Are Alpine Bank Voles Larger and More Sexually Dimorphic because Adults Survive Better?

Abstract

Microtine body weights vary temporally and geographically: they are low before the onset of winter and are highest in northern cyclic populations, particularly at peak densities, as well as in island populations. Adaptive explanations have focused on the relationships between body weight, age-specific reproductive and survival rates as well as on population dynamics. We live-trapped five populations of bank voles for up to five years in the French Alps in subalpine habitats: two spruce forests, a deciduous forest, a hedgerow network, and a boulder field recolonized by birch and spruce. Population dynamics were non-cyclic, characterized by seasonal fluctuations. Densities were generally high, with little variability among years. Survival rates were higher (0.80 to 0.90 per month), both during summer and winter, than reported from other populations of bank voles in Europe, or of microtines in general. Survival rates varied little among years, despite large variation in snow depth and duration of snow cover, and were similar between males and females. Sex ratio was generally biased towards males (average proportion of males: 0.59). Body weights were higher than for lowland or northern European populations, both for adults in the spring and for subadults at the onset of winter. The latter difference was particularly large, autumn body weights being 50% higher than in these other populations. Difference between autumn and spring weights was therefore much reduced compared with northern populations. Moreover, sexual dimorphism was the inverse of what is known for other populations in which males are smaller than females, males being larger than females during the breeding season in all populations studied here. We suggest that the overall larger body weight is a consequence of a higher survival rate selecting for lower reproductive effort and a higher somatic allocation, and that the reversed sexual dimorphism is a consequence of more intense sexual selection among males, deriving from a larger body size, and possibly also from a male-biased sex ratio. We discuss the importance of food, habitat heterogeneity and predation in shaping life histories and population dynamics of microtines. Lower impact of predation, possibly resulting from the absence or scarcity of weasels, is one possible cause for the observed higher survival rates, and consequently larger body size of the alpine bank vole.