The Evolution of Cooperative Hunting

Using a series of game-theoretical models, we develop two major predictions concerning the evolution of cooperative hunting. First, we specify the conditions under which individuals should hunt in groups rather than solitarily. When a group captures only a single prey per hunt, the expected benefits from cooperation rarely outweigh the advantages of hunting alone, since the prey must be divided between group members. Cooperation to capture the same prey can confer sufficient mutual benefit only when solitaries have low foraging efficiency; otherwise, each individual would do better to hunt separate prey. In contrast, species that capture multiple prey in a single hunt are not faced with similar disadvantages from grouping and should not be similarly constrained to hunt alone by high individual foraging efficiency. However, these predictions are not well supported by comparisons of individual hunting success in species that hunt alone and in groups. Consistent with previous reviewers, we consider that cooperative hunting in many species is more often a consequence of gregariousness than its evolutionary cause. However, cooperative hunting does often appear to be an important cause of grouping in species that take multiple prey. Second, we specify the conditions under which group members should hunt cooperatively. A temptation to cheat is prevalent in species that capture prey large enough to be scavenged by noncooperative companions. Cheaters can thereby avoid the costs of hunting and prey capture. Cooperation is most likely when an individual has a low probability of capturing a large prey by itself. Under these conditions, a second hunter can sufficiently increase the pair's chances of capturing the prey to overcome his own costs of participating in the hunt. However, the temptation to cheat increases with group size, since each additional hunter is decreasingly likely to overcome his costs of hunting. In contrast, there is no temptation to cheat when prey size is so small that only the successful hunter can feed from the captured prey. Data from 28 studies of group hunting generally conform to these predictions. Group members always cooperate when hunting small prey. When groups hunt large prey, cooperation mostly occurs when solo hunting success is very low, whereas cheating appears to be common when solo hunting success is high and group size is large. Although repeated hunting of a single large prey often conforms to the conditions of the iterated prisoner's dilemma, we have little evidence that cooperation has evolved from a "tit-for-tat" hunting strategy. We specify the theoretical effects of kinship, individual differences in hunting ability, and behavioral dominance on the evolution of cooperative hunting of single large prey. We show that "tit-for-tat" is unlikely to be found where there are extreme individual differences in hunting ability.