Noisy and gregarious on the beach, northern sea lions (Eumetopias jubatus) are found in cold coastal waters of the North Pacific. They gather between May and July in large breeding groups on remote shores, such as those of the Aleutian Islands and the Pribilof Islands, depicted here. Males fiercely defend their harems, which can include 10 to 30 females.

Unlike most other pinnipeds, or fin-footed marine mammals, northern sea lions wean their pups slowly, sometimes nursing as long as two years. Mothers teach pups to dive and hunt, and adults often gather in cooperative groups to attack large schools of fish.

On all fours? As if sauntering down the beach, a sea lion tucks its hind fins underneath its body to get around. True seals wriggle on their bellies to maneuver on land. © Lewis S. Trusty / Animals Animals

Seal or sea lion?
They look like seals, but sea lions actually belong to a different pinniped family.

Flippers and ears are the most obvious external features that separate true seals (phocids) and sea lions (otariids). Both groups are pinnipeds, having flippers for feet. A true seal's fins have fur on both top and bottom, and they always point backwards. In contrast, the undersides of a sea lion's flippers are bare for traction on land, and they can tuck into a forward-pointing position.

CONSERVATION: Mysterious Decline
Soon after large commercial fishing vessels began trawling the North Pacific in the 1960s, the northern sea lion population rapidly declined. By the late 1990s more than 80 percent of the sea lions had disappeared. Some experts suspected that the fishing boats were competing with sea lions for fish; others said the animals were the unintended victims of trawl nets.

Common roots
Pinnipeds are more closely related to land-based carnivores than to other marine mammals.

Sea lions, together with walruses and true seals, are pinnipeds, a Latin term that refers to their finlike feet. Pinnipeds evolved from terrestrial carnivores at least 30 million years ago, when their first recognizable fossils appear in marine sediments. Among these land-based relatives, a group of doglike mammals that includes bears, raccoons, and weasels is most closely allied with pinnipeds.

Although they are designed for life in water, pinnipeds have retained some of the characteristics of their terrestrial ancestors. Seals and sea lions, for example, haul out on land or ice to give birth, breed and shed their fine hair. These traits distinguish pinnipeds from other marine mammals such as whales and manatees, which live out their lives entirely in water.