Sexual selection for increased male body size and protandry in a spider

Female-biased sexual size dimorphism (SSD) is found in many organisms yet is poorly understood. Spiders in general, and web-building species in particular, typically have strongly female-biased SSD. We investigated the causes of SSD in the web-building spider Stegodyphus lineatus. Females are slightly, but significantly, larger than males. Large females are more fecund but the selection pressures on male body size are not clear. Males were introduced on to the webs of virgin and mated females and we also conducted competition experiments between males. Large males did not have longer copulations, nor did they mate more successfully with virgin females than small males did; however, they were more successful with previously mated females and remated more often. They also won more fights, and were more successful at obtaining prey from the female's web. Indiscriminate mating by virgin females, however, conferred a fitness advantage on early maturing males. We suggest that the female's mating strategy selects for protandry, which results in female-biased SSD, despite the selection for large body size in males. Indiscriminate mating by females and a trade-off between time to maturation and male body size may be important in understanding the evolution of female-biased SSD.