A major goal of evolutionary research is to elucidate the processes involved in the evolution of group versus solitary living, by examining the selective forces driving or constraining a particular type of social system. Species with intermediate group living tendencies are particularly interesting because they offer insight into evolutionary transitions and, in particular, how different selective environments may modulate group living behaviours. This dissertation explores the different factors that shape the social strategies of the western black widow spider, Latrodectus hesperus, a facultatively group-living spider. Over a period of four years, I characterized the social structure of coastal British Columbia populations of L. hesperus, and show that individuals live either solitarily or in groups depending on the time of year, their reproductive status, and age. I then used an experimental approach to investigate the dynamics and adaptive value of facultative group-living behaviours, by testing different hypotheses about the decisions involved in social interactions, web building, microhabitat settlement, movement, and foraging. Several factors were manipulated and shown to influence the patterns of group living in L. hesperus, including individual nutritional state, prey availability, group size, population density, and neighbour proximity. Spiders adjusted their group living behaviours according to changes in these factors, and responded strategically to the presence and proximity of conspecifics. Based on the results of these experiments, I developed a model of group living in spiders that specifically considers the dynamic and strategic nature of social interactions in the context of frequency-dependent selection. The research presented in this thesis furthers our understanding of the evolution of social behaviour by providing new evidence on the mechanisms that promote and regulate facultative group-living behaviours.
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