Male Contest Dynamics in New Zealand Sheetweb Spiders (Cambridgea plagiata)

Abstract

Sexual selection has repeatedly driven the evolution of exaggerated ornaments and weaponry that are used to compete for access to mates. Competition for mates often takes the form of violent contests that may have major consequences such as injury and even death. A variety of traits can determine the outcome of such interactions, and males must develop strategies to make decisions on whether to persist or retreat from a given contest. Spiders are considered a model system to study contest dynamics, but current contest literature is biased towards jumping spiders (Salticidae), a unique group in part due to their excellent vision and diurnal lifestyle. Additionally, local population dynamics for spiders are often overlooked as most studies stage contests within a laboratory setting. While beneficial in allowing fine scale control, it raises questions on how applicable these results are to behaviours occurring in the wild. The endemic sheetweb spider Cambridgea plagiata provides a novel study species to examine sexual selection and is ideal for studying contest dynamics, as males exhibit enlarged chelicerae that are utilized as weapons during contests over access to females. In this study, I firstly examine contest dynamics by staging contests between males in a wild population to determine what traits predict the winner of a contest, as well as how rival males assess each other. Secondly, I investigated the allometry of various morphological traits to examine the evolutionary consequences of sexual selection within C. plagiata. Finally, I used population surveys in order to collect relevant ecological data comparing with C. foliata - and examined population dynamics through time, adding a broader context to the competitive landscape in which these contests occur. Staged contests provided evidence that chelicerae size is the key predictor for the outcome of a fight, and that the relative difference between rival chelicerae size determined contest duration. My results could not unambiguously distinguish between self and mutual assessment models of contest behaviour for this species. Morphological analyses confirmed that males exhibit positive allometry for chelicerae length, and additionally showed compensation in their forelegs to potentially offset the costs of relatively large weaponry. Surprisingly, population surveys found that the local population is consistently female-biased over the breeding season, but that males also exhibited precopulatory guarding of females. I additionally found key ecological differences between C. plagiata and C. foliata, including substrate type, prey type, and web size. This research is valuable by not only contributing the first ecological, behavioural and morphological data for this data deficient genus, but by also adding phylogenetic diversity to current spider contest literature. As C. plagiata is both a web-building and nocturnal species (and is therefore likely not to be reliant on vision), conclusions drawn from this research may be useful in generalizing more broadly to other spider species than the current literature on jumping spiders.