Resource Consumption Variance Within and Among Individuals: On Coloniality in Spiders

Abstract

Several models for the economics of sociality demonstrate that the probability distribution of an individual's resource consumption could depend on the size of its social group. When groups share food clumps discovered by any member, expected resource consumption may increase or decline with group size, but the individual's resource consumption variance will ordinarily decline as group size increases. Hence, the formation and dissolution of social groups may represent risk—sensitive responses to foraging success. A series of field studies report greater mean prey consumption and decreased prey consumption variance as group size in colonial spiders increases. Several hypotheses attempt to explain this effect. Coloniality may promote acquisition of information concerning temporal variation in prey availability. Colony membership may permit individuals to steal food when the number of captured prey varies spatially. Finally, colonality can enhance food acquisition because prey (that might otherwise escape) ricochet from one spider's web to another group member's web. Our purpose was to quantify each hypothesis and ask if the resulting models predict reduced resource consumption variance as group size increases. We model each mechanism and conclude that stealing prey is the simplest explanation for variance reduction in spider colonies. We emphasize that variance among individuals need not provide a good estimate of the within—individual resource consumption variance, and it is the latter variance to which risk sensitivity responds.