The School: Polar Bear Adaptations
A weekly dose of education in the ocean.
Polar bears (Ursus maritimus) are the Arctic’s apex predator, and the largest four-legged predator in the world. Unlike other bears, they are marine mammals, and have adapted to thrive in one of our planet’s most extreme environments, the Arctic Ocean.
Polar bears are the newest of the six bear species that exist today, arising about 150,000 years ago, when grizzly bears living in Siberia and Alaska adapted to living in and around the Arctic Ocean. Because bears that live on sea ice don’t leave many remains (carcasses have a tendency to sink to the bottom of the ocean), and top predators are much less abundant than species at lower trophic levels, piecing together a polar bear fossil record is tough, according to Dr. Andrew Derocher, professor of biology at University of Alberta, and Chair of the IUCN’s Polar Bear Specialist Group.
Grizzly bears (Ursus arctos) are polar bears’ closest relatives. Photo by Steve Hillebrand/US Fish and Wildlife Service, via Wikimedia Commons, Public Domain.
Varying hypotheses have emerged on when the bears differentiated from grizzlies – anywhere from 70,000 to 250,000 years ago. The oldest polar bear fossil is 130,000 years old, and was found in Svalbard, well known for its present day polar bear population. One hypothesis posits that the polar bears differentiated during an exceptionally warm period in earth’s history, about 134,000 years ago, when the bears would have followed receding ice northward, causing them to separate from grizzlies. According to Derocher, “an ancestor common with bears also gave rise to the seals, walrus, and sea lions about 26 million years ago.”
Polar bears are thought to have arisen about 150,000 years ago, from Siberian and Alaskan grizzly bears. Derocher explains how this likely happened in his recently published book Polar Bears, A Complete Guide to Their Biology and Behavior.
“It is easy to imagine grizzly bears on land expanding into an area where the climate was a bit cooler or perhaps a cooling period occurred. Sea ice along shore is difficult to distinguish from land. A hungry grizzly coming out of its winter den would find marine mammal carrion a welcome food source. Eventually some grizzlies started wandering a bit farther onto the sea ice. Ringed seals, which are an older species at about 5 million years old, might have been abundant in the nearshore ice. Given that seals had not seen any predators in their recent evolutionary history, they were probably incredibly naïve. Antarctic seals have no natural predators. You can walk right up to them without their paying much attention to you. Grizzlies that moved onto the ice likely found an abundant early spring food source in these seals, and bears that did it more often possibly had more success rearing young. Presumably, lighter colored bears fared better at catching seals until white bears became the norm.”
(Copyright 2012, The Johns Hopkins University Press. Reprinted by permission of the author and the publisher).
The white bears may have stayed out there, and gradually become separated from the other grizzlies who remained terrestrial omnivores that were best suited to temperate climates. Over generations the seal-hunting bears adapted to being on the ice, and a whole new type of bear emerged: Ursus maritimus, the polar bear, a marine mammal and pure carnivore.
In support of this hypothesis, grizzlies in the Northwest Territories of Canada can still be observed wandering out onto the sea ice to hunt seals and scavenge carcasses of marine mammals.
Polar bears developed highly specialized characteristics for extreme cold, for an aquatic environment and for hunting needs specific to the Arctic.
In the next three posts, we will explore each of these in more detail.