Avoiding the tragedy of the commons: Improved group-feeding performance in kin groups maintains foraging cooperation in subsocial Stegodyphus africanus spiders (Araneae, Eresidae).

Cooperation involving shared resource systems is prone to 'the tragedy of the commons', where individuals act in their own self-interest to exploit the resource in a manner that is detrimental to the common good of all group members. Directing cooperation towards kin provides a solution to this problem and predicts the differential performance depending on the relatedness of group members. In subsocial spiders, juveniles live in transient groups that cooperate in hunting and communal feeding. Prey capture is costly in terms of risk of injury and investment of venom and digestive enzymes, and therefore presents a situation where individuals may attempt to avoid costly interactions and exploit the resource acquired by other group members. We tested the prediction that individuals differentiate participation and/or investment in cooperative prey capture and extra-oral digestion (injection of digestive enzymes into prey prior to the initiation of extraction of nutrients) in response to the relatedness of group members with whom they interact, in the subsocial spider Stegodyphus africanus. The performance of groups and interactions over prey attack in groups of either related or mixed kin spiderlings were determined over a period of 4 weeks. We show that kin groups attack the prey significantly faster, recruit individuals to form feeding groups faster, extract prey body mass more efficiently and experience less antagonistic interactions than groups of mixed relatedness, which ultimately translates into an elevated growth rate. These results indicate that related individuals are more willing to take risks and invest in communal digestion when foraging with kin, as predicted by inclusive fitness theory as a solution to the tragedy of the commons.