The ecological significance of sexual size dimorphism in two predatory stoneflies: Isogenoides frontalis and Isogenoides olivaceus.

Abstract

Sexual Size Dimorphism (SSD) occurs in many animal species, however the ontogenetic patterns and ecological ramifications of this phenomenon are not well documented. In hemimetabolous aquatic insects SSD can arise early in development especially among taxa with non-feeding adults whose material and energy required for reproduction must be accrued prior to emergence. The objective of this dissertation was to determine how SSD develops over an insect's ontogeny given the constraints placed upon it by its physical and biotic environment. Sex-specific growth patterns and behavior were analyzed during the ontogeny of two stream-dwelling, predatory stoneflies: Isogenoides frontalis and I. olivaceus (Plecoptera: Perlodidae). A life history study of the two species revealed that SSD appears just one month after eclosion. Males and females of both species displayed very different growth patterns. Female growth rates often exceed male growth rates during development. Sexual differences in diet contributed to a larger mean daily ration among female I. frontalis and suggested that the sexes followed distinct foraging strategies. Female diets breadths were greater in both the size and type of prey consumed. Males tended to be more size-selective in their diets. Laboratory feeding trials confirmed that nymphal females expended greater foraging effort to obtain a greater daily ration than males. Conversely, males appeared to limit their exposed foraging activity within a much shorter time frame. Studies of Isogenoides adults suggests fecundity-selection may be partially responsible for the SSD in these species. Potential selective forces acting on male size were less clear. Isogenoides frontalis is slightly protandrous, however, advantages of early male emergence within the mating system are yet unknown. This study suggests that sex-specific constraints on Isogenoides reproductive size lead to sex-specific foraging strategies and risk-taking in the nymphal stage.