SEASONAL CHANGE IN BODY-MASS AND SIZE OF TAIL OF NORTHERN BEAVERS

Loss of body mass and change in size of tail during winter were examin ed for beavers (Castor canadensis) at 33 beaver colonies in two Nation al Parks an Minnesota and Wisconsin during 7 years. These indicators o f condition were used to assess the effects of winter on beavers and d etermine if location of colony and presence or absence of young influe nce changes in condition. During summer (May-September), beavers alway s gained mass and their tails increased in size. During winter (Novemb er-April), young-of-the-year increased in body mass and size of tail, but yearlings and adults decreased in both measures. Changes in mass a nd size of tail during winter did not differ between sexes. Overwinter ing condition did not differ between Parks. Seasonal change in body ma ss was not different among years, but was different among colonies. Ch ange in size of tail was different among years and colonies. Greater l osses in both measures of condition were observed during winters that were longer than average. Yearlings and adults lost more mass in colon ies with young than in colonies that over-wintered without young. Thes e findings highlight the importance and variable effect of weather, si te of pond, age, and composition of colony for overwintering condition of beavers, and may indicate why the study of winter ecology of beave rs has been enigmatic.