Just over a year ago, more than 150 polar bears amassed on a remote island off the north coast Siberia to devour a dead bowhead whale that had washed ashore. It was the largest swarm of polar bears ever recorded feasting on a stranded whale — but events such as this could become more common in a warmer world.
That is the suggestion of a fascinating paper published this week in Frontiers in Ecology and Evolution.
In the not-so-distant geologic past, the paper argues, polar bears might have survived rising temperatures by trading life as ice-bound seal hunters for longer stints as land-bound whale scavengers. And some bears might make that ecological switch again as human-driven climate change causes Arctic sea ice cover to crash.
Dwindling sea ice means the 26,000-odd polar bears scattered across the high Arctic will have to trek further and expend more energy to hunt the seals that are currently their primary food source. But the study notes that whale carcasses are the “largest parcels of organic matter in the ocean”, which is scientist-speak for basically “a hundred free extra-large Domino's pizzas”.
While those blubbery snacks typically end up at the bottom of the sea, on occasion the buildup of gases inside a dead whale’s putrefying carcass can buoy it to shore. Groups of bears have been seen noshing on these carcasses from Siberia to Svalbard.
But do dead whales wash up often enough to truly sustain polar bears, rather than provide the occasional free lunch?
In some places, the answer might be yes.
Pulling data on polar bears’ seal consumption rates in the Canadian Arctic, the authors modelled a hypothetic population of 1000 bears and found they’d consume some 26,400 seals over the springtime foraging season. Considering the average blubber and meat content of a whale carcass, the researchers figure this is equivalent to about 20 dead bowheads. Another eight bowhead carcasses would be required to get those hypothetical bears through summer.
Those numbers, the researchers note, compare favourably to the nearly 50 total large whale carcasses estimated to wash up on the coastlines of Alaska and eastern Siberia each summer, meaning whale carcasses “are at least theoretically capable of greatly augmenting, or even replacing for periods” a seal-based diet.
The authors hypothesise that a switch to whale scavenging helped some polar bears pull through warm periods such as the Eemian 130-115,000 years ago, during which Arctic sea ice extent was greatly diminished.
Lead study author Kristin Laidre, a marine biologist at the University of Washington, cautioned that the study only looked at whale stranding rates in a few parts of the Arctic, and that in other habitats, there might not be a buffet of dead whales ready and waiting to replace seals in polar bear diets.
“Greenland, which is very mountainous, is not a place where we get a lot of stranding,” Laidre told us. “There’s definitely parts of the Arctic where this is unlikely or does not occur.”
What’s more, even if Earth’s climate history tells us polar bears have survived an ice-impoverished Arctic in the past, that doesn’t necessarily mean they’ll be able to adapt to the rapid warming taking place today, which is probably without precedent in the species’ less than million-year history.
Commercial whaling in the 19th and 20th century vastly reduced many large whale populations compared with their estimated historical abundances, adding another wrinkle that doesn’t favour polar bears’ ability to adapt. All of this “makes it really hard to use history to project the future,” Laidre said.
“We don’t expect carcasses to replace seals in diets of polar bears, but in a few places they may play a role,” she continued.
So if you start to hear more stories about polar bears snacking on giant whales, just remember this is nature doing its best to adapt to our mess.