Variation in selection, phenotypic plasticity, and the ecology of sexual size dimorphism in two seed-feeding beetles

This chapter reviews recent studies on two seed beetle species with contrasting sexual size dimorphism (SSD): males are larger in Stator limbatus and females are larger in Callosobruchus maculatus. Male ejaculate size clearly affects male fitness in S. limbatus via both fecundity and sexual selection, but not in C. maculatus. Female S. limbatus lay more eggs when mating with large males, reflecting either direct nutritional benefits obtained from male ejaculates or increased female allocation of resources following mating with large males. This difference in patterns of sex-specific selection between the species contributes to explaining the difference in SSD between the two species. However, variation in SSD, both within and among species of seed beetles, is also caused by differential phenotypic plasticity of the sexes. The implications of this for overall patterns such as Rensch's rule are discussed.