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A winter under water

By Sorcha Masterson,

Whistler Naturalists

If you took a paddle down the River of Golden Dreams or if you spent some time examining the trees during a fall picnic in Rainbow Park, you may have seen evidence of Whistler’s resident beaver ( Castor canadensis ) population. Most of the beaver’s late summer and fall activities are related to their winter survival. Unlike the mammal’s discussed in a recent Naturespeak article, beavers do not hibernate in the winter and therefore must have adequate shelter and food supplies to survive until spring.

Beavers are amazing builders; their best-known structure is the dam. Beaver dams create deep ponds that provide year-round underwater access to the lodge, allow for safe water access to food supplies and provide storage for winter food. A beaver will build its dam where the noise of moving water in the stream is greatest. Sticks are embedded vertically to serve as anchor prongs in the mud. Sticks, leaves, stones, etc… are laid around the anchors. Finally, the beaver uses mud from the stream bottom to provide a watertight seal.

Beavers, like other rodents, construct a den or lodge for shelter and protection against predators. Dens are burrows in a riverbank and lodges are built in the water. The interior layout of both consists of one or more underwater entrances, a feeding chamber and a dry nest. Lodges can be up to 2 metres in height and 5 metres in diameter, with size dependant on family numbers, pond size, water level and number of years of occupation. Over winter there can be six or more beavers in the lodge, including parents yearlings and kits. Lodges are similar to dams in that they are a tangled pile of sticks, mud, and stones. The tunnels and chambers are excavated once the pile is complete.

In late fall as freezing weather begins, the lodge is plastered with mud, except around the air intake near the top. The mud results in a concrete-like outer shell which the beaver’s enemies – like the wolf, wolverine, or lynx – cannot break through as they approach the lodge on the winter ice. The mud shell also increases the lodge’s insulation capability.

Beavers are vegetarians, maintaining a herbaceous diet (ferns, algae, aquatic plants, etc…) in the spring, summer and fall before switching to a winter woody diet. Each fall, a family of six beavers will have to acquire a half hectare of dense aspen, poplar, birch and/or willow trees for its winter food supply, totalling approximately 50 kg.

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