Climate change is lengthening the time that some parts of the Far North go without sea ice, which polar bears depend on to hunt their preferred prey: plump, calorie-rich seals. When the ice melts in summer, bears move ashore and face two options. They may rest and slow down to a state close to hibernation, or they may forage for alternative foods such as berries, bird eggs, and small terrestrial animals.
Scientists who tracked 20 polar bears in Manitoba, below the Arctic Circle at the southern end of the animals' range, found that which option the polar bears chose didn't make much difference. Foraging bears generally got enough calories from their small meals to replenish the energy they expended searching for them, but not enough to maintain their body mass.
“Terrestrial foods are not suitable for prolonging the period that polar bears can survive on land,” said Anthony Pagano, a wildlife biologist with the US Geological Survey and lead author of a study based on the research, published on Tuesday in Nature Communications.
In western Hudson Bay, the ice-free period is now three weeks longer than in the 1970s, and polar bears now spend about 130 days on land during the year. Scientists estimate that in the future there will be five to ten more days without sea ice each decade.
The question of whether polar bears can survive for longer periods on land has become politicized at times as the creatures became a symbol of climate change.
A 2015 assessment by the International Union for Conservation of Nature found a high probability that the global polar bear population will decline by more than 30 percent by 2050. This local population in Hudson Bay may already has been reduced by half, from an estimated 1,200 bears in the 1980s to about 600 bears in 2021.
Nearly all of the bears tracked in the new study lost weight, and two individuals were on track to starve to death before sea ice returned.
Anecdotal observations of individual polar bears eating ducks, geese, seabird eggs and even caribou on land have offered hope that the animals could adapt to a warmer world. But research that simply documents what polar bears eat hasn't been enough to determine whether the bears get enough calories from that food to help them survive longer periods without sea ice.
For this study, Dr. Pagano and his colleagues traveled to Wapusk National Park in northern Manitoba. Over three summers, they captured 20 polar bears and attached video cameras to their collars to give them bear-eye views of their days.
The scientists weighed the bears, took blood samples and measured their breathing to obtain detailed images of their body conditions, activity levels and energy expenditure. They recaptured each bear after about three weeks, retrieved the cameras, and repeated their measurements.
Putting cameras on polar bears is a new technique, and watching the video was “amazing,” Dr. Pagano said. ““Seeing what a polar bear actually does in the wild was really rewarding.”
Six of the bears (fewer than scientists expected) appeared to rest and fast, while the others foraged for food and some even swam long distances.
Foraging bears were seen primarily eating grass, seaweed, and berries, and occasionally bird carcasses, bones, caribou antlers, eggs, and small mammals. Two of the swimmers found carcasses of seals and beluga whales, but were unable to eat much while swimming in open water.
Regardless of whether the bears fasted or foraged, all but one lost similar amounts of weight. The scientists calculated an “expected starvation date” for each bear based on how much body fat and muscle it had, and how much energy it was estimated to expend each day.
Most were predicted to be fine until sea ice returned in November, but two young females, which tend to be the smallest polar bears, had predicted famine dates before then, and a few others were close to that time. (The researchers had to leave in September and don't know what ultimately happened to the bears.)
Dr. Pagano noted that the study did not include females with puppies, who burn much more energy while nursing. The researchers included some pregnant bears, but they left before giving birth.
These findings are “what we feared and what we hoped we wouldn't see,” but also in some ways expected, said Melanie Lancaster, a conservation biologist who specializes in Arctic species at the World Wildlife Fund.
Dr. Lancaster, who was not involved in the study, cautioned that these 20 bears only represent one population in one region. “Polar bears are not experiencing the effects of climate change uniformly across the Arctic,” he said. At higher latitudes, where thicker sea ice persists for several years, polar bears still do well.
But for this declining population in Hudson Bay, the individual variability the researchers found is significant, said Gregory Thiemann, an associate professor at York University in Toronto who studies Arctic carnivores but was not involved in this research.
Each polar bear tried to cope in their own way, but the variation shows that there is no winning solution. “It paints a collective picture that this is a group of bears at the edge of their biological limits,” he said.