Polar bears obviously have big appetites. Any mammal that can outweigh a grand piano must be a hearty eater, especially in the Arctic. But these huge carnivores require even more food than we thought, scientists report in a new study — and that doesn't bode well for their ability to cope with dwindling Arctic sea ice.
The plight of polar bears is well-known, as is the reason behind their misfortune. Climates around the world are growing warmer with unusual speed, fueled by greenhouse gases from human activities, and the Arctic is heating up twice as quickly as the rest of the planet. That's causing a dramatic decline of Arctic sea ice, which is shrinking at a rate of 13.2 percent per decade, according to NASA.
Polar bears are good swimmers, but their bodies aren't built for aquatic pursuit, so they get most of their food by ambushing seals from sea ice. Less ice means fewer places to hunt, and thus fewer chances to eat. The decline of Arctic sea ice has coincided with falling polar bear populations in parts of their range — around the Beaufort Sea, for example, the U.S. Geological Survey (USGS) estimates polar bear populations have dropped by about 40 percent in the last decade alone.
Shrinking ice, shrinking bears
For the new study, published in the journal Science, researchers looked at the physiology behind polar bear woes. They focused on adult females without cubs, monitoring the bears' behavior, hunting success and metabolic rates during spring hunts in the Beaufort Sea. (The bears wore collars that recorded video, locations and activity levels, while metabolic tracers revealed how much energy each bear used.)
"We've been documenting declines in polar bear survival rates, body condition and population numbers over the past decade," says first author Anthony Pagano, a Ph.D. candidate at the University of California-Santa Cruz, in a statement. "This study identifies the mechanisms that are driving those declines by looking at the actual energy needs of polar bears and how often they're able to catch seals."
It turns out being a polar bear requires more energy than previously thought. The bears' metabolic rates averaged more than 50 percent higher than previous studies had predicted, the researchers report. On top of that, more than half of the bears lost at least 10 percent of their body mass during the study, which means they weren't catching enough fatty prey to satisfy their bodies' energy demands.
And this happened at a critical time of year, Pagano points out: "This was at the start of the period from April through July, when polar bears catch most of their prey and put on most of the body fat they need to sustain them throughout the year."
Spring is a good time to hunt because there's more sea ice, which naturally retreats every summer and fall before slowly returning in winter. But it's also when polar bears can hunt young ringed seals that have recently weaned, and are thus easier to catch. By fall, Pagano explains, the seals are older, wiser and wilier.
"It's thought that bears might catch a couple per month in the fall," he says, "compared to five to 10 per month in the spring and early summer."
'They need to be catching a lot of seals'
Earlier studies had tried to estimate polar bears' metabolic rates and energy needs, the study's authors note, but relied largely on speculation. Because polar bears are mainly ambush predators, for example, they often seem to require minimal energy for catching food. And even when a bear is in a hunting slump, some researchers have speculated it can conserve energy by turning down its metabolic rate.
"We found that polar bears actually have much higher energy demands than predicted," Pagano says. "They need to be catching a lot of seals."
By some estimates, the ongoing loss of Arctic sea ice could drive polar bears extinct by 2100. Stopping that will require a much broader effort to slow down climate change, but in the meantime, Pagano says, new methods of studying polar bears in the wild are helping us understand these iconic creatures like never before. And only by learning how they work can we hope to help them survive.
"We now have the technology to learn how they are moving on the ice, their activity patterns, and their energy needs," he says, "so we can better understand the implications of these changes we are seeing in the sea ice."