Polar bears are the largest of all bear species, while the grizzlies that live on Alaska's North Slope are the smallest brown bears in that state - some no bigger than the black bears that try to break into garbage cans on the hillsides around Anchorage. So should the two ever encounter each other, the seal-eating denizens of Arctic ice might be expected to have the advantage, right? Actually, not so much. In fact, according to a recent study, not at all.
In an article for Alaska Dispatch News, Yereth Rosen spoke with Susanne Miller of the U.S. Fish and Wildlife Service, who explained that diminishing amounts of summer and fall sea ice in the southern Beaufort Sea off the North Slope have forced polar bears to spend more time ashore, grabbing whatever food they can on land. Among their targets are bowhead whale scraps left by Inupiat whalers on the shore by villages such as Kaktovik.
Miller and colleagues resolved to study the polar bears that visited these "bone piles" to see if they could ascertain any behavioral changes as a result of not being on the sea ice; but, she told Rosen, they soon found they had a problem: Grizzly bears got in the way.
"Brown bears just showed up and polar bears left," she said.
The North Slope is not an area of high brown bear concentration, and the grizzlies that do live there tend to be smaller than those farther south; conditions are much harsher and food rarer than in many other parts of the species' range, and the bears generally depend on plants and a smattering of mostly lean prey animals.
So a concentration of fatty whale remnants is a welcome indulgence that naturally attracts grizzlies in the area as it does polar bears. But because the arrival of the former frequently meant the departure of the latter, Miller and her colleagues switched the focus of their study to the interspecies interactions around the bone pile.
The scientists observed a total of 137 encounters between the two bear species, polar bears reacted submissively, even though the grizzlies did not obviously act aggressively toward them; in roughly 50 percent of the encounters, grizzlies displaced the polar bears completely, writes Rosen, even though, in Miller's words, "they look like they're about half the size of the polar bears."
The reason why can perhaps be determined from differences in the two species' behavior and ecology. Brown bears are naturally territorial, fiercely defending areas that have food and females from interlopers whose areas have less of either. Polar bears are not, given that they inhabit a constantly shifting mosaic of ice floes. Indeed, although polar bear males will occasionally cannibalize cubs, and older males will display dominance over younger ones should they converge at a kill, instances of intraspecific aggression in polar bears are rare - outside of mating season, when males will fight ferociously over females.
One of the study's co-authors, Richard Shideler of the Alaska Department of Fish and Game, told Rosen that, "I think it's attitude ... (Grizzlies are) more aggressive in terms of bear-bear interaction." Scent, however, may also play a role, with Miller noting that even a brown bear carcass on the bone pile was enough to spook some polar bears.
As well as being an intriguing insight into bear behavior, the study is particularly relevant given that decreasing sea ice means that polar bears are likely to spend greater time ashore along the North Slope and to come into greater contact with grizzlies, and perhaps into competition with them, in the future.