Competitive foraging in the social spider Stegodyphus dumicola

Reproductive skew models have greatly enhanced the study of sociality but are applicable only to societies where the aim of the members of the group is to reproduce. In ‘foraging societies’, where the aim of the members of the group is to grow, quite different parameters will govern the form that the societies take. We examined factors that influence the structure of foraging groups in the social spider Stegodyphus dumicola (Eresidae). In particular, we examined food distribution and consumption within groups, and how this related to predictions derived from two models. One model indicates that colonies in which individuals compete for resources via interference competition are more likely to survive than those in which individuals divide resources using scramble competition. The second model predicts the proportions of hunters, cheaters and scavengers expected in foraging groups. We found that food was not distributed evenly among group members, and that spiders that fed primarily on the head and thorax of the prey during the middle of a feeding event gained the most body mass. Spiders even lost mass if they fed only in the last hour of a foraging event. Large spiders had a competitive advantage (via interference competition) in obtaining preferred positions, and could ingest food faster than small spiders. Distributing food among colony members in this manner could cause large size differences between colony members, as predicted by the models. The implications of competitive foraging for sociality are discussed.