Polar bears are a fitting avatar for the dire consequences of climate change, living right at the heart of where the Earth has been warming the fastest. Now, two new studies add to a swelling pile of evidence that polar bears aren’t handling their rapidly changing environment very well. The new research suggests that polar bears are finding it harder to catch enough seals to meet their basic energy demands, and that their territories are contracting, isolating the bears as sea ice retreats to the north.
The lifestyle of polar bears is quite different from that of other bear species. They travel over vast, icy home ranges in search of food, largely by swimming. All this athleticism hinges on that food. Not keeping the metabolic fires burning could be disastrous for a polar bear’s survival prospects. And as sea ice—which is disappearing at a rate of 14 percent per decade—continues to wane, the fear has been that polar bears might lose access to the seal prey they depend on.
So a team of researchers investigated how the bears were spending their energy out on the ice, where they spend most of their time.
“We started this study to get a much better understanding of the basic biology of these animals,” said lead author Anthony Pagano, a wildlife biologist with the USGS who conducted this study as part of his PhD thesis at UC Santa Cruz.
The hope was to use that information to determine how many seals the bears need to eat, and how changing ice conditions may influence their energy demands.
“We’ve been using satellite transmitters on polar bears dating back to the late 1980s, so we know a lot about their movement patterns,” Pagano told Earther. “But what we don’t know is what they’re doing in those locations.”
Because polar bears hang out in remote, often inaccessible areas of sea ice, direct observations of how they spend their time and energy have been very difficult. Thankfully, modern technology can provide a perspective on these majestic carnivores that wasn’t possible decades ago.
The research team tracked the activity, hunting behavior, and metabolic rates of nine female polar bears in their field site on the Beaufort Sea, along Alaska’s far northern coastline. They did this over an eight to eleven day period for each of several spring seasons. Bears had their metabolic rate calculated via doubly labeled water—a method that involved immobilizing the animals, and injecting them with a solution that had a known quantity of a specific oxygen isotope When the bears were recaptured after the study period, the researchers took a blood sample to determine how much of the isotope had been expelled in CO2. The less that was left, the higher the metabolic rate.
The polar bears were then fitted with a tracking collar outfitted with accelerometers and a video camera—a device that’s basically like a giant FitBit with a GoPro and GPS slapped on it. This system allowed the researchers to see everywhere a bear went, everything it did, and how much energy it spent for about ten days.
Their results, published today in the journal Science, show that polar bears need lots of high-fat prey to keep up with a surprisingly high metabolic rate—more than 50 percent higher than previous estimates—and that, depressingly, many bears are running on fumes.
Five out of the nine bears lost body mass over the duration of the study, with video data showing that prey was harder and harder to come by. Bears recorded eating freshly killed seals gained mass, while those seen scavenging on seal or whale carcasses lost mass. Most dropped more than 10 percent of their weight, or about 18 to 20 kilograms (40 to 44 pounds), over the ten-ish day period the bears were being tracked out on the ice.
“That’s a pretty amazing amount of mass to lose over a ten day period. It really reinforces the high energy demand these animals have and their reliance on the ability to catch energy-dense seals,” said Pagano.
The degree to which the bears are struggling is particularly troubling considering that this study took place in the spring, when polar bears tend to catch most of their prey and put on the bulk of their fat for the year.
Though Beaufort Sea polar bears are already operating on razor thin margins, current sea ice trends—ice is becoming thinner, and breaking up and retreating earlier in the spring—are set to make things even harder. This dovetails with another recent study’s findings.
Although a recent study predicted polar bears will drop by a third in number in the next few decades as a result of warming, it still isn’t well-known how waning sea ice will affect how polar bear populations move, how they interact with other groups of bears, and how they access prey and suitable habitat. Changes in the availability of habitat and routes to mating partners can just as easily screw with polar bears at the population scale as food deficits do to individual bears. So, in Canada’s Baffin Bay, researchers used a combination of satellite telemetry, data from marked, individual bears, and genetic data to track changes in the bears’ home range size, their seasonal movements, and breeding connections to neighboring populations over 25 years.
Their findings were published in the journal Ecology and Evolution last month, and show that bears in Baffin Bay are becoming increasingly isolated from other populations, with shrinking home territories.
The researchers found that summertime range area contracted by 70 percent between the 1990s and 2000s, and that in the 2000s, bears were less likely to leave Baffin Bay. Over the decades, their overall range shifted several degrees latitude northward (even more so in the summer months), evidently following the northward-receding sea ice.
The local bears also became more genetically differentiated from their neighbors—the result of less cross-breeding, in turn the result of sea ice corridors between populations vanishing earlier in the year. Isolation increases the risk of local extinction, because without bears moving in and out of the area, population-level crises like inbreeding and disease become more prevalent.
These multiple lines of evidence point to a population of apex predators being pushed into an untenable corner, potentially giving us a preview of what may befall polar bears in other Arctic regions as sea ice disintegrates. Polar bears face hunger, exertion, shriveling hunting grounds, and more limited breeding opportunities. It’s not known how many can weather the changes that are already here, but if these two recent studies are any indication, that number may be much smaller than we want to admit.