|MadSci Network: Zoology|
This is a really interesting question. I'd never thought about it before, but it makes sense that the temperature inside the den would be warmer than outside so the bear could stay alive. It turns out that what keeps the inside of the bear's den warm is the bear itself. The bear's body tempera- ture is somewhere around 100 degrees F normally, and drops a few degrees when the bear sleeps through the winter. In a grizzly bear's den, when the outside temperature is only about 20 degrees, the temperature of the den right around the bear is about 70 degrees. So the bear's body heat is making the den 50 degrees warmer than the outside air. Near the entrance of the den or in the back of the den away from the bear, the temperature is closer to the outside temperature (around 25-30 degrees). In The Track of the Grizzly, Frank Craighead describes how he and his colleagues obtained these measurements by putting temperature-sensitive transmitters (basically a tiny machine that records the temperature and then sends that information out to be picked up by a receiver, the same way a radio station sends out a signal to be picked up by your radio) into a bear's den. A NASA satellite orbitting the earth received the temperature information from the transmitters and relayed it to the scientists!
A polar bear's den is a different story. They live in a much colder climate, so their body heat can only bring the temperature of their dens up to about freezing. Considering that their dens are made of snow, that seems pretty good!
Thanks for your question.
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