Over-winter activity and body temperature patterns in northern beavers

Transmitters were implanted in three adult, one yearling, and one kit beaver (Castor canadensis) in northern Minnesota and upper Michigan to determine if core body temperature (Tb) declined in winter. Beaver Tb and activity were automatically recorded from late autumn through early spring. In late October through early November, mean daily Tb of adults was 36.3 °C (range 34.5–37.6 °C), with a mean daily Tb fluctuation of 1.4 °C. From fall to winter adult beaver Tb declined at a mean rate of 0.01 °C/day, and the mean daily winter (17 November – 15 April) Tb was 35.3 °C (range 32.5–38.8 °C). Mean daily winter Tb fluctuation was 2.5 °C. In early March, mean adult daily Tb began increasing at a rate of 0.03 °C/day. Tb and activity were positively correlated, except for the kit beaver, which did not undergo a significant Tb decline during winter. The yearling beaver Tb declined at a slower rate than adult Tb, and its Tb was intermediate between that of the kit and the three adults. Continued body growth in winter in young beavers is probably enhanced by high body temperature. For adult beavers, reduced body temperature in winter may be an adaptation that facilitates survival during extreme resource scarcity.