Fitness consequences of group living and the loss of dispersal in spiders

The evolutionary transition from solitary to group living in spiders implies the progressive loss of natal dispersal behaviour. This process is likely mediated by the fitness consequences of two interrelated factors: the "choice" that individuals make to stay or disperse from their natal group and, as a result of this, the group size they experience at different life stages. In this study I investigated the consequences of these factors in three components of female lifetime reproductive success in the social spider Anelosimus guacamayos (Araneae: Theridiidae). By regularly recording changes in spider counts and nest proliferation events in a total sample of 105 naturally occurring colonies, my collaborators and I found: (1) an overall higher survival probability for philopatric females relative to emigrants, (2) some survival improvement related to group size for philopatric females living in colonies smaller than ~20 individuals, and (3) an increase in the offspring survival probability, mainly associated with group size rather than being merely related with philopatry or dispersal. We found no effect of either dispersal or group size on the probability of female reproduction. Despite the fitness costs of dispersal, its occurrence in group living spiders, such as A. guacamayos, is best explained by the negative density-dependent effects of group living. On the other hand, the rapid improvement in offspring survival is a straight forward explanation for the evolution of group living because it does not require active cooperation between the spiders staying in their natal nest. Because mother spiders sometimes die before their offspring are self-sufficient, the first benefit of group living is most likely a geometric decrease for each adult that is added to the group in the chances of ending up in a nest with dependent offspring but no adults.Science, Faculty ofZoology, Department ofGraduat