As Ice-Free Periods Lengthen, Polar Bears May Face a Higher Risk of Starvation
Michelle Milliken
Ice-free periods are lengthening in the Arctic, with the annual freeze-up in fall happening nearly a month later than it was in 1979. That means that polar bears are forced to wait even longer before they can head back to the ice, where they’re best able to catch their primary food, the energy-rich seal. A new study shows that as polar bears spend more time on land with little access to seals, their risk of starvation may go up.
Research recently published in Nature Communications tracked the daily energy expenditure, diet, weight, and behavior of 20 polar bears in the western Hudson Bay region of Manitoba during three late summer weeks in the ice-free period. The bears were weighed before and after the study period, and their activity was tracked with collars equipped with GPS and cameras.
It’s thought by some that with more time on land, polar bears may act more like grizzlies and either hibernate during this time of limited food availability or adapt to eating more of a land diet. Bears were observed doing both, but all but one still lost weight, at an average of 2.2 pounds per day. The only one that didn’t drop weight had found a marine mammal carcass on land. This suggests that neither foraging nor hibernating are enough to help the animals survive.
Charles Robbins, study co-author and director of the Washington State University Bear Center, says, “Neither strategy will allow polar bears to exist on land beyond a certain amount of time. Even those bears that were foraging lost body weight at the same rate as those that laid down. Polar bears are not grizzly bears wearing white coats. They’re very, very different.”
During the study, the bears on opposite ends of the spectrum were found to have a five-fold difference in energy expenditure, between those that went into a hibernation-like state and those that moved around the most. Three of the more active bears went for long swims, and two of them found possible food in the water. One came across a beluga whale but spent very limited time trying to eat it, mostly using it as a buoy, while another found a seal and tried to bring it ashore but couldn’t. The researchers say this suggests they’re not very good at eating in the water.
Those that foraged ashore ate items including berries, other plants, birds, bones, antlers, and caribou carcasses. While these foods did provide energy to the animals, they expended more energy trying to get to them. In all, 70% of the bears had a higher daily energy expenditure than the average expected from hibernation. This indicates that they were more apt to forage to address limited food availability. This was also more common among younger bears with less experience with low-ice season. The researchers say their findings show that the risk of starvation may increase, especially for subadults, as their time onshore increases.
Anthony Pagano, lead author and research wildlife biologist at the U.S. Geological Survey Polar Bear Research Program, says, “As polar bears are forced on land earlier, it cuts into the period that they normally acquire the majority of the energy they need to survive. With increased land use, the expectation is that we’ll likely see increases in starvation, particularly with adolescents and females with cubs.”
To read more of the study, click here.
