Monday, May 20

Is that polar bear eating enough? Try a collar with a camera.

Climate change is lengthening the period of time that parts of the Far North remain free of sea ice, which polar bears rely on to hunt their favorite prey: fat, calorie-rich seals. When the ice melts in the summer, bears move onto land and are faced with two options. They may rest and slow down until they reach a state close to hibernation, or they may search for alternative food such as berries, bird eggs and small land animals.

Scientists tracking 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 only enough calories from their small meals to replenish the energy expended finding them, but not enough to maintain body mass.

“Terrestrial foods are not adequate to extend the period that polar bears can survive on land,” said Anthony Pagano, a wildlife biologist at the U.S. Geological Survey and lead author of a research-based study, published Tuesday in Nature Communications.

In western Hudson Bay, the ice-free period is three weeks longer than in the 1970s, and polar bears currently spend about 130 days on land during the year. Scientists estimate that, going forward, there will be five to 10 more days without sea ice every decade.

The question of whether polar bears can survive for longer periods on land has sometimes been politicized as the creatures have become 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 have already shrunk by half, from around 1,200 bears in the 1980s to around 600 bears in 2021.

Nearly all of the bears tracked in the new study lost weight, and two individuals were on the verge of starvation before the 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 can adapt to a warmer world. But research simply documenting what polar bears eat hasn’t been enough to understand whether bears get enough calories from that food to help them survive longer periods without sea ice.

For this study, Dr. Pagano and colleagues traveled to Wapusk National Park in northern Manitoba. Over the course of three summers, they captured 20 polar bears and attached cameras to their collars to provide a birds-eye view of their days.

The scientists weighed the bears, took blood samples and measured their breathing to paint a detailed picture of their body condition, activity levels and energy expenditure. They filmed each bear after about three weeks, retrieving the cameras and repeating the measurements.

Putting cameras on polar bears is a new technique and watching the video was “amazing,” Dr. Pagano said. Watching what a polar bear actually does in the wild was really rewarding.”

Six of the bears (fewer than scientists expected) appeared to be resting and fasting, while the others were feeding and some even swam long distances.

Foraging bears have been seen primarily eating grass, algae, and berries, with occasional bird carcasses, bones, caribou antlers, eggs, and small mammals. Two of the swimmers found carcasses of seals and beluga whales, but they couldn’t eat much while swimming in open water.

Regardless of whether the bears were fasting or foraging, all but one lost similar amounts of weight. The scientists calculated an “expected starvation date” for each bear based on the amount of body fat and muscle he had and the estimated amount of energy he would consume each day.

Most were expected to be fine until the sea ice returned in November, but two young females, which tend to be the smallest polar bears, had predicted starvation dates before then, and a few others were close to that period. (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 cubs, 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 not to see,” but also somewhat expected, said Melanie Lancaster, a conservation biologist specializing in Arctic species at the World Wildlife Fund.

Dr Lancaster, who was not involved in the study, cautioned that these 20 bears represent only 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 many years, polar bears still fare well.

But for this declining population in Hudson Bay, the individual variability 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 has tried to do it in their own way, but the variation proves that there is no one-size-fits-all solution. “It paints a collective picture that this is a group of bears that have reached their biological limits,” she said.