BRAS D’OR BIOSPHERE: Beaver tails and the next generation

By: Dr. Annamarie Hatcher

In the Mi’kmaw calendar, October is “Wikewiku’s,” or animal fattening month.

Animals that will hibernate throughout the winter put on weight at this time of year and birds that migrate south add fat as fuel for their long flights. One of our biosphere mammals that is fattening up at this time of year might surprise you.

The beaver (Mi’kmaq: “kopit”), that iconic Canadian animal, neither migrates nor hibernates. In fact, if conditions are right, it continues to munch on layers of wood over the winter as they are available. As a sort of insurance and as a boost to the next generation, the older individuals also lay down fat stores in the autumn which round out their silhouettes in a most unusual place, their tails. In fact, the voluptuous nature of a female beaver’s tail is a good predictor of the future health of unborn offspring.

Beavers can be found in most of the freshwater rivers, streams and wetlands in the Bras d’Or Lake Biosphere. That is where they make their homes. The younger adults, searching for a suitable place to call their own, will also swim in the salty waters of the estuary. However, they do not stay or feed there. In fact, beavers have been sighted in ocean waters off Nova Scotia’s eastern shore. They are in transit, swimming toward islands that have freshwater habitat.

Our beavers are semi-aquatic, nocturnal vegetarians. Most of their diet is made up of tree bark and cambium (soft tissue growing under the bark) and they prefer willow, maple, birch, beech, poplar trees, and alder shrubs. They build dams as a protection against predators and to provide easy access to food during winter. During the night, they often are “busy as a beaver,” carrying mud and stone building materials on their forepaws and timber between their teeth. The ponds created by their dams help isolate the beavers’ lodges where they live during the winter. During the autumn, they cover their lodges with mud which will freeze during the winter, creating a concrete-like surface. That must inhibit any predators hoping for a mid-winter feast of beaver.

Unlike other rodents, beavers mate for life and stay together for many breeding seasons. In the safety of a winter lodge, you can find parents and the adolescent offspring which were born last season. In the biosphere, beavers breed in January-February. The males have to be on the ball because the females are in estrus for only 12 to 24 hours. The females deliver between two to six newborn kits about three-and-a-half months later. Unlike most other rodents, beaver pairs are monogamous, staying together for multiple breeding seasons. They start reproducing when they are about three years old. When the new litter is born, the adolescents are launched into the world on their own and they seek their own space to build a dam and a lodge.

This is an image of the Canadian nickel design by Gerald Gloade of Millbrook.

There are often strong feelings about the landscape changes that a beaver’s building activities can produce, particularly among landowners who discover new areas of flooding. However, Mother Earth often adapts to these changes in a very positive way. For example, beaver ponds can enhance groundwater recharge by acting as storage reservoirs. They provide critical habitat for many species of resident and migrating waterfowl. They provide habitat and food for salmon and trout during dry periods and severe winters. Long standing beaver dams can increase diversity of trees and shrubs in forested areas near the water’s edge.

In some cases, this enhancement has been shown to have a greater impact than the negative effect of the beavers’ harvesting activities. Beavers are being re-introduced in many areas because of their beneficial impacts on stream ecosystems.

Now, let’s get back to that tail. The beaver’s tail can be up to 30 centimetres long and is a multi-use body part. It has a tough, thick outer layer of skin which has very little hair. It is flexible and supported by a strong muscular framework which makes it useful as a rudder for swimming and a prop for sitting. It is also used to signal other beavers. When alarmed, they use their tails to slap the water, warning others to take refuge under the water.

In autumn, female beavers are at their chubbiest, along with many other animals of the biosphere. Their voluptuous tails decrease in volume two-fold from autumn to spring as they metabolize the fat reserves. That fat reservoir feeds the developing embryo in the overwintering, pregnant females. It has been shown that the females with the fattest tails in autumn are most likely to produce a healthy litter in spring.

So, as you hike near beaver ponds in the biosphere this autumn, imagine what the landscape will look like in mid-winter and picture the family of beavers safe in their lodge. They will have a stash of drowned tree branches and a store of fat to sustain them until spring.

Dr. Annamarie Hatcher is a consulting ecologist and a board member of the Bras d’Or Lake Biosphere Reserve Association.