Polar bears are at risk of starvation in longer ice-free seasons in the Arctic, despite their ability to adapt their diet and behaviour, scientists have warned.
Climate change is increasing the time some areas of the Arctic are ice-free each year, forcing polar bears – which are specialised to use the ice to hunt their preferred food of seals – to spend longer on land in those regions.
Polar bears monitored by scientists in a new study tried different strategies to maintain energy reserves on land, including resting, scavenging and foraging, but almost all lost weight rapidly.
The scientists warned the bears, particularly youngsters, were at increased risk of starvation with the greater amount of time they had to spend on land.
The study, led by researchers from Washington State University and US Geological Survey, monitored 20 polar bears in the western Hudson Bay region of Canada, onshore over several weeks in August to September between 2019 and 2022.
Hudson Bay has seen the ice-free period increase by three weeks from 1975 to 2015, reducing the amount of time polar bears can spend on the ice, where they catch and eat seals to bolster their energy reserves.
Polar bears are now spending around 130 days on land near Hudson Bay, whereas they used to spend 100 to 110 days onshore in the area.
The individual polar bears were weighed and assessed for their energy expenditure before and after the study period and fitted with GPS tags and cameras.
The assessments revealed they had a wide range of strategies on land, from hibernation-like resting to conserve energy, to actively foraging for food on land and even swimming tens of kilometres.
Bears consumed berries, vegetation, birds, bones, antlers, seal and beluga whale, but there was very little benefit from foraging, as 19 of the 20 animals lost weight during the period, dropping an average of 1kg (2.2lbs) a day.
Only one increased its weight, after likely finding a marine mammal such as a seal or beluga carcass on land, the researchers said.
Two female bears were predicted to starve to death before the average November 30 freeze-up of the Hudson Bay, the study found.
Lead author Anthony Pagano, research wildlife biologist with the US Geological Survey polar bear research programme, said: “We found a real diversity of bear behaviours, and as a result, we saw a diverse range of energy expenditures.
“The terrestrial foods did give them some energetic benefit, but ultimately, the bears had to spend more energy to access those resources.
“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,” he said.
He added: “Our results suggest that polar bears cannot alter their behaviour or energetics in ways that can prevent weight loss when summering on land and that, in most cases, the resources available on land are insufficient to counteract weight loss.”
This means suggestions the Arctic’s top predators could act like their grizzly bear cousins by resting or foraging for food on land are unlikely to work.
Charles Robbins, director of the Washington State University Bear Centre and co-author of the study, said neither strategy of resting to conserve energy or foraging would 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,” he said.
The paper was published in the journal Nature Communications.