Sex-Specific Foraging Behaviours and Growth Rates in Juveniles Contribute to the Development of Extreme Sexual Size Dimorphism in a Spider

Extreme sexual size dimorphism (SSD) in temperate species is expected to be proximally caused, at least partially, by sex-specific growth rates due to the limited time available for growth and reproduction. Hence sex-specific foraging strategies are predicted to mediate differential growth rates. However, little is known about how sex differences in foraging behaviour and growth trajectories relate to the expression of pronounced SSD. Here we tested for sex-specific foraging strategies and growth rates in juveniles of the highly size dimorphic orb-web spider Argiope aurantia under natural conditions. In a number of web sites, we estimated web height, web size (size of the prey capture area), mesh size, stabilimentum (web decoration) size and length, vegetation density and prey availability. Over four days in the field we also measured spider growth and web site tenacity. Independently of body size, females exhibited faster growth rate than males. When body size and condition were controlled for, we found that females built larger webs, and at sites with greater prey availability compared to males. Males built webs with significantly larger and longer stabilimenta independent of web size. These results indicate that extreme female-biased SSD in A. aurantia is at least partially the result of sex-specific growth rates already in early juvenile stages mediated by sex-specific web design and placement to allow for greater foraging success of females compared to males. We discuss these findings in the context of SSD evolution, and consider whether the sex-specific behaviours detected are more likely consequences or causes of the evolution of extreme SSD.