Sea Otters and Polar Bears: Marine Fissipeds

Two Alaska marine mammals are neither pinniped nor cetacean: the polar bear and sea otter. They are both fissipeds, “split-footed” members of the order Carnivora, and are more closely related to terrestrial carnivores, like weasels, than seals or whales. Evolutionary newcomers to the marine environment, these species lack many of the physiologic adaptations to marine life seen in pinnipeds and cetaceans. Both species are considered marine mammals under U.S. laws because of the roles they play in the marine environment.

Polar bears, in the bear family (Ursidae), spend most of their lives associated with marine ice and waters. Although competent swimmers, they are the marine mammal least adapted to aquatic existence. They rest, mate, give birth, and suckle their young on the ice, and as such, are vulnerable to reductions in the extent and duration of sea ice.

Sea otters, in the weasel family (Mustelidae), live a primarily marine life: they rest, mate, give birth, and suckle their young in the water. Their hind limbs are webbed for swimming, but their front paws are padded with separate, clawed digits. They lack blubber, but are insulated by air trapped in their thick fur, which is densest among all mammals.

Taxonomic Relationships of Alaska Fissipeds

fissipedia taxonomic tree

Identifying Swimming Mammals

Some terrestrial (land-based) mammals can be confused with marine mammals when seen swimming in ocastal marine waters. Although their silhouette head profiles may be similar, the swimming behavior of terrestrial and marine mammals differs and may be used to distinguish the two groups.

Most terrestrial mammals like bears, deer, and moose rarely submerge, and their backs may be visible while they are swiming. Beavers, mink, and river otters may submerge momentarily but resurface within seconds. Although river otters swim and roll at the surface like sea otters, they never float on their backs. Continued observation of a swimming mammal's behavior may be required to distringuish terrestrial frommarine species.