Group foraging

Group or social foraging is foraging that is cohesive with individuals remaining close to one another while foraging, generally exhibiting collective movements (e.g., flock, herd, school). Such group foraging is widespread, but patchy, across the animal world.

Group foraging involves decisions made by individual animals, some being the same as decisions made by solitary foragers but in a group context, others being unique to group foraging. Decisions unique to group foraging include group membership (i.e., whether to forage solitarily or as part of a group and whether to transfer from one group to another), scrounging for food vs producing one’s own, and scanning for potential predators. We focus on the first two of these decisions, but do not include scanning as this decision has so far received little attention.

Because the outcomes of decisions made by individual members of foraging groups depend on actions taken by other group members, foraging group behavior should be viewed as a ‘game’ that reaches an equilibrium when all individuals adopt the same strategy and no individual can do better by deviating from it. In this sense this strategy can be viewed as optimal.

Attempts to understand group membership have generally focused on group size and hypothesized that it will maximize some individual benefit such as rate of food intake per individual or anti-predation. However, this approach is theoretically unsound as net individual benefit will include multiple combined benefits and costs and, at equilibrium, net individual benefits should be equal across different group sizes. It also fails empirically as the max benefit group size generally differs from the observed average group size. On the other hand, when a game-theoretic approach has been taken, good agreement has resulted between expected and observed group size.

A game-theoretic approach has been successfully applied to the ‘producer-scrounger’ game. It has, for example, predicted that the proportion of individuals playing ‘scrounger’ should increase with increasing group size and decrease with finder’s advantage, in agreement with observations.

To increasingly understand group foraging behavior, we should maintain the game theoretic approach, but recognize the distinction between evolutionarily and behaviorally stable strategies, and focus on the behavioral and cognitive mechanisms involved.