Nepotism mediates enforced cooperation in asymmetric negotiations
Abstract
Cooperative breeding is the source of the most complex social organisations known among animals. In cooperative breeders, dominant and subordinate group members typically exchange different commodities among each other, which involves an incessant negotiation process. For example, helpers may trade alloparental brood care against safety and resource access in the dominants’ territory. The crucial question is how in this continual bargaining process the conflict of fitness interests is resolved between the unequal partners, so that maintaining the cooperative interaction is optimal for all parties involved. Relatedness between breeders and helpers can alleviate the conflict of fitness interests between them, but evidence is accumulating that direct fitness benefits are pivotal for the evolution of such social systems. To evaluate the relative importance of direct and indirect fitness effects, here we experimentally disturb the negotiation process between dominant breeders and related or unrelated subordinates in a cooperative cichlid by simulating transgression from the helpers while allowing or preventing the ability of breeders to respond to this dereliction of duty in a full factorial design. Our results show that coercion by breeders is crucial for the performance of alloparental egg care by subordinate helpers, but that kinship reduces the importance of coercion as predicted by theoretical models. By experimentally manipulating both the behaviour of all involved parties and their responses to each other, we unravelled the interaction between the most fundamental selection mechanisms responsible for the evolution of complex social systems.