Studying large carnivorous mammals is not without challenges, especially when they live in remote locations. Although footprints have been used before, the use of GPS collars in combination with cameras has given researchers the opportunity to learn more about how polar bears (Ursus maritimus) use land for hunting, and the impacts this will have on their health and population as sea ice continues to shrink.
Normally, polar bears hunt ringed in this region of Canada’s western Hudson Bay (Pusa hispida) and bearded seals (Erignathus barbatus) of the ice sheet, usually in late spring and early summer, when the seals are giving birth. As sea ice diminishes, polar bears are forced to spend more time hunting on land for various prey. Researchers studied 20 individual polar bears for three weeks each August and September from 2019 to 2022.
The Arctic marine ecosystem is experiencing not only loss of sea ice, but also reduced size and thickness, causing the bears to spend more time on land. Current research shows that the ice-free period in Hudson Bay increased by three weeks from 1979 to 2015, meaning that polar bears are on land for about 130 days.
Some predictions suggest that 24 percent of adult male Arctic animals would starve to death if that period were extended to 180 days.
“As polar bears are forced onto land earlier, the period during which they normally acquire most of the energy they need to survive is shortened,” said lead author Anthony Pagano, a wildlife biology researcher at the US Geological Survey Polar Bear Research Program and former researcher. Postdoctoral researcher from Washington State University, in a statement seen by IFLScience. “With increased land use, it is expected that we will likely see an increase in famine, especially among adolescents and women with cubs.”
The team wanted to know whether the polar bears reduced their energy consumption on land during this ice-free period. They measured energy expenditure, diet, behavior, activity, exercise speed, blood chemistry and body composition of eight adult females, five adult males, four subadult females and three subadult males.
The team discovered major differences in each of the polar bears. Adult and subadult females spent about 13 percent of their time feeding, mostly eating berries. Three individual bears spent 10 to 16 percent of their time swimming, and although two of the three found carcasses to feed on during these swimming periods, they each spent minimal time feeding them, despite the high energetic demands it swimming involves.
This suggests that the bears have difficulty feeding while in the water, and one bear was observed via CCTV trying to bring a seal carcass back to shore, but dropped it while swimming.
Recently, a polar bear in Alaska died after eating a bird carcass infected with bird flu.
A polar bear sleeps on land in Western Hudson Bay, with its collar camera attached.
Image credit: Anthony Pagano
“No strategy will allow polar bears to persist on land beyond a certain period of time. Even the bears that were foraging lost their body weight at the same rate as the bears that were lying down,” said Charles Robbins, director of the Washington State University Bear Center and co-author of the study.
Despite the different foraging and resting strategies, 19 of the 20 bears examined weight loss: between 8 and 36 kilograms (17-79 pounds) over the three-week period. On average, this indicated a weight loss of about 1 kilogram per day (2.2 pounds). Only one bear gained weight after finding a dead marine mammal carcass on land. This indicates that as sea ice continues to shrink and bears spend longer on land, they will be at greater risk of starvation.
This problem is not unique to Canada. In Norway, the image of a sleeping polar bear won an award and helped reduce sea ice around the world.
The research has been published in Nature Communications.