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Beavers in winter

Posted: November 18, 2011 - 12:00am  |  Updated: November 18, 2011 - 12:03am
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Beaver lodge  Kathy Hocker
Kathy Hocker
Beaver lodge

O n an October exploratory excursion, we encountered a well-built beaver lodge, one we hadn’t known about before. The sides were freshly packed with mud, effectively waterproofing and insulating the lodge for winter. I needed to refresh my failing memory on what it might be like inside a beaver lodge in winter, so back to the literature I went.

Beavers seldom venture into the open air outside the lodge in winter, when ice covers their ponds, so for months a family of beavers breathes “indoor” air, using oxygen and generating carbon dioxide. Beaver lodges have underwater entrances, and mud seals the walls, so air exchange is effected through a ventilation hole in the roof. Apparently this roof vent is sufficient to keep carbon dioxide from building up and allow an influx of oxygen, because when researchers measured the levels of those gases inside an occupied lodge, they stayed nearly constant.

Temperatures inside a well-built lodge also do not vary much. For example, when outside temperatures drop to minus 20 degrees Celsius (about minus 4 degrees Fahrenheit), inside temperatures remain just above freezing. Thick walls obviously conserve more body heat than thin walls, so inside temperature varies more if the walls are thin.

If beavers had to live outdoors in winter, for example at an air temperature of minus 20 degrees Celsius, their metabolic rate would almost double, compared to that at normal lodge temperatures. They might not be able to eat enough to stay alive under those conditions.

Beavers don’t hibernate, so they need a supply of energy throughout the winter months.

Although beavers make a pile of cut branches in front of the lodge as a winter cache of food, the cache does not contain enough food for a whole family of beavers (adults, yearlings, and kits) if they eat as much as they do in summer. Instead of eating lots of cached bark and roots, wintering adults reduce their energy demands by lowering their metabolic rate and body temperature and conserve energy by not moving around very much. The adults put on large amounts of fat in fall, partly in the body cavity and under the skin, but especially in the tail: the amount of fat in the tail in winter can be ten times what it is in summer. This stored fat is used up over the winter, so the adults lose weight.

In contrast, the kits and yearlings maintain their body temperature and metabolic rate, which is higher than that of adults, and they keep on growing. So they eat a lot, and the adults often bring the sticks into the lodge for them to eat.

Just inside the entrance of a lodge is a ledge to which beavers bring sticks from the cache—the winter dining room, so to speak. Here a wet beaver drip-dries as it eats the bark from the branch. A slightly higher ledge is a dry sleeping place where the family spends most of its time in winter. Adults, yearlings, and kits all huddle on the sleeping platform, dozing and occasionally grooming, and sometimes ducking outside to retrieve a cached stick or to defecate and urinate.

If one is very quiet, and the winds aren’t howling nor the rains pounding down, it is sometimes possible to stand next to an occupied lodge and hear the residents chewing on retrieved sticks.

• Mary F. Willson is a retired professor of ecology.

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