Polar bears are great swimmers. Their thick fur sends the water in easy ripples off their bodies and insulates them in subzero oceans. Big paws mean they paddle well. Still, they stick to short-distance swimming. Anything more than a few kilometres drains body fat and exhausts them.
But in 2011, a female polar bear swam for nine days nonstop to find a place to rest and eat. The journey took her across 400km of ocean. Her cub died and she lost 20% of her body weight.
According to research coming out of the University of Alberta and Environment and Climate Change Canada last week, such desperate voyages are becoming increasingly normal. This was mostly a result of the warming climate shrinking the extent of Arctic sea ice. The ice provides a habitat for polar bears.
At the release of the research, lead author Nicholas Pilford said: “The pattern of long-distance swimming by polar bears shows the fingerprint of climate change.”
The study – Migratory Response of Polar Bears to Sea Ice Loss: To Swim or Not to Swim – was published in the journal Ecography. The researchers started tracking polar bears’ movement in Hudson Bay and in the Beaufort Sea along the north of Canada and Alaska in 2004. Tracking collars were put on 117 adult females and 18 cubs. No adult males were included because their necks are too large to fit collars, but work is being done to track them by satellite.
Between 2004 and 2014, the team found that the number of polar bears swimming more than 50km at a time had grown from 25% to 69%. This corresponded with a time when the mass of the Arctic ice shelf shrunk dramatically, hitting a record low in 2012.
Pilford says that when he started research on polar bears in the 1980s he could go to the Beaufort Sea and see ice from the shore all year round.
“In recent years, the ice has retreated several hundred kilometres offshore by September,” he says.
With less ice, he says, polar bears must travel further to hunt and find a habitat, and changing sea currents have also moved their food further away: “This … may have serious implications for populations of polar bears living around the Arctic Basin.”
Pilford’s colleague, Andrew Derocher, told the Washington Post that older and younger polar bears, as well as underweight bears, were dying in increased numbers because of changing climate.
Their research showed that mothers tried to swim less, so they could keep their offspring out of the water.
“With cubs – if they have to undergo a long-distance swim – it’s basically a death sentence,” says Derocher.
Polar bears are listed as critically endangered by the 2015 Red List of Threatened Species, which is curated by the International Union for Conservation of Nature (IUCN). It says there are only 26 000 polar bears left in the wild.
A study released last year by the IUCN found a “high probability” that these numbers would decline by 30% by 2050 as their ice habitat retreats.
Dena Cator, of the IUCN’s species survival commission, said: “You should consider polar bears to be the canary in the coal mine.”
Loss of sea ice
With the Arctic seeing the quickest increase in its average temperature and a rapid loss in sea ice, their fate was tied to that of the planet, she added. “There is a high risk of extinction and the threat is serious.”
Another study on the effect of changing climate on polar bears was released last week by the United States Geological Survey. This looked at their ability to adapt to their food source moving.
Isotopic Incorporation and the Effects of Fasting and Dietary Lipid Content on Isotopic Discrimination in Large Carnivorous Mammals was published in the journal Physiological and Biochemical Zoology. It said that research needed to be done so that decisions could be made on the conservation of polar bears.
The article said: “Science can sometimes be a slow process, and climate change is happening rapidly. Anything we can do to quickly gain information about how polar bears respond will help managers make critical decisions for protecting them in the wild.”
Data went as far back as 1980, with isotopes of the food polar bears eat taken from fur and blood samples. This found that consumption patterns had changed dramatically in the past decade. In East Greenland, polar bears have started eating more hooded seals, because their usual prey – ringed seals – have changed habitat.
But even this source has become increasingly difficult for polar bears, because less ice means they have fewer places to catch food and to eat it. Less food has come to mean more starvation and less energy to cover long distances to hunt.