The Arctic is rapidly losing sea ice due to the climate crisis and is expected to disappear completely by the summer of the 2030s. With this in mind, an international research team led by the Alaska Science Center has been studying how polar bears (Ursus maritimus) behave in the face of this thaw.
To do this, they used collars with video cameras and GPS to track 20 polar bears during the Arctic summer season – August to September – between 2019 and 2022 in western Hudson Bay, Canada. The ice-free period in this region increased by three weeks between 1979 and 2015, allowing bears to stay on land for about 130 days over the past decade.
The authors monitored their daily energy expenditure, changes in body mass, diet, behavior and exercise. Their goal was to find out what these ice-top predators ate and did during the long periods of time they spent on land when their preferred prey, seals, were out of reach.
We were able to document their behavior, their weight changes, their energy costs and their movements and thus determined that the bears exhibited different behaviors and strategies.
Karyn Rode, researcher at the Alaska Science Center
“We were able to document their behavior, their weight changes, their energy costs and their movements and determined that the bears exhibited different behaviors and energy strategies on land in the summer, but these did not prevent weight loss.” 19 of the 20 bears in our study lost body mass,” Karyn Rode, a researcher at the Alaska Science Center and co-author of the study published in the journal Nature Communications, tells SINC.
These tactics to expend less energy include fasting, restricting exercise, and eating berries and birds. All were conducted regardless of age, sex, reproductive stage – pregnant females were also observed – or initial fat content. However, they could not prevent themselves from losing an average of one kilogram per day. Even those who foraged lost weight just as quickly as those who lay down. Only one of them gained weight after stumbling across a dead sea mammal.
“Our results support previous studies that indicate that polar bears lose more and more weight during longer summer stays on land,” emphasizes the researcher.
Support conservation efforts
As sea ice continues to decline, understanding these adaptive behaviors is critical to conservation efforts aimed at supporting polar bears in a rapidly changing ecosystem.
Between late spring and early summer, polar bears use the sea ice as a platform to hunt seals, especially when they are giving birth and weaning their pups.
There are some populations – of the 19 known throughout the circumpolar Arctic – that have access to the carcasses of marine mammals such as whales on land, which would help them offset weight loss. But in Hudson Bay, bears currently lack such marine resources on land and are likely to be negatively impacted by extended ice-free periods.
Terrestrial foods provided them with some energetic benefit, but ultimately they had to expend more energy to access these resources
Anthony Pagano, wildlife research biologist
“Environment and Climate Change Canada conducted a long-term research program that helped us closely track these animals for about three weeks while they were on land,” explains the scientist.
Anthony Pagano, a wildlife expert with the US Geological Survey’s Polar Bear Research Program, notes that “terrestrial food provided them with some energetic benefit, but ultimately they had to expend more energy to obtain these resources.”
The study required the collaboration of multiple organizations and agencies across international borders to successfully collect the data needed to better understand the behavior and energy of polar bears as they spend the summer on land.
“With increasing land use, it is expected that hunger will increase, particularly among adolescents and females with young,” the expert concludes.
Reference:
Anthony M. Pagano et al. “Energy and behavioral strategies of polar bears on land with implications for survival during the ice-free period.” Nature communication.