Abstract

Group size has been shown to provide survival benefits across different species, particularly in cooperative breeders. However, group formation can also lead to reproductive conflict, especially when resources for raising offspring are limited. This conflict has often been studied as a struggle between dominant and subordinate individuals, with debates on whether subordinate reproduction results from negotiation or a lack of control by dominants. Yet, the ecological context where dominants are incentivized to allow others to reproduce remain underexplored. Additionally, accepting immigrants can increase group size and group member survival, but may heighten reproductive competition. In turn, reproductive concessions may serve as incentives for dispersers to join a group, influenced by supply-demand dynamics in a biological market fashion. We developed a stochastic theoretical model to explore ecological conditions favouring low reproductive skew and/or immigration acceptance. The results suggest that plural breeding species, i.e. cooperative breeding species characterized by low reproductive skew, likely evolved in harsh, fluctuating environments, where individuals gain greater fitness benefits by mitigating group extinction risks through shared reproduction and immigration rather than monopolizing breeding opportunities. Reproductive concessions evolve in highly adverse environments even in the absence of relatedness structure, but kin selection benefits allow these concessions to evolve across a broader range of ecological landscapes. The model also shows that high-relatedness groups tend to discourage immigrant acceptance, preferring to share reproduction among kin. Furthermore, alloparental care to breeders' offspring facilitates immigration acceptance while hindering the evolution of reproductive concessions. Combining the model results with a comparative study, we demonstrate how harsh fluctuating environments, that are expected to become more common in the future, influence the evolution of plural breeding by shaping reproductive strategies to enhance inclusive fitness benefits and reduce group extinction risks.