Sex-specific selection and sexual size dimorphism in the waterstrider Aquarius remigis


Ferguson Department of Biology, Concordia University, 1455 de Maisonneuve Blvd. W., Montreal, Quebec, Canada H3G 1M8. Tel.: +1 514 848 3397; fax: +1 514 848 2881; e-mail:


We estimated selection on adult body size for two generations in two populations of Aquarius remigis, as part of a long-term study of the adaptive significance of sexual size dimorphism (SSD). Net adult fitness was estimated from the following components: prereproductive survival, daily reproductive success (mating frequency or fecundity), and reproductive lifespan. Standardized selection gradients were estimated for total length and for thorax, abdomen, genital and mesofemur lengths. Although selection was generally weak and showed significant temporal and spatial heterogeneity, patterns were consistent with SSD. Prereproductive survival was strongly influenced by date of eclosion, but size (thorax and genital lengths in females; total and abdomen lengths in males) played a significant secondary role. Sexual selection favoured smaller males with longer external genitalia in one population. Net adult fitness was not significantly related to body size in females, but was negatively related to size (thorax and total length) in males.