Reproductive versus ecological advantages to larger body size in female snakes, Vipera aspis

Body size can influence an organism's microevolutionary fitness either via ecological factors (ecological selection) or changes in reproductive output (sexual or fecundity selection). Published studies on sexual dimorphism in reptiles have generally focussed on sexual-selective forces on males, unde r the implicit assumption that the intensity of fecundity selection in fema les (and hence, overall selection on female body size) is likely to be rela tively consistent among lineages. In this paper, we explore the degree to w hich larger body size enhances ecological attributes (e.g., food intake, gr owth, survival) and reproductive output (reproductive frequency, litter siz e, offspring size, offspring viability) in free-ranging female aspic vipers , Vipera aspis. The less-than-annual reproductive frequency of these animal s allows us to make a direct comparison between females in years during whi ch they concentrate on "ecological" challenges (especially building energy reserves) versus reproductive challenges (producing a litter). Because fema le snakes have limited abdominal space to hold the clutch (litter), we expe ct that fecundity should depend on body size. However, our data show that l arger body size had a more consistent effect on ecological attributes (such as feeding rates and "costs of reproduction") than on reproductive output per se. Indeed, total reproductive output was maximised at intermediate bod y sizes. These results suggest that variation in female body size among and within species (and hence, in the degree of sexual dimorphism) may be driv en by the ecological as well as reproductive consequences of body size vari ation in both sexes.