Abstract
A major aim in evolutionary biology is to understand altruistic help and reproductive partitioning in cooperative societies, where subordinate helpers forego reproduction to rear dominant breeders' offspring. Traditional models of cooperation in these societies typically make a key assumption: that the only alternative to staying and helping is solitary breeding, an often unfeasible task. Using large-scale field experiments on paper wasps (Polistes dominula), we show that individuals have high-quality alternative nesting options available that offer fitness payoffs just as high as their actual chosen options, far exceeding payoffs from solitary breeding. Furthermore, joiners could not easily be replaced if they were removed experimentally, suggesting that it may be costly for dominants to reject them. Our results have implications for expected payoff distributions for cooperating individuals, and suggest that biological market theory, which incorporates partner choice and competition for partners, is necessary to understand helping behaviour in societies like that of P. dominula. Traditional models are likely to overestimate the incentive to stay and help, and therefore the amount of help provided, and may underestimate the size of reproductive concession required to retain subordinates. These findings are relevant for a wide range of cooperative breeders where there is dispersal between social groups.
Highlights
Altruistic helping behaviour occurs throughout the animal kingdom despite costs to helpers’ direct fitness
A range of factors has been identified to explain the evolution and maintenance of this phenomenon, including both direct fitness benefits, such as inheritance of the breeding position [3,4], and indirect fitness benefits obtained through helping a relative [5,6]
Traditional models predicting the level of help and reproductive skew in cooperative breeders often make a key assumption: that a subordinate helper’s only alternative to staying and helping in its current group is to leave and breed solitarily [13,14,15,16,17,18]
Summary
Altruistic helping behaviour occurs throughout the animal kingdom despite costs to helpers’ direct fitness. Future studies are encouraged to include the following: partner choice rather than partner control, where sanctioning of uncooperative partners is replaced by partner switching [12,23 –25]; outside options beyond solitary breeding [9,10]; asymmetric relationships where the exchange of behaviours is more valuable for one of the parties [21,26,27]; and n-player interactions not achievable in traditional 2-player cooperative games [21,28] These modifications can be achieved by invoking biological market models [21]. If helpers have alternative options available that offer higher fitness payoffs than solitary breeding, the incentive to stay and help in their current groups may previously have been overestimated. We ask the following questions. (i) Do available nesting options include high-payoff alternatives? Alternative options will affect the predictions of existing models only if they offer a higher payoff than solitary nesting. (ii) Do alternative options differ from observed choices in ways that should affect direct and indirect fitness, such as inheritance rank and relatedness to the dominant? We predict that alternative options are inferior to observed choices: in a biological market, individuals are expected to assess their options and make the choice that offers the highest payoff [10]. (iii) Is it costly for dominants to reject an additional cooperative partner? We expect help to be in high demand because productivity and group survival increase with the number of helpers in P. dominula [3,22], so we predict that rejecting a joiner represents a cost to dominant breeders
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