Naturespeak

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.

The winter food cache is placed in deep water close to the lodge or den. The edible foliage is held below the beaver pond’s surface by a thick top layer of branches cut from non-preferred trees and shrubs. This layer protrudes above the pond’s surface where snow provides an insulating layer that prevents water from freezing around the winter food cache. Throughout the winter, the beavers will bring sticks from this cache into the feeding chamber of the lodge, feeding on the cambium, or growing, layer of tissue just under the bark. Beavers may also obtain the thick roots and stems of nearby aquatic plants, such as pond lilies and cattail, for food.

The woody diet ingested by the beaver over winter, consists mainly of cellulose, which is indigestible by most mammals. However, the beaver has a cecum, which is a pouch between the large and small intestine in which colonies of micro-organisms digest 30 per cent of the cellulose that the beaver consumes. Without these micro-organisms the beaver would not be able to obtain enough energy from its winter diet to carry out its basic life functions. Beavers obtain additional energy by eating their fecal pellets, thus running them through the digestive process a second time. To further conserve energy, adult beaver’s body temperature will drop by1º C throughout the winter.

Beavers, like other animals, must prepare for their winter in the water by insuring they will have adequate food and shelter until spring. Then they set in for a long, dark winter under the ice. But boredom will not ensue as beavers mate in the winter, in the water, and 3-4 kits will be born in the family lodge in the late spring!