One fall a young beaver, probably a two-year-old kicked out by its parents, built a small lodge in the old mill pond below our house. On cold January days when temperatures were below zero, I looked at the snow-covered lodge and wondered if the beaver was still alive. But when the ice melted in late March, it was swimming around again.
Mortality rates are higher among young, lone beavers than established adults. Winter is especially daunting: no sooner had the mill pond beaver taken up residence, than it had to prepare for months of cold and food scarcity. How did it survive?
One answer is by storing food. All fall, the beaver cut aspen, willow, alder, and red-osier dogwood and made a huge pile of sticks underwater, anchored in the mud on the bottom of the pond. It kept up this activity until the pond became fully iced over.
Beavers will typically collect food as late into the winter as they are able. Naturalist Patti Smith, who recently authored The Beavers of Popple’s Pond: Sketches from the Life of an Honorary Rodent, has observed beavers’ herculean efforts to extend the tree-cutting season. One beaver bit the edge of the ice closing in on the last hole of open water in the pond. She saw beavers swim under the ice and push it up with their backs. They also climbed on top of the ice and pushed it down with their forepaws.
Last winter, said Smith, water levels dropped in one pond and the ice collapsed. The beavers continued to live in their lodge, but created a tunnel through the muck to the pond shore, where they emerged from under the ice, cut trees, and hauled the branches back beneath.
Other adaptations help beavers survive winter. They put on body fat during the fall, providing insulation as well as stored energy. In particular, a beaver’s tail is designed to store fat and shrinks in size over the winter as the fat is used up. Thick fur also insulates a beaver from the cold. As a beaver grooms, it spreads oil from anal glands over its fur, waterproofing its coat and enabling it to swim without getting its body wet.
Beaver lodges provide shelter from cold temperatures. To construct a lodge, beavers pile up cut branches, debris, and aquatic vegetation, and coat the conical structure with mud, except for the peak, left uncoated for ventilation. This mixture freezes like cement, making the lodge impenetrable to most predators and remarkably well insulated.
Within the lodge is an excavated chamber, located above the water level, which typically has at least two tunnel entrances/exits opening underwater. The chamber has two levels: a shelf a few inches above the water used for feeding and drip-drying of fur and a higher, drier sleeping platform cushioned with shredded wood fibers and grasses. The thick walls of the chamber (two to three feet at the bottom), the beaver’s body heat, and snow on top of the lodge help maintain a stable temperature inside. In one study, temperatures inside an Ontario beaver lodge in January and February hovered around 32 degrees, while the outside temperature averaged between 19 and minus 6 degrees.
An advantage that our mill pond beaver lacked, as a single animal, was a family to share warmth. Beaver families typically consist of five or six animals: an adult male and female, yearlings, and kits born the previous spring. They huddle on the sleeping platform together. The adult beavers reduce their energy demands by decreasing their metabolic rates and activity levels in winter. The young beavers, however, are still growing, and must maintain their metabolic rates. To do this, they need to eat a lot. Adults often retrieve sticks from the food cache and bring them into the lodge for the young to eat.
Researchers who placed video cameras inside beaver lodges found that beavers sometimes share their winter abodes with muskrats. Perhaps, speculated Smith, beavers tolerate the freeloaders because the muskrats’ body heat helps warm the lodge. (As far as I could tell, there was no evidence of muskrats in our mill pond).
This winter, ski or snowshoe onto a beaver pond if the ice is safe. If you stand quietly beside the lodge, you may hear beavers squeaking or gnawing sticks inside.
Susan Shea is a naturalist, conservationist, and freelance writer who lives in Brookfield, Vermont. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: firstname.lastname@example.org