Variation in Sexual Size Dimorphism and Maximum Body Size Among Adder Populations: Effects of Prey Size

Abstract

(1) Geographic variation in maximum body size of male and female adders, Vipera berus, was documented over one mainland locality and six groups of islands in the Baltic Sea. Males varied more in body size among localities than females, although not significantly so. (2) Geographic variation in prey (field vole, Microtus agrestis) body size explained 68% and 40% of the variation in maximum body size of male and female adders, respectively. Body size of adders increased with body size of prey. (3) Adders were smaller on islands where there were three prey species than where there were two. It is suggested that selection for fasting endurance where there are few prey species and a high risk of starvation may have produced this pattern. (4) Growth rates of individual adders were faster where mean field vole body weight was large (47 g) than where it was small (26 g). Maximum body size of adders was large where growth rate was fast and vice versa. (5) Female adders were larger than males at all localities. Females also had faster growth rates than males. (6) The degree of sexual size dimorphism of adders varied among localities and was negatively correlated with size of males. There was no relationship between sexual dimorphism and size of females. (7) There was no significant relationship between sexual size dimorphism of adders and mean body size of field voles. Nor was there any relationship between sexual dimorphism and number of prey species or size distribution of field voles. (8) I argue that optimal body size for survival is locally determined by prey availability and size of prey items. However, due to the fecundity advantage of large size in females, female adders deviate from the optimum size for survival, and more so when this optimum size is small. Thus, local variation in properties of the food resource, e.g. prey size, can give rise to variation in sexual size dimorphism.