Retreating Arctic sea ice is causing polar bears to swim more often and narwhals to make more inefficient dives to escape killer whales and human disturbances, forms of travel that require three to four times more energy than walking on ice or normal dives, respectively.

These changes in behavior, newly quantified in a scientific review published Wednesday in Journal of Experimental Biology, highlight the additional stress put on both species as they simultaneously experience fewer hunting opportunities. A decline in the population of these apex predators would “lead to rapid changes in the entire Arctic marine ecosystem,” according to the authors.

The Arctic has been warming about twice as quickly as the rest of the world, and the effects can be seen in the polar region’s shrinking sea ice. The average amount of Arctic sea ice in the month of September, when the ice extent is at an annual minimum, has fallen more than 13% per decade since 1979. 

Two marine-mammal researchers — Anthony Pagano, a postdoctoral researcher studying polar bears at San Diego Zoo Global, and Terrie Williams, an ecophysiologist at the University of California, Santa Cruz — compiled recent studies on polar bears and narwhals to determine how the ice-reliant species are being affected on a physiological level.

This approach can produce causes and other insights that are unattainable from large-scale population studies, Pagano explained.

“From a physiology perspective, we measure their energy expenditure, and we look at how much energy they're able to gain,” he said. “That's ultimately going to impact their ability to raise young and survive.”

Polar bears are listed as “vulnerable” by the International Union for Conservation of Nature, which concluded there is a high risk of large reductions in population if Arctic sea ice continues to melt. Narwhals are of “least concern” with about 123,000 living adults, according to the IUCN’s 2017 assessment.

Polar bears are inefficient swimmers compared with other semi-aquatic and marine mammals, the researchers said, so they burn more than three times as much energy while swimming compared with walking on ice.

And polar bears are swimming for longer periods of time to reach more sea ice or land, the researchers determined, as accelerating sea ice retreat in the spring is cutting short their crucial window of seal-hunting.

The review highlighted one female polar bear that swam 287 miles in 10 days — with seemingly little or no rest — across the Beaufort Sea, north of Canada. The endeavor took place in August 2012, one month before the Arctic experienced its lowest-ever September extent of sea ice.

“It's this combined effect of a lower ability to catch seals during this critical time in the spring and summer, combined with this increase in movements of having to walk further as the ice retreats further but also having to make these really extreme, long-distance swims," Pagano said.

More polar bears are expected to spend time on land as Arctic sea ice is predicted to continue shrinking or even disappear during September by 2050, but, "Few resources exist on land within the polar bears’ range” to replace seals, according to Pagano and Williams. They said that blubber from a single seal provides the same amount of calories as 1.5 caribou, 37 of the fish Arctic char, 74 snow geese, 216 snow goose eggs or 3 million crowberries.

Many of these other food sources would require chasing prey, which expends energy and exacerbates the animals' caloric deficit, Pagano said. When on sea ice, polar bears often hunt seals while exerting little movement by sitting at a breathing hole and waiting for the seal to resurface for air.

Narwhals regularly dive under sea ice for several minutes at a time to hunt fish, and they have evolved responses that lower their metabolism underwater. But when they flee killer whales or are disturbed by ships and other human activity, they dive but remain in shallow water, stroke quickly and retain a high metabolism.

These “flight dives” burn more than three times as much energy as the regular dives, according to the marine-mammal researchers, and they will likely occur more often as Arctic sea ice recedes. Killer whales are becoming more prevalent in the Arctic as sea ice melts and prey on narwhals, which are slow swimmers and have fewer places to hide with less sea ice.

Human-caused seismic activity also spooks narwhals and will likely become more common as fossil-fuel production enters the Arctic, seizing on new drilling locations exposed by the disappearing ice.

“The sudden increase in anthropogenic disturbance as humans progressively advance into areas that were historically ice-bound for much of the year is problematic for a naïve species like the narwhal that has lived in relative isolation from industrial activity in the Arctic,” the researchers wrote.

The warming climate is also making breathing holes in the ice less stable and predictable, causing some narwhals to become trapped and die under the ice, according to the review. Changing temperatures and overfishing affecting populations of narwhals’ prey puts the marine mammal at higher risk of not consuming enough energy.

More research needs to be done on the changing behavior of both species, Pagano said, including polar bears’ rates of seal-catching and how narwhals respond to disturbances from humans.

The article, “Physiological consequences of Arctic sea ice loss on large marine carnivores: unique responses by polar bears and narwhals,” was published Feb. 24 in Journal of Experimental Biology. The authors of the study were Anthony Pagano, San Diego Zoo Global; and Terrie Williams, University of California, Santa Cruz.