Optimal body weight of Brandt's voles for winter survival

Winter survival is an important fitness component of non-hibernating small mammals in northern latitudes. Body sizes are related to many life history traits influencing the fitness of animals. Counterbalancing selective forces of survival selection may optimize autumn body weight to maximize winter survival of non-hibernating small mammals. Brandt's voles (Lasiopodomys brandtii) are non-hibernating and live in groups year round. We live trapped Brandt's voles in an enclosure and estimated weekly survival probabilities and daily proportional body weight growth rates of the voles from September 2003 to March 2004. Autumn body weight as an individual covariate explained about 43% of variation in autumn-spring survival of the voles. Survival of females and males peaked at body weight of about 33 g and 51 g, respectively, supporting stabilizing survival selection on body sizes of Brandt's voles. However, breeding selection may reduce the optimal body size of female voles. Brandt's voles did not lose body weight during the autumn and winter probably to enhance winter survival. Therefore, Brandt's voles adapt to the energetically demanding winter environments with optimal body size and maximized winter survival.