Punishment controls helper defence against egg predators but not against fish predators in cooperatively breeding cichlids

Helping behaviour in some cooperative breeders is apparently maintained by a combination of coercion and reciprocity. In such pay-to-stay systems, alloparental brood care of subordinate group members functions as a service to dominants, which tolerate subordinates based on how much help they provide. Cooperative territory defence is a key task of cooperative breeders, but it is unknown how territory defence by subordinates is socially regulated. Diverse costs and benefits associated with defending the territory against different threats suggest that these defence behaviours may be maintained through divergent selection regimes, and they might be regulated through different social processes. In the cooperatively breeding cichlid fish Neolamprologus pulcher, unrelated subordinates help defending the territory against egg predators even if they do not participate in reproduction and therefore do not suffer direct or indirect fitness costs through predators of eggs. This behaviour has therefore been interpreted as altruistic. Subordinates also defend the group territory against predators of juveniles and adults, which might at least partly reflect their own direct fitness interests and could be maintained through mutualistic interactions among group members. Here, we directly compared the regulation of these two types of defence behaviours and tested whether they are enforced by breeders. We prevented subordinates from defending the territory against egg-predators or predators of adults and observed whether they received more aggression in response to this treatment. We found that subordinates received more aggression from breeders after withheld defence against egg-predators, but not after withheld defence against fish predators. This suggests that territory defence against egg-predators by helpers is enforced by breeders and hence subject to negotiations and trading, whereas defence against fish predators is likely based on mutualistic fitness benefits.

部分合作繁殖（cooperative breeding）物种的帮助行为，显然是通过胁迫与互惠的共同作用得以维持。此类“付费居留”（pay-to-stay）系统中，从属群体成员的异亲育幼（alloparental brood care）行为可作为对优势个体的服务，优势个体将依据从属个体提供的帮助程度容忍其留在群体中。合作性领地防御是合作繁殖物种的核心任务之一，但目前尚不清楚从属个体的领地防御行为如何受到社会调控。针对不同威胁开展领地防御所涉及的多样成本与收益，表明这类防御行为或通过分化选择机制（divergent selection regimes）得以维持，且可能受不同社会过程调控。在合作繁殖的丽鱼科鱼类新亮丽鲷（Neolamprologus pulcher）中，与其无亲缘关系的从属个体即便未参与繁殖，不会因卵捕食者承受直接或间接的适合度（fitness）成本，仍会协助领地防御以抵御卵捕食者，因此该行为被视作利他行为（altruism）。从属个体还会防御群体领地，抵御捕食幼体与成体的捕食者，这类行为至少部分契合其自身的直接适合度利益，或可通过群体成员间的互利互作（mutualistic interactions）得以维持。本研究直接对比了这两类防御行为的调控机制，并检验它们是否由繁殖个体施加约束。我们通过实验阻止从属个体分别抵御卵捕食者或成体捕食者，并观察它们是否会因该处理行为遭受更多攻击。实验结果显示，当从属个体未参与抵御卵捕食者时，会遭受来自繁殖个体的更多攻击；而未参与抵御鱼类捕食者时则无此现象。这表明，协助者对卵捕食者的领地防御行为由繁殖个体强制约束，因此涉及群体内的协商与利益交换；而针对鱼类捕食者的领地防御，则大概率基于互利的适合度收益。