Exaggerated male weaponry in New Zealand sheet-web spiders (Cambridgea)

Abstract

The goal of this thesis is to investigate the role of sexual selection in the evolution of exaggerated weapons as well as the implications of that exaggeration. Male New Zealand sheetweb spiders (Cambridgea spp.) possess chelicerae (jaws) which are significantly longer than those of conspecific females and these weapons are used by males in agonistic contests for access to potential mates. Using this genus and, in particular, the North Island species, Cambridgea foliata, I study the exaggeration of male chelicerae in the context of morphology, behaviour and phylogeny to gain a more total understanding of how sexual selection can influence the characteristics of male weaponry. Firstly, I establish that males use their chelicerae in agonistic contests and identify how the structure of male-male contests in C. foliata drives selection for exaggerated weaponry. I then identify how cohabitation between males and females provides additional benefits to males for winning contests. This increases the strength of selection on traits which help males win contests. I develop and present the first molecular phylogeny for the genus. Using this, I apply phylogenetic comparative methods to examine how weapon and body traits covary across the genus. Finally, I analyse how components of chelicerae anatomy vary across males and between adult males, adult females and penultimate males to investigate how exaggerated morphology relates to functional performance in males, and to better estimate the likely costs of exaggeration. The outcomes of this research suggest that sexual selection has generated a positively allometric relationship between chelicerae length and body size not only in C. foliata but in nearly all Cambridgea species studied. This is in spite of my finding that chelicerae length is not a strong predictor of fight outcomes, compounded by evidence that natural selection is acting against exaggeration. I propose a relatively novel explanation for how positive allometry in male weaponry could arise under sexual selection via intrasexual contests and my multi-faceted approach to this group provides a more comprehensive view of the costs and benefits of weapon exaggeration than is afforded to most species.