to depths of more than 6,500 feet looking for food.