Risk allocation and competition in foraging groups: reversed effects of competition if group size varies under risk of predation

Abstract

Animals often feed more quickly when in larger groups. This group-size effect is often explained by safety advantages for groups but an alternative explanation is that animals feed faster in larger groups because of greater scramble competition for limited food. We show that predation risk enhances the group-size effect if groups vary in size. By contrast, competition leads to the group-size effect only when individuals feed in groups of constant size. When individuals feed in groups that vary in size, the best strategy for dealing with competition is to feed intensely when in smaller groups and feed little when in larger (more competitive) groups. In all situations, the effects of competition interact with the effects of predation risk in a simple multiplicative way. Our results suggest that scramble competition is not a general explanation for the group-size effect on vigilance in situations where group size changes relatively rapidly.

Foraging competition, vigilance and group size in two species of gregarious antelope

“…Such group dynamics have been suggested to affect the effect of group size on foraging rates, with a predicted decrease in foraging rates in larger groups when the group size is dynamic (Bednekoff & Lima 2004). However, group size has been reported as directly affecting foraging decisions in browsing impala (Fritz & De GarineWichatitsky 1996), suggesting that both foraging competition and foraging rates may indeed increase with increasing group size in this species.…”

Trophic Niche Width, Shift, and Specialization of Dascyllus aruanus in Toliara Lagoon, Madagascar

“…All these factors may enhance individual diet specialization, potentially reducing resource competition between individuals within groups (Svanbäck and Bolnick, 2005). Intra-group competition linked to group size (Bednekoff and Lima, 2004) and group density could lead to a high degree of specialization among individuals, which could ultimately affect individual body condition. Such interactions may also be greater among animals living within a limited home range, such as highly site-attached coral reef fishes.…”

Innovative problem solving in nonhuman animals: the effects of group size revisited

“…Although there has been a substantial effort to model the effects of group size on foraging behavior in nonhuman animals (Shaw et al 1995;Lima 1998a, 1998b;Giraldeau and Caraco 2000;Bednekoff and Lima 2004), to our knowledge, there have been no previous attempts to model the effects of group size on innovative problem solving. Here, we use a theoretical modeling approach to simulate how the likelihood of a group solving a hypothetical 1-action extractive foraging task varies with group size under different facilitation and inhibitory scenarios.…”

“…First, if the presence of other individuals allows for antipredator vigilance to be shared, as has been found in the context of foraging behavior (Elgar 1989;Beauchamp and Livoreil 1997;Beauchamp 1998;Lima and Bednekoff 1999;Beauchamp and Ruxton 2003;Bednekoff and Lima 2004), then problem-solving efficiency should increase in the presence of other individuals. Second, if the presence of other individuals socially facilitates approach and exploration, either via reduced neophobia or increased scramble competition, then individuals should also have higher solving probabilities in the presence of others (Coleman and Mellgren 1994;Visalberghi et al 1998;Visalberghi and Addessi 2000).…”