ARE ALPINE BANK VOLES LARGER AND MORE SEXUALLY DIMORPHIC BECAUSE ADULTS SURVIVE BETTER

Microtine body weights vary temporally and geographically: they are lo w before the onset of winter and are highest in northern cyclic popula tions, particularly at peak densities, as well as in island population s. Adaptive explanations have focused on the relationships between bod y weight, age-specific reproductive and survival rates as well as on p opulation 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 genera l. Survival rates varied little among years, despite large Variation i n snow depth and duration of snow cover, and were similar between male s and females. Sex ratio was generally biased towards males (average p roportion of males: 0.59). Body weights were higher than for lowland o r northern European populations, both for adults in the spring and for subadults at the onset of winter. The latter difference was particula rly large, autumn body weights being 50% higher than in these other po pulations. Difference between autumn and spring weights was therefore much reduced compared with northern populations. Moreover, sexual dimo rphism was the inverse of what is known for other populations in which males are smaller than females, males being larger than females durin g 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 allo cation, and that the reversed sexual dimorphism is a consequence of mo re intense sexual selection among males, deriving from a larger body s ize, and possibly also from a male-biased se?: ratio. We discuss the i mportance of food, habitat heterogeneity and predation in shaping life histories and population dynamics of microtines. Lower impact of pred ation, possibly resulting from the absence or scarcity of weasels, is one possible cause for the observed higher survival rates, and consequ ently larger body size of the alpine bank vole.