Seasonal Change in Body Mass and Size of Tail of Northern Beavers

Abstract

Loss of body mass and change in size of tail during winter were examined for beavers ( Castor canadensis ) at 33 beaver colonies in two National Parks in Minnesota and Wisconsin during 7 years. These indicators of condition were used to assess the effects of winter on beavers and determine if location of colony and presence or absence of young influence changes in condition. During summer (May–September), beavers always gained mass and their tails increased in size. During winter (November-April), young-of-the-year increased in body mass and size of tail, but yearlings and adults decreased in both measures. Changes in mass and size of tail during winter did not differ between sexes. Overwintering condition did not differ between Parks. Seasonal change in body mass was not different among years, but was different among colonies. Change in size of tail was different among years and colonies. Greater losses in both measures of condition were observed during winters that were longer than average. Yearlings and adults lost more mass in colonies with young than in colonies that over-wintered without young. These findings highlight the importance and variable effect of weather, site of pond, age, and composition of colony for overwintering condition of beavers, and may indicate why the study of winter ecology of beavers has been enigmatic.