By Jason Sandefur/Courthouse NewsFebruary 26, 2021
(CN) — Rapidly melting Arctic ice is forcing two seemingly dissimilar apex predators to adapt their diets and behaviors faster than these cold-climate mammals can cope, scientists say.
At first glance, polar bears and narwhals seem to have little in common except for their reliance on Arctic ice to provide the prey and hunting opportunities they require to survive. But both are highly evolved to live and hunt where most mammals would die. They both grow larger than their ancestors did, with layers of blubber to insulate them from the cold and altered metabolism to keep them warm.
But these iconic polar species now face an uncertain future as global warming causes catastrophic sea ice loss, driving polar bears onto land and narwhals to swim for their lives to avoid the killer whales that now have access to previously ice-locked stretches of ocean, according to a study published in the Journal of Experimental Biology.
Major ice loss forces both species to alter their behaviors in fundamental ways that increase the amount of energy they burn by as much as 400% — a rate few species could sustain.
For polar bears, hunting land animals requires more energy but provides significantly fewer calories with each successful kill than their preferred prey ringed seals, which are full of calorie-rich blubber that helps keep the bears warm.
“A polar bear would need to consume approximately 1.5 caribou, 37 Arctic char, 74 snow geese, 216 snow goose eggs (i.e. 54 nests with 4 eggs per clutch) or 3 million crowberries to equal the digestible energy available in the blubber of one adult ringed seal,” said study authors Anthony Pagano of San Diego Zoo Global and Terrie Williams of the University of California, Santa Cruz. “Few resources exist on land within the polar bears’ range that could compensate for declines in seal feeding opportunities.”
While hunting land animals not only cuts calories from polar bears’ diets, it also demands much more effort, whether through tracking creatures or swimming greater distances to find food. By contrast, polar bears typically ambush fatty ice seals, saving valuable energy for keeping warm.
“Polar bears on land without access to marine mammal prey are at an increased risk of starvation,” the study authors wrote.
For narwhals, changes in ice cover challenge their highly specialized muscles that store most of the oxygen needed to dive deeply for Greenland halibut and Arctic cod. However, this adaptation also increases their proportion of slow-twitch muscle fibers, making these deep-sea hunters much slower than they would otherwise be. This trade-off places narwhals at increased risk of being hunted in open water by their new neighbors thanks to climate change: killer whales.
“The high costs of diving for narwhals, coupled with the loss of reliable breathing holes upon which they depend, due to unpredictable sea ice shifts, have led to the mammals becoming trapped beneath the ice,” the study authors wrote.
The loss of sea ice also disrupts the timing of narwhal migration, exposing their young to increased predation from killer whales.
Narwhals also struggle to adapt to the growing presence of humans in a rapidly melting Arctic. Narwhals are sensitive to underwater ship noise from cargo ships looking to exploit newly opened routes. Such “seismic” disturbances alter essential behaviors that enable prolonged dives, causing narwhals to expend up to 300% more energy when they hunt.
The scientists warn that the decline of both apex predators will “lead to rapid changes in the Arctic marine ecosystem.”
“The physiological specializations of these predators, whether hunting on top or below the sea ice, are ill suited for a rapidly warming Arctic.”
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