The influence of siblings on body condition in a social spider: is prey sharing cooperation or competition?

Siblings living together compete with each other for resources, yet they may also cooperate to maximize their inclusive fitness. In social spiders, siblings share prey and may both compete and cooperate to obtain this resource. In the laboratory, the social huntsman spider, Delena cancerides, readily shares prey captured by other colony members; however, these spiders only occasionally share prey in the field, making the importance of prey sharing to their social system difficult to assess directly. We examined the importance of prey sharing indirectly by measuring the body condition of spiders from 90 colonies at the time of collection. We compared body condition to colony demographics to determine whether the patterns were consistent with the hypothesis that younger spiders benefit from sharing prey captured by older siblings. We tested several alternative hypotheses that might also explain associations between condition and the presence of siblings. We further conducted a laboratory experiment to directly determine whether feeding on prey captured by older siblings improves the condition of younger spiders. Younger spiders collected from the field were heavier in the presence of older siblings, but there was no effect for older spiders or for any spider with younger siblings. Laboratory spiders gained access to additional prey captured by older siblings. We rejected the alternative hypotheses and concluded that younger spiders indeed benefit from the presence of older siblings. This system provides evidence that the exploitation of others' resources can provide a benefit of group living and act as a form of cooperation.