Group-size dependent punishment of idle subordinates in a cooperative breeder where helpers pay to stay

In cooperative breeding systems, dominant breeders sometimes tolerate unrelated individuals even if they inflict costs on the dominants. According to the ‘pay-to-stay’ hypothesis, (i) subordinates can outweigh these costs by providing help and (ii) dominants should be able to enforce help by punishing subordinates that provide insufficient help. This requires that dominants can monitor helping and can recognize group members individually. In a field experiment, we tested whether cooperatively breeding cichlid Neolamprologus pulcher subordinates increase their help after a forced ‘idle’ period, how other group members respond to a previously idle helper, and how helper behaviour and group responses depend on group size. Previously, idle helpers increased their submissiveness and received more aggression than control helpers, suggesting that punishment occurred to enforce help. Subordinates in small groups increased their help more than those in large groups, despite receiving less aggression. When subordinates were temporarily removed, dominants in small groups were more likely to evict returning subordinates. Our results suggest that only in small groups do helpers face a latent threat of punishment by breeders as predicted by the pay-to-stay hypothesis. In large groups, cognitive constraints may prevent breeders from tracking the behaviour of a large number of helpers.

在合作繁殖系统中，优势繁殖者有时会容忍非亲缘个体，即便这些个体对其造成代价。根据「付费居留（pay-to-stay）」假说，其一，从属个体可通过提供帮助抵消这些代价；其二，优势繁殖者应能够通过惩罚帮助行为不足的从属个体来强制其提供帮助，这要求优势繁殖者能够监控帮助行为并可单独识别群体成员。在一项野外实验中，我们针对合作繁殖的饰颊新亮丽鲷（Neolamprologus pulcher）开展了相关测试：检验从属个体在被迫「闲置」一段时间后是否会提升其帮助行为、其他群体成员会对此前处于闲置状态的帮助者作出何种反应，以及帮助者行为与群体反应如何随群体规模变化。结果显示，相较于对照组帮助者，此前处于闲置状态的帮助者会表现出更强的顺从性并遭受更多攻击，这表明惩罚确实用于强制提供帮助。尽管小群体中的从属个体遭受的攻击更少，但它们相较于大群体中的同类，提升帮助行为的幅度更大；当从属个体被暂时移除后，小群体中的优势繁殖者更有可能驱逐返回的从属个体。我们的研究结果表明，仅在小群体中，帮助者才会面临如「付费居留」假说所预测的、来自繁殖者的潜在惩罚威胁，而在大群体中，认知限制可能会阻碍繁殖者追踪大量帮助者的行为。