By Roberta Attanasio, Blog Editor
It’s celebration time for the iconic polar bear, the poster child of climate change. Or better, it’s a different type of celebration. Every year, February 27—International Polar Bear Day—highlights the challenges that this big, charismatic creature faces in a warming Arctic, and becomes a day of action to reduce carbon emissions. The nineteen subpopulations of polar bears spend their winter on sheets of frozen ocean water, which melt and retreat in warmer months, to then advance again in the fall. As ice keeps melting because of higher temperatures, their natural habitat shrinks, affecting their capacity to travel, hunt, and breed. In other words, their lives are tied to the annual spread and shrinkage of Arctic sea ice.
Last December, heartbreaking images of an apparently starving polar bear—visibly emaciated—captivated public attention, and raised awareness of climate change’s effects on polar bears’ foraging ability. Polar bears, the top predators in the Arctic marine ecosystem, eat mostly ringed seals—full of fat and high in calories. Seals poke breathing holes in thin ice, and then surface at different intervals of time to breathe. Bears stalk the seals, waiting at the holes to attack them once they surface.
Seals depend on ice, and when the ice retreats, polar bears follow the ice, traveling long distances to keep their food source handy. Despite the effort, the supply of their favorite and most nourishing food eventually dwindles and then disappears during the warmer season. Therefore, polar bears build most of their fat reserves during a mere few months, which allows them to survive during the lean period. The no-food season lasts typically 3 or 4 months. However, global warming is delaying formation of the ice pack, leading the bears to wait longer for their seals—seals are not available in ice-free conditions. Andrew Derocher, a polar bear scientist at the University of Alberta in Edmonton, told onEarth that while a few bears may be able to scrape out a living while eating a less than optimal number of seals, down at the genetic level the species is just too dependent on the high-fat marine mammal diet.
Interestingly, most information currently available on the behavior of polar bears on sea ice derives from findings obtained more than 40 years ago by Ian Stirling, a Canadian scientist, combined with local traditional knowledge from Arctic indigenous peoples. Stirling collected his data by observing the bears at distance, using a spotting scope positioned on a cliff. However, results from a recent study that relied on technological advancements provides unique insight on a variety of polar bears’ behaviors. It also highlights the challenges polar bears face because of increasingly longer ice-free seasons. The study was designed to measure the energy expenditure of the bears, and then link the measurements with the behavior of the bears on the sea ice.
Researchers monitored nine female free-ranging polar bears over 2 years off Alaska in the Beaufort Sea, where there are demonstrable declines in abundance, survival rates, and body condition of bears. They placed on the bears high-tech tracking collars, consisting of small video cameras mounted to GPS-equipped leather and plastic straps, and then measured patterns of daily activity, field metabolic rates, body condition—including weight and foraging success while the polar bears prowled the spring sea ice. They found that these animals are basically high energy, fat-burning machines, whose proper functioning requires consumption of high-fat prey. When activity levels are higher because the bears have to work harder and move around more to find scarce food, the high metabolic rates result in an energy deficit and the bears lose weight fast.
The Arctic sea ice is expected to undergo continuous decline and fragmentation in the next years. Therefore, we can expect polar bears to be exposed to increased energy demands due to the additional activity needed to find their seals, leading to deteriorating body condition and declined survival. No one knows for sure what caused the sadly crumbling appearance of the polar bear in the heartbreaking images that captivated public attention last December, but we can imagine they’re an apt representation of what climate change will likely bring to the Arctic in a not so distant future.