Competitive foraging in the social spider Stegodyphus dumicola

Reproductive skew models have greatly enhanced the study of sociality but a re applicable only to societies where the aim of the members of the group i s to reproduce. In 'foraging societies', where the aim of the members of th e group is to grow, quite different parameters will govern the form that th e societies take. We examined factors that influence the structure of forag ing groups in the social spider Stegodyphus dumicola (Eresidae). In particu lar, we examined food distribution and consumption within groups, and how t his related to predictions derived from two models. One model indicates tha t colonies in which individuals compete for resources via interference comp etition are more likely to survive than those in which individuals divide r esources using scramble competition. The second model predicts the proporti ons of hunters, cheaters and scavengers expected in foraging groups. We fou nd that food was not distributed evenly among group members, and that spide rs 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 the y fed only in the last hour of a foraging event. Large spiders had a compet itive advantage (via interference competition) in obtaining preferred posit ions, and could ingest food faster than small spiders. Distributing food am ong colony members in this manner could cause large size differences betwee n colony members, as predicted by the models. The implications of competiti ve foraging for sociality are discussed. (C) 1999 The Association for the S tudy of Animal Behaviour.