The role of threats in animal cooperation

Abstract

In human societies, social behaviour is strongly influenced by threats of punishment, even though the threats themselves rarely need to be exercised. Recent experimental evidence suggests that similar hidden threats can promote cooperation and limit within-group selfishness in some animal systems. In other animals, however, threats appear to be ineffective. Here I review theoretical and empirical studies that help to understand the evolutionary causes of these contrasting patterns, and identify three factors-impact, accuracy and perception-that together determine the effectiveness of threats to induce cooperation.

Figure 1.

(a) Fitness surplus and bargaining set in two-player cooperative association. Both parties benefit from cooperation if they can negotiate a settlement u*, which lies in the bargaining set (the thick black line). In the case shown, the two parties are of equal ‘bargaining power’, so u* is located at p_A = p_B = 0.5. (b) Fitness payoffs accruing to player A (solid line) and player B (dotted line) as a function of the negotiated outcome u*.

Figure 2.

Effective and ineffective threats in nature. (a) The threat of eviction: in the coral-dwelling goby Paragobiodon xanthosomus subordinates adjust their growth to avoid triggering the threat of eviction by dominants [31,32]. (b) The threat of departure: studies of the cleaner fish Labroides dimidiatus and its clients suggest that the threat of departure can deter cleaners from ‘cheating’ (that is, feeding on client tissue rather than their ectoparasites [–51]). (c) The threat of attack: subordinates of the queenless ant Dinoponera quadriceps appear to be deterred from challenging the dominant female by the threat of attack from nestmates. Dominant females mark challengers (such as female 14 in this photo) with a chemical that singles them out for ‘immobilization’ by other workers [66]. (d) An ineffective threat: in banded mongooses Mungos mungo, dominant females limit reproductive competition by evicting subordinate females from the group, but females do not exercise pre-emptive reproductive restraint to avoid eviction [39]. In this species, evicted females are often permitted to return, and even non-breeders are sometimes evicted, two factors that reduce the effectiveness of the threat of eviction as a deterrent. Photos: (a) Marion Wong; (b) Maxi Eckes; (c) Thibaud Monnin; (d) Roman Fuller.

Figure 3.

Effectiveness of threats to enforce cooperation in multi-member groups. The figure shows the results of a three-player model in which a single dominant can evict one of the two subordinates if either of them defects (redrawn from [39]). The figure shows the zones for which the threat is effective versus ineffective at inducing cooperation, as a function of accuracy with which it is targeted and the cost of being evicted. The cost C of being evicted is measured relative to the benefit B of defecting but escaping eviction. The targeting parameter z varies from 0 (implying that subordinates are equally likely to be evicted whether they choose to cooperate or defect) to 1 (implying that a subordinate who cooperates is never evicted). The case where z = 1 applies to groups that exhibit a hierarchical structure, such that each individual monitors and punishes its immediate subordinate (labelled ‘chain of command’ in the figure). As the accuracy of targeting declines, the threat of eviction rapidly becomes ineffective at enforcing cooperation. See [39] for details of the model.