The evolution of cooperative breeding through group augmentation

Some individuals (helpers) in cooperatively breeding species provide allopa rental care and often suppress their own reproduction. Kin selection is cle arly an important explanation for such behaviour, but a possible alternativ e is group augmentation where individuals survive or reproduce better in la rge groups and where it therefore pays to recruit new members to the group. The evolutionary stability of group augmentation is currently disputed. We model evolutionarily stable helping strategies by following the dynamics o f social groups with varying degrees of subordinate help. We also distingui sh between passive augmentation, where a group member benefits from the mer e presence of others, and active augmentation, where their presence as such is neutral or harmful, but where helping to recruit new group members may still be beneficial if they in turn actively provide help for the current r eproductives ('delayed reciprocity'). The results show that group augmentat ion (either passive or active) can be evolutionarily stable and explain cos tly helping by non-reproductive subordinates, either alone or leading to el evated help levels when acting in concert with kin selection. Group augment ation can thus potentially explain the weak relationships between relatedne ss and helping behaviour that are observed in some cooperatively breeding s pecies. In some cases, the superior mutualistic performance of cooperativel y behaving groups can generate an incentive to stay and help which is stron g enough to make ecological constraints unnecessary for explaining the stab ility of cooperatively breeding groups.