Researchers tracked 20 polar bears with GPS and fitted cameras to their necks to observe their activities, and found that despite a varied diet of eggs, berries, antlers and beluga bones, the polar bears who live along the western Hudson Bay coast and come ashore when sea ice is absent increasingly struggle to sustain themselves.
Eggs, berries, antlers and beluga bones – polar bears that spend time on land may dine from a remarkably varied menu. But, as scientists discovered after poring through hours of video footage taken from the bears’ own perspective, it is rarely enough to satisfy.
The results of the study, published Tuesday in the journal Nature Communications, suggest that polar bears that live along the western Hudson Bay coast and come ashore when sea ice is absent are increasingly hard-pressed to sustain themselves.
While time spent on land is an annual occurrence for polar bears in the region, longer summers caused by climate change are delaying their return to the ice every fall. Only then, when the ice reforms, can the bears resume hunting for seals, their primary food source during the rest of the year.
Anthony Pagano, a biologist with the U.S. Geological Survey, and his colleagues conceived an elaborate scheme for tracking the movements of 20 individual bears together with their calorie intake.Anthony Pagano/Supplied
Previous fieldwork has long shown that polar bears can exploit a broad range of resources while on land. But just how much the terrestrial diet can serve as a substitute has been a matter of scientific debate.
The new study, conducted in northern Manitoba by a team of U.S. and Canadian scientists, shows with unprecedented detail that the bears’ summer sojourn is far from easy living.
“The bears we were monitoring were losing one kilogram per day on average,” said Anthony Pagano, a biologist with the U.S. Geological Survey based in Alaska and the study’s lead author. “It’s kind of a ticking clock – how long they can survive while losing that much weight.”
To better understand how well polar bears get by on land, Dr. Pagano and his colleagues tracked the movements of 20 individual bears together with their calorie intake. Since it would normally be impossible to closely follow and test such large predators over time, the plan called for anesthetizing the bears and outfitting them with video collars – small cameras that hang like a pendant behind each bear’s chin, providing a close and occasionally bloody view of every meal.
Video collars provided a close view of every meal the polar bears consumed.Joy Erlenbach/Supplied
“Our motivation was to quantify what the bears are doing,” Dr. Pagano said. “How much energy they’re spending while they’re on land relative to how much energy they might be gaining from anything that they eat.”
The study was conducted over three seasons between 2019 and 2022. The researchers followed eight males and 12 females of varying ages for about 20 days each. During that time, the video collars transmitted footage in five-second bursts every two minutes, providing ample views of what the bears were eating.
While the videos include many hours of bears at rest, conserving energy, there are also scenes of foraging, hunting small animals or scavenging along the shoreline as well as bears meeting and engaging in play.
“Some of it is just amazing,” Dr. Pagano said.
The collars transmitted GPS co-ordinates allowing scientists to map the bears’ movements. In one case, a younger female travelled 330 kilometres during the study, including an extended period swimming on the open water during which she used a beluga whale carcass as a buoy overnight before heading west and returning to shore at Nunavut. At the other end of the spectrum, a male bear spent nearly all of his time at rest and moved only 15 kilometres.
The study showed it didn’t matter which survival strategy the bears employed: Nearly all ended up thinner at the end of the observation window than they were at the beginning. Those that moved less found less to eat. Those that moved more used up more calories in their quest for nourishment.
“They were basically compensating for the energy they were spending, but they weren’t getting any benefit from it,” Dr. Pagano said.
To quantify this, Dr. Pagano and his colleagues included accelerometers with the video collars to record total body motion. This information was compared with metabolic data showing how much total energy the bears expended based on carbon dioxide production, which was obtained using isotopes administered to the bears when they were outfitted with their collars and then measured when they were recaptured.
Over all, 19 of the bears lost between 5 per cent and 11 per cent of their total body mass. Bears that were younger also appear to fare less well. The one exception was a male that gained 32 kilograms. Because that bear’s collar stopped transmitting six days before recapture scientists did not see its final meals, but blood samples suggest it likely feasted on the fat-rich carcass of a whale or seal that had washed up on shore shortly before it was recaptured.
Currently, the bears spend about 130 days on land during the summer, roughly three weeks longer than they would have in the 1980s. Previous estimates suggest that adult male bears could reach starvation levels by the time climate change pushes their time on land up to 180 days, Dr. Pagano said.
“Our hope is that the data we collected through this study will help to evaluate some of those estimates,” he added.
Jane Waterman, a behavioural ecologist at the University of Manitoba in Winnipeg who was not involved in the study, said the results show the toll the summer can take on sub-adult bears that are growing and need to maintain muscle mass.
While the main limitation of the study is its small sample size, Dr. Waterman praised the rich information the researchers were able to obtain on each individual polar bear – a species that is among the most challenging to study in the wild.
“Here you’ve got detailed data on exactly what they’re doing,” she said. “That, to me, is phenomenal.”
A new study by U.S. and Canadian researchers tracked the movements and caloric intake of 20 polar bears using GPS and video collars. The footage revealed just how difficult ice-free months can be for polar bears on the western coast of Hudson Bay.