Producers, scroungers and foraging group structure

The producer-scrounger model of Barnard gr Sibly (1981, Anim. Behav., 29, 543-550) predicts that at a certain ratio of producers and scroungers in a foraging group the payoff curves for individuals of the two phenotypes intercept. At this point all the group members obtain equal payoffs. In this paper, the original model is elaborated further by allowing individuals to differ not only in their food-finding abilities but also in their abilities to compete for the food once it is found. As in Barnard & Sibly's original model foraging individuals are assumed to share information about the whereabouts of food patches and to benefit from patches found by others. Here it is determined under which combinations of searching and sharing characters food intake rate in mixed-phenotype groups would be equal for producing and scrounging individuals. In the present model the payoffs are much affected by both the finder's advantage (the fraction of prey available only to the finder of the food-patch) and the combination of the foraging characters of the two phenotypes. Generally the intake rate of producers increases with their presence in the group and with increases in the finder's advantage. Our model suggests that the equal payoffs arise only when the relative food-searching and competitive abilities of producers and scroungers fall within a narrow range of options acknowledging also the finder's advantage. Moreover, such combinations are predicted to be unstable if only foraging benefits are called for in group formation, this being a common feature of social foraging models incorporating foraging role asymmetries. (C) 1996 The Association for the Study of Animal Behaviour