Polar bears are built to withstand some of the coldest temperatures on the planet. Their brown and black bear cousins avoid the winter cold by digging dens and sleeping. But, except for pregnant females, polar bears spend the arctic winter outside where temperatures could be -40° F (which equals-40 °C) and windy. That’s too cold for humans. You could go outside, but only for only a few minutes with every part of your body completely covered. And if you didn’t wear goggles, your eyelashes would freeze and break off if you touched them.
Polar bears are warm-blooded like us with a body temperature of about 98°F/37°C. But they are invisible to night-vision goggles that pick up the infrared rays that warm-blooded creatures, including humans, give off. Why? Nature has given polar bears enough insulation to prevent body heat from escaping. They are toasty warm and comfortable in the frigid arctic.
Their heat insulation is in several layers. Under their skin, there is a 4-inch (21.5 cm) layer of fat. Next to the skin is a dense layer of woolly fur that also keeps heat in. The fur you see is a thick layer of long, colorless guard hairs that shed water quickly after a swim. They are stiff and transparent and hollow. In the arctic sunlight, the hairs act like mirrors and reflect white light, which acts as camouflage against the snow so the bears are not seen by their prey. Polar bear skin is actually black, so that it can absorb the invisible warm infrared rays of the sun and the bear’s own body heat, both of which are reflected back by the guard hairs.
Most warm-blooded animals raise their body temperatures through exercise. Polar bears hunt seals, which they don’t often chase. They prefer to sit at the edge of an ice floe and wait for dinner to arrive. At best, they’ll lumber after a seal at four and a half miles (7.25 km) an hour, raising their body heat to 100°F (38°C). When that happens, they go for a swim to cool off.
Cold won’t kill off the polar bears, but global warming can. As polar ice disappears, so does the hunting ground for seals. Not so cool!
Close up, the polar bear guard hairs are transparent. This allows the infra-red light (heat) from the sun to pass through them to be absorbed by the black skin under the hairs. The hairs also act like mirrors , reflecting back to the skin any infra-red radiation escaping from the bears body so it can be reabsorbed. Thus, the insulation is just about perfect with no infra-red radiation escaping. The hairs are also coated with oil so they drain quickly after a swim.
Vicki Cobb's This Place Is Cold shows how the latitude of Alaska affects the lives of the plants, animals and people who live there. It is gloriously illustrated by Barbara Lavallee, a long-time Alaskan resident and artist.
Vicki is a member of Authors on Call—she can visit your classroom with interactive videoconferencing: Read more about her here.
MLA 8 Citation
Cobb, Vicki. "The Way Polar Bears Keep Warm Is Cool." Nonfiction Minute, iNK Think Tank, 29 Nov. 2017, www.nonfictionminute.org/The-Way-Polar-Bears-Keep-Warm-Is-Cool.
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