Abstract

Extreme sexual size dimorphism (SSD) is relatively rare in animal species. Males are much smaller than females for example in anglerfish, some barnacles, and various spiders, Spiders (Araneae) are unique because this is the only free-living terrestrial taxon where extreme SSD is common. The factors involved in the evolution and maintenance of extreme SSD are still poorly understood. Spiders also exhibit a "shape" dimorphism, males having relatively longer legs than females. In this thesis, I estimate selection on adult males of the highly dimorphic orb-weaving spider Argiope aurantia to evaluate hypotheses about the adaptive significance of male size and shape in spiders. I use a multivariate approach to distinguish selection targeting different body components. My results suggest that selection tends to favour longer legs during mate search, and that this may result in net selection for overall larger males. During mating, the pattern of selection on males depends on the type of female with which they mate. Most males mate opportunistically with a moulting female when she is defenceless and are under strong selection for large body size due to interference competition over access to females. If males mate with mature, post-moult females, they face an 80% chance of a cannibalistic attack by the female, but this does not result in selection on male body size. However, the longer males stay in copula, the more likely they are to be cannibalised. Relatively longer legs in males were not favoured during either form of mating and thus probably constitute an adaptation during mate search. Current hypotheses about the adaptive significance of small male size in spiders either predict a small male advantage during mate search or during mating with cannibalistic females, or assume that sexual selection for large size due to interference competition is absent or weak. Neither of these assumptions and predictions were upheld in my study.