Seals are divided into two families—the eared seals (the sea lions and fur seals), and the earless seals (also called true, or hair, seals). Eared seals have small, protruding earflaps on the side of the head. Earless seals have no earflaps. (All seals have internal ears.)

A seal's body tapers from the chest towards the tail, which is very short or nearly absent. The digits of a seal's four limbs are modified into flippers —the digits of the “hands” and “feet” are webbed and resemble paddles. Flippers are very useful in steering and maneuvering in water. Earless seals have fur-covered flippers; eared seals have flippers covered only with thick skin.

Seals are warm-blooded animals—that is, their body temperature remains constant regardless of the outside temperature. All seals have a layer of blubber under the skin that acts as an insulator against the cold. Sea lions, which are generally found in warmer climates than other seals, have little additional protection. Earless seals, on the other hand, have a circulatory system that limits the quantity of blood that flows through the skin and thus reduces the amount of body heat lost from the skin. The fur seals, in addition to blubber, have a two-layer covering of fur. The upper layer consists of coarse guard hairs; the dense lower layer is made up of short, fine hairs and acts as an additional insulator.

Seals have large, roundish eyes that in some species are huge in proportion to the size of the skull. The eyes have thick, spherical lenses that enable the seals to see well under water. Unlike most other mammals, seals do not have tear ducts. Thus when seals are observed on land they look as though they are crying because their tears run out of the eye onto the face. Seals have rather long whiskers around the mouth.

Adult seals weigh from about 130 pounds (60 kg) to more than 8,000 pounds (3,600 kg), depending on the species and individual. They range in length from less than 4 1/2 feet (1.4 m), for the Baikal seal, to 20 feet (6 m), for the southern elephant seal. The bulls (males) are usually larger than the cows (females), except in some species of earless seals. The lifespan of some species is up to 40 years.

In many species, the male has a thick layer of fur, resembling a mane, around the neck. Seals vary in color depending on the species, and sometimes members of the same species have different coloration. In general, they are brownish or grayish, often with various shadings, blotches, or stripes.

How Do Pinnipeds Measure Up?

Pinnipeds are all big animals, but they do vary in size. In length, pinnipeds can vary from about 4 feet (1.2 meters) to about 16 feet (4.9 meters). They may weigh from about 110 pounds (65 kilograms) to about 8,800 pounds (4,000 kilograms).

Ringed seals, which look plump, are among the smallest pinnipeds. Cape (South African) fur seals are longer and heavier than many kinds of fur seals that live in other places. They are the largest fur seals.

Elephant seals are the only pinnipeds that are longer and heavier than walruses. Elephant seals are the largest pinnipeds in the world. Southern elephant seals live near Antarctica. They are even larger than Northern elephant seals, which live near the southwestern United States and Mexico.

Adult male pinnipeds are usually larger than adult females.