A polar bear mom and two cubs swim in ice-cold water.

Humans can't last long in very cold water, so how do polar bears do it?

© Dick and Valerie Beck/Polar Bears International

9/23/2020 3:51:33 PM

Polar Bear Questions: How Can Polar Bears Survive Swims in Icy Waters?

By Dr. Thea Bechshoft

Q: I recently received this intriguing question from Jonas: “How can polar bears survive long swims in ice-cold water when humans can’t?”

A: The answer can be summed up in one wobbly little word: fat! Polar bears are exceptionally well adapted to the cold Arctic environment, and are able to stay toasty and keep up their 37° C body core temperature, even when a snowstorm is raging in the middle of the polar night.

What keeps a bear from freezing when above water is primarily/first and foremost its fur—but under water, the hair no longer offers much isolation, and the bear must instead rely on the fat layer it has accumulated under its skin to stay warm. This fat layer can be up to 11 centimeters thick and is especially prominent on the bear’s rump. How thick this layer of fat layer becomes obviously depends on how successful a hunter the individual bear is: The more seals it eats, the fatter the bear will be. And, as I’ve said before, a fat polar bear is a happy polar bear! On a side note, a thick layer of fat likely also helps keep the bear naturally buoyant in the water.

Polar bears are generally strong swimmers, and longer swims are not uncommon (especially as climate change is creating a more fragmented sea ice landscape). But recent research shows that swimming is much more energetically costly for polar bears than walking. In addition, long swims are risky for young cubs, who are not yet very well insulated and may ultimately die if they are too cold for too long, as might happen during a lengthy swim. Some polar bear mothers solve this by giving their cubs a piggyback ride when swimming between ice floes!

The most extreme polar bear swim on record is that of an adult female who swam for nine days straight, a total of 687 km (426 miles). Very impressive—however, the swim also cost her the life of her cub, who started the journey with her. In addition, the mother bear lost 22% of her body fat, which tells you that although this swim was physically possible for her, it took a heavy toll.

Compared to polar bears, humans are very poorly insulated: We have almost no fat layer under our skin, which is the main reason we don’t last very long in cold water, quick polar plunges aside!

Dr. Thea Bechshoft is a staff scientist for Polar Bears International based in Aarhus, Denmark. She is the author of the popular Polar Bear Questions page on Facebook, republished here with permission. Have a question for Thea? Send her a message via her page. 

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