Abstract
Life abounds with examples of conspecifics actively cooperating to a common end, despite conflicts of interest being expected concerning how much each individual should contribute. Mathematical models typically find that such conflict can be resolved by partial-response strategies, leading investors to contribute relatively equitably. Using a case study approach, we show that such model expectations can be contradicted in at least four disparate contexts: (i) bi-parental care; (ii) cooperative breeding; (iii) cooperative hunting; and (iv) human cooperation. We highlight that: (a) marked variation in contributions is commonplace; and (b) individuals can often respond positively rather than negatively to the contributions of others. Existing models have surprisingly limited power in explaining these phenomena. Here, we propose that, although among-individual variation in cooperative contributions will be influenced by differential costs and benefits, there is likely to be a strong genetic or epigenetic component. We then suggest that selection can maintain high investors (key individuals) when their contributions promote support by increasing the benefits and/or reducing the costs for others. Our intentions are to raise awareness in—and provide testable hypotheses of—two of the most poorly understood, yet integral, questions regarding cooperative ventures: why do individuals vary in their contributions and when does cooperation beget cooperation?
Highlights
Fitness is enhanced by forwarding more gene copies to following generations than conspecifics from the same population [1,2]
Individual contributions to cooperation vary dramatically across taxa and increases in either the number or contributions of investors can be associated with an unexpected increase in the contribution of other investors
Our primary aim using a broad, selective, case study approach is to show that examples of each are neither limited nor exceptional
Summary
Fitness is enhanced by forwarding more gene copies to following generations than conspecifics from the same population [1,2]. McDonald et al [51] reported positive responses to targeted playback-induced increases in provisioning rates of unrelated helpers and breeding males, suggesting positive matching Together, these studies hint at intriguing complexity in how individuals respond to each other’s investments in cooperation and highlight a pressing need to conduct further targeted experiments in conjunction with appropriate analytical techniques [52,53], in order to understand the stark among-individual variation in contributions to ‘public goods’ in cooperative breeders. Among the pastoralist Nyangatom of southwest Ethiopia, membership of small-scale raids is variable with a small number of men participating in the majority of raids [74] These studies reveal substantial inter-individual variation in contributions to a particular kind of cooperation in which potential costs and benefits are high. The apparent ubiquity of leaders cross-culturally, suggests they can function to motivate other individuals to contribute more to conflict
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