Does the resource dispersion hypothesis explain group living ?

Whether animals live in groups can be influenced by numerous direct benefits, such as cooperative hunting, predator avoidance or alloparental care, as well as by direct disadvantages, such as elevated parasite burdens or infanticide risk [1]. Here, we focus instead on an alternative factor, namely, how the availability of RESOURCES (see Glossary) in the environment influences grouping behavior. Species from across the animal kingdom commonly partition available space to defend exclusive TERRITORIES, the size and shape of which represent some economic optimum [2,3]. These cannot be constructed at random, however, because resources (such as food, water and shelter) do not occur randomly in space. Behavioral ecologists have long recognized a relationship between social behavior and the distribution and predictability of resources [4–7], and increasing empirical evidence points to resource-based explanations of social organization in a variety of species [8–13]. However, there are few well-developed general models that explicitly consider these factors [14–16]. Increases in overall resource abundance leads to increases in habitat quality per unit area, thus resulting in higher animal densities, and typically smaller ranging areas [13,17]. Resource abundance in itself does not necessarily affect group size, however, because rich territories are typically contracted (or split), such that individuals maintain the former per-capita intake rate [18]. But if resources are HETEROGENEOUS in space or time, then this is no longer necessarily true – proportional increases in average resource abundance might not enable the territory to shrink, because larger areas are still needed to encompass the temporal and spatial variability of these resources. Thus, it is the pattern of resource availability, in both space and time, that influences group size [7,10,19]. Therefore, we need specific theories to model how this occurs, and to predict relationships between resource distribution and social organization. Here, we discuss one such theory, the resource dispersion hypothesis (RDH). The RDH is a hypothetical model that suggests that, where resources are patchily distributed over space and/or time, the economics of exploiting these PATCHES enable several individuals to share resources over a common area, satisfying their resource needs without imposing large costs on each other (Box 1; Fig. 1). The basic idea is that, even a single animal using patchy resources will have to defend a large enough area to be sure that, with some CRITICAL PROBABILITY, at least one ‘ripe’ patch will be available to satisfy its resource requirements. According to the RDH, given a certain PATCH RICHNESS and DISPERSION, this same area defended by the original resident (PRIMARY ANIMAL) is predicted to have an excess of Does the resource dispersion hypothesis explain group living?