COSTS OF REPRODUCTION INFLUENCE THE EVOLUTION OF SEXUAL SIZE DIMORPHISM IN SNAKES.

Abstract

Most explanations for sex differences in adult body size have focused on differences between the two sexes in the degree to which larger body size enhances reproductive success. For example, the general tendency for females to grow larger than males has been attributed to production of larger litters by larger females (Darwin 1871; Trivers 1972; but see Shine 1988), and the correlation between large male size and mating systems with male-male competition has led many authors to conclude that larger male size enhances male reproductive success in such systems (Darwin 1871; Clutton-Brock et al. 1977; Shine 1978; Berry and Shine 1980). Because each of these hypotheses deals only with the relationship between body size and reproductive success within one sex, no prediction can be made about the relative sizes of the two sexes (Greenwood and Adams 1987; Shine 1987). The degree of size dimorphism is presumably determined by the relative intensities of selection for large size in the two sexes, as well as the degree of genetic correlation between the sexes for alleles associated with body size (Arnold 1985). It is entirely likely that large size enhances reproductive success in both sexes, but more strongly in one than in the other. In order to predict the actual direction or degree of sexual size dimorphism within a population, we need data on the effects of body size on reproductive success in both sexes. Such data are rarely available (Clutton-Brock 1988), so the balance between selective forces acting on conspecific males and females has rarely been evaluated. A comparative analysis of sexual size dimorphism and mating systems in snakes reveals that females grow larger than males in most species, but not in species that show male-male combat behavior (Shine 1978). The same pattern has been noted in many other animals, including other reptiles, and generally has been attributed to sexual selection for large male body size in taxa with male-male combat (Shine 1979; Berry and Shine 1980; Fitch 1981). Nonetheless, there are many exceptions to this generalization, and the most puzzling of these are species in which male-male combat occurs but in which females attain larger sizes than males. Species that fail to conform to the usual pattern may tell us a great deal about the factors responsible for those general patterns. In this paper, we use detailed demographic and ecological data on one of these atypical species (the adder Vipera berus) to test among potential explanations for female-larger dimorphism in a species with male-male combat. Our analyses suggest that sex differences in costs of reproduction may strongly influence optimal body sizes in each sex, and hence may affect the direction of sexual size dimorphism. We focus here on the potential costs and benefits of larger body size in terms of reproductive success, especially, fecundity selection in females and sexual selection in males.