Imagine trying to walk with sharp shards of ice between your toes, cutting into your feet and causing pain with every step you take. For polar bears this is yet another tragic consequence of the northern ice cap melting, say scientists from the University of Washington (UW).
The injuries to the polar bears’ pads were discovered when the health of two populations in Canada and Greenland were being studied by the university.
Researchers noticed that some of the bears in their study groups seemed to be having trouble with their feet. "The two most affected bears couldn't run - they couldn't even walk very easily,” says Kristin Laidre from the UW Applied Physics Laboratory.
When they sedated the animals to have a closer look, they found large ice blocks, up to 30cm in diameter, stuck to their pads. Walking on these had caused deep, bleeding lacerations.
“When immobilising them for research,” says Laidre, “we very carefully removed the ice balls. The chunks of ice weren't just caught up in the hair, they were sealed to the skin. When you palpated the feet it was apparent that the bears were in pain.”
The two polar bears groups have been observed since the 1990s but this is the first time injuries such as these have been seen. In the Kane Basin population, located between Canada and Greenland, 31 of 61 polar bears showed evidence of ice-related injuries.
In the second population, in East Greenland, 15 of 124 polar bears had similar lesions. The cause, say the scientists, is undoubtedly climate change and fluctuating temperatures, coupled with the nature of polar bears’ feet. Unlike other species of bear, polar bears have bumps on their foot pads to help their grip on slippery ice. But these also give a good adherent surface for slushy snow which freezes solid when the temperature drops. It’s a problem that is also affecting sled dogs in the North.
The current shift from a climate that used to remain well below freezing year-round to one with frequent freeze-thaw cycles is to blame. "As strange as it sounds,” says Laidre, “with climate warming there are more frequent freeze-thaw cycles with more wet snow, and this leads to ice build-up on polar bears' paws."
There are two other possible explanations put forward by the boffins, both also related to a warmer Arctic. Higher temperatures make for thinner sea ice. This allows seawater to seep into and thaw the snow on top which then clumps on the bears’ feet and refreezes.
The second hypothesis is that spells of warm weather melt the surface of the snow, which then hardens into a crust when it gets colder. The polar bears tread on the crust and cut their paws as they sink past the sharp edges.
Whatever the reason, the possibilities all stem from the same cause - a gradual warming of the high Arctic. Asked what can be done to help the polar bears, Laidre has a simple response: "We can reduce greenhouse gas emissions and try to limit climate warming."
The study was published in the journal Ecology by Kristin L Laidre and Stephen N Atkinson: Icing‐related injuries in polar bears (Ursus maritimus) at high latitudes. 2024
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