Polar bears might avoid extinction by hunting land animals

Hudson Bay, that giant bite taken out of northeastern Canada, is projected to have fewer and fewer icy days as the planet grows warmer. By the time the ice-free season lasts for half the year – 6 months – between a quarter and half of all adult male polar bears are expected to die of starvation. That’s because, according to conventional wisdom, polar bears need sea ice to hunt newly born ringed seals.

Historically, polar bears have relied upon the nutrition they gain from hunting those seal pups to sustain them through the ice free summer until the bay again solidifies each autumn and they can get back to the hunt. If an expanded ice-free season means that polar bears won’t get enough to eat, they’ll be forced onto shore during the summer without enough fat layered between their bones and skin. They’ll be too weak to mate, and the polar bear population will decline.

But what if polar bears were more resilient than some have thought? What if they were shrewd enough, skilled enough, to hunt other animals during the ice-free summer? Could that allow them to offset the nutrition lost by being deprived of ice? A pair of zoologists from the American Museum of Natural History thinks so. Linda J. Gormezano and Robert F. Rockwell argue this week in the journal PLoS ONE that if polar bears are able to modify their dietary decisions and hunting tactics, they could find enough nutrition from terrestrial sources to offset the loss of baby seals.

There is some evidence that polar bears could hunt terrestrial animals and forage on terrestrial plants. For one thing, the bears are, at heart, opportunists. They’ll eat what they can, and there is evidence of that going back to some of the earlier natural history records for the species.

To figure out whether adult male polar bears could sustainably survive six months without ice, Gormezano and Rockwell first calculated how many prey items of each sex and age class a single adult male polar bear would have to eat over the course of that season to prevent starvation.

Second, they calculated how much potential nutritional energy is available to those bears from snow geese, eggs, and caribou, three terrestrial sources of food that polar bears have not historically incorporated into their diets in significant amounts.

Last, they combined those two estimates to infer how many adult males could be saved by the amount of added nutrition available, and whether it would be enough to maintain the species.

Adult male caribou provide the most bang for the buck, for example. A polar bear would need to eat just five of them during the summer, around one per month, to survive until seal pups were once again on the menu. Alternatively, a hungry bear could survive on one caribou calf every 8-9 days or so.

The researchers discovered that there are more than enough calories on land to sustain the hungry polar bears of the Hudson Bay, and the caribou and snow goose populations are strong enough to sustain the added pressure of polar bear predation. In fact, the caribou and goose populations have been increasing in recent years. That’s excellent news for a group of hungry bears, though it’s not clear how they’ll respond to future climate-related changes.

“If caribou herds continue to forage near the coast of Western Hudson Bay when bears come to shore earlier each year, they are likely to become a crucial component of the bears’ summertime diet,” Rockwell said in a prepared statement.

What the researchers did not explicitly account for was the energy required to hunt those animals. For goose eggs, the energetic cost is minimal. And it turns out that polar bears use the same low-energy ambush strategy for hunting caribou as for hunting their seals. That, too, is a hopeful sign that these terrestrial food sources could save the iconic white bears.

It’s hopeful news for a species that many have considered doomed by climate change, a group of animals racing towards extinction, but it’s not an entirely free pass. For one thing, the bears have to actually make it to land before the ice melts away each season. For another, their ability to consume enough terrestrial prey might depend on how quickly they make it to shore. Those who arrive too late might miss the caribou calving season, for example, or arrive after all the goslings have hatched from their eggs.

Still, these calculations suggest that, under the right conditions, polar bears just might be able to scrape by. – Jason G. Goldman | 11 September 2015

Gormezano LJ, Rockwell RF (2015) The Energetic Value of Land-Based Foods in Western Hudson Bay and Their Potential to Alleviate Energy Deficits of Starving Adult Male Polar Bears. PLoS ONE 10(6): e0128520. DOI: 10.1371/journal. pone.0128520.

Header image: A mother polar bear and her cub are near the coast of Western Hudson Bay. Copyright AMNH/R. Rockwell, used with permission.