The evolution of parental and alloparental effort in cooperatively breeding groups: when should helpers pay to stay?

We used a reproductive skew framework to consider the evolution of parental and alloparental effort in cooperatively breeding groups. The model provides the first theoretical treatment of rent payment (the "pay-to-stay" hypothesis) for the evolution of helping behavior of subordinates. According to this hypothesis, not all helping behavior is kin selected, but group members help in order to be allowed to stay in the group and potentially gain breeding positions later in life. We show that reproductive concessions may be replaced by complete skew and voluntary, costly alloparental effort by subordinates once future prospects are included in fitness calculations. This suggests that incomplete skew observed in long-lived species is not due to dominant control over reproduction. Rent payment is predicted to occur when relatedness between subordinate and dominant is low, survival is high, ecological constraints are at least moderately tight, and retaining nonhelping subordinates harms the dominant's fitness. Rent may also be required from related subordinates if helping is very costly (leading to low voluntary helping effort) and ecological constraints are moderately tight. However, related subordinates do not need to have a positive net effect on the dominant's direct fitness to be accepted as group members. We also consider compensatory responses of dominant group members as a potential threat to the stability of renting behavior. If dominants trade off parental effort against their own survival, they may selfishly reduce their own parental effort as a response to increased help. As this improves their own survival, the prospects of territorial inheritance diminish for the subordinate, and subordinates should hence be less willing to accept the rent agreement. However, we show that compensatory responses by "lazy" parents prevent group formation only in borderline cases.