Ice is starting to form on ponds, and days are numbered when beavers can be out grooming themselves on land or eating freshly-cut branches. As long as the ice is thin enough to crack by swimming up under it and bumping their heads against it, they will do so, for soon it will be thick enough to lock them into their pond. Life under the ice is challenging. At 32 degrees F. a beaver’s resistance to heat loss in water is about 1/8th of that in air at the same temperature. This is due to the fact that its fur is compressed in the water, allowing the insulating air between the hairs to escape — a beaver’s pelt accounts for about 24% of its total insulation in water and body fat is responsible for the rest. Heat is also retained through a beaver’s tail and hind legs, which serve as heat exchangers. In the summer, a beaver can lose 25% of its body heat through its tail, but it only loses 2% in winter. Even so, it’s no wonder beavers risk getting a headache in order to see the sun for the last time until spring.