Blueberries, plants and a few animal carcasses: this is the polar bear’s “sea ice free” diet. When sea ice is slow to form, the King of the Arctic has to find calories on land, or rest to avoid energy expenditure. But whatever strategy is adopted, it is not enough to prevent weight loss and avert the risk of starvation. This is the finding of a study published on February 13 in Nature Communications.
The difficulty polar bears have in feeding themselves as the sea ice disappears is a subject that has been discussed on several occasions. We’ve all seen images of these polar plantigrades rummaging through rubbish dumps, creating a complicated cohabitation with humans and a dangerous one for the bears. The same goes for the images of gaunt bears looking for food. And yet, while it was known that the loss of sea ice was synonymous with a diet for these animals, the eating habits of this symbolic Arctic mammal when on land were a mystery.
To shed light on the subject, a team of American and Canadian scientists conducted a study on the habits of 20 bears, equipping them with GPS and cameras. And the results aren’t very encouraging, revealing a rather meagre diet for an animal that should be ingesting over 10,000 calories a day (20,000 for large males).
Resting and reducing activity to conserve energy, or trying to find food on land or at sea, the studied polar bears all tried different strategies. To no avail. Of the twenty animals observed, nineteen lost weight at a rate of 0.4 to 1.7 kilos per day.
Researchers measured changes in daily energy expenditure (DEE), diet, behavior, movement and body composition on 20 polar bears. Conducted in Wapusk National Park, west of Hudson Bay in the Manitoba region of Canada, the study covered three-week periods from August to September, between 2019 and 2022.
The studied sample consisted of 13 adult bears (8 females without cubs and 5 males) and 7 subadult bears, aged between 2 and 4 years (4 females and 3 males). The animals weighed between 147 and 566 kg, and total energy expenditure over the study period ranged from 75,000 to 467,000 kcal.
The results of the study show that the bears adopted different behaviors, some minimizing their movements, sometimes drastically as if they were in hibernation, others spending their energy, on land as if they were on ice, to find food. Three of the subjects even swam distances ranging from 54 to 175 km. Using GPS, the scientists also noticed that the subjects moved variably, from 15 to 329 km depending on the individual. Females were also found to move more than males, as they spent more time feeding.
A varied but insufficient diet
During the observation period, the polar bears ate berries, vegetation, birds, bones, seals and belugas. The animals consumed were carcasses, not catches. A real slimming diet: nineteen bears lost between 4% and 11% of their body mass in just three weeks.
Only one bear increased his body mass during the study period, gaining 32 kg in less than a month. An increase in mass similar to that of polar bears feeding on ringed seals on sea ice. Scientists believe that the bear had probably fed on the carcass of a large marine mammal that it had found on land. However, this windfall is likely to be the exception rather than the rule: “Such opportunistic events have been hypothesized to have enabled polar bears to survive past interglacial periods but are predicted to be less of a resource during the current Anthropocene due to lower abundances of whale populations.”, notes Anthony M. Pagano, a research biologist with the US Geological Survey Alaska Science Center and lead author of the study.
In addition, video recordings suggest that polar bears, despite their status as marine mammals, may have difficulty feeding in the water. On two occasions, female bears were filmed close to a submerged carcass of a beluga whale or a seal, but failed to feed on it. “The subadult female found a beluga carcass and was only observed feeding on it for 35 s over the 6 h that she was periodically observed near it.”, the authors note. “Instead, she appeared to use the carcass more as a buoy to rest upon.” The same goes for an adult female who came across a seal carcass that she was unable to bring to shore.
While other bear species have diversified their diet to the point of becoming omnivores, polar bears have specialized in hunting seals, particularly ringed and bearded seals, which live on sea ice. “Polar bears acquire the majority of their energy resources during a brief period in the late spring and early summer when seals are giving birth to and weaning their pups. Climate warming is increasing the duration that some areas of the Arctic are ice free, which in turn forces polar bears in these regions to move to land.”, explain the researchers.
For a long time, it was thought that polar bears could adapt on land by diversifying their diet. This study shows the opposite. Polar bears need sea ice, and without it, the chances of these animals disappearing are far greater than those of them evolving and surviving on an omnivorous diet.
Mirjana Binggeli, PolarJournal
Featured image: Michael Wenger
More on the subject
