The common ancestor of pinnipeds diverged from terrestrial bears during the Miocene Period, about 20-25 million years ago. Today, these semiaquatic mammals are represented by three families: true seals, eared seals (sea lions) and walruses.
Once composed of numerous species inhabiting tropical, temperate and arctic life zones, the walrus family has two surviving members, the Atlantic and Pacific walruses; both are Arctic residents. The Atlantic walrus was once found as far south as Cape Cod but is now restricted to to the Arctic of northeastern Canada, Greenland and northwest Russia. The Pacific walrus, a bit larger and much more numerous, summers on and along the Chukchi and Beaufort Seas but winters south of the Bering Strait; male groups may remain in the more southern areas throughout the year.
Both species rely on ice flows, rocky islands and isolated beaches to "haul out" in order to rest, warm up or to give birth. Walruses breed in late winter and a single calf is born in spring of the following year (twins are very rare); a quarter of this prolonged gestation results from delayed implantation of the embryo. Females nurse their young for up to two years and are thus impregnated no more often than every 2-3 years. Though their life span may reach 50 years, this low birth rate makes the walrus population especially vulnerable to external forces such as hunting pressure and alterations in their habitat.
The warming of Earth's climate is sure to affect the welfare of these colonial creatures. Though hunting restrictions have led to a rebound in the world population (currently estimated at 250,000, of which 80% are Pacific walruses), global warming has already had an impact on Arctic ice formation and may adversely affect the bivalve mollusks on which these pinnipeds feed. In addition, a reduction of their haul-out options may make them more vulnerable to predators (killer whales, polar bears and humans), especially during the calving period. Of course, time will tell.
