Latitudinal variation in snowshoe hare (Lepus americanus) body mass: A test of Bergmann’s Rule

The relationship between body size and latitude have been the focus of dozens of studies across many species. However, results of testing Bergmann’s Rule – that organisms in colder climates or at higher latitudes possess larger body sizes – have been inconsistent across studies. We investigated whether snowshoe hares (Lepus americanus) follow the Rule by investigating differences in body mass using data from six published studies and from data of 755 individual hares captured from ten populations across North America covering 26° of latitude. We also explored alternative hypotheses related to variation in hare body mass, including winter severity, length of growing season, elevation, and snow depth. We found body mass of hares varied throughout their range, but the drivers of body mass differed based on geographic location. Females in northern populations followed Bergmann’s rule, whereas males did not. In northern populations male mass was related to average snow depth. In contrast, in southern populations body mass of both sexes was related to length of the growing season. These differences likely represent variation in the drivers of selection. Specifically, in the north, a large body size is beneficial to conserve heat because of low winter temperatures, whereas in the south, it is likely due to increased food supply associated with longer growing seasons.