Evidence for Rensch's rule in an orb-web spider with moderate sexual size dimorphism

Abstract

Background: The sexes are often subject to different selection regimes. In some cases, these generate sexual size dimorphism. Rensch’s rule postulates that male body size is more variable than female body size. The rule applies to many but not all animal groups. In spiders, sexual size dimorphism is extreme in cases of female gigantism and male dwarfism. These more widely studied species with extreme reversed sexual size dimorphism do not follow Rensch’s rule. Question: Do orb-web spiders with moderate sexual size dimorphism follow Rensch’s rule? Organism: The bridge spider, Larinioides sclopetarius, with moderate sexual size dimorphism and high developmental plasticity. We tested its sex specific variation in size and age at maturity. Methods: We experimentally varied food supply during spider development. We measured leg length (as a measure for body size), body weight, and development time for 267 male and female spiders from 15 matrilines. Results: Females reacted to decreasing food levels by prolonging developmental time and adding extra moults. Their body size was unaffected by the food supply. However, males that developed with a poor diet, despite adding extra moults and increasing their age of maturation, matured at smaller sizes than did well-fed males. Thus our results support Rensch’s rule because male body size was affected by diet whereas female body size was not.