Helping effort in a dominance hierarchy

In many cooperatively breeding species, group members form adominance hierarchy or queue to inherit the position of breeder.Models aimed at understanding individual variation in helpingbehavior, however, rarely take into account the effect of dominancerank on expected future reproductive success and thus the potentialdirect fitness costs of helping. Here we develop a kin-selectionmodel of helping behavior in multimember groups in which onlythe highest ranking individual breeds. Each group member caninvest in the dominant's offspring at a cost to its own survivorship.The model predicts that lower ranked subordinates, who havea smaller probability of inheriting the group, should work harderthan higher ranked subordinates. This prediction holds regardlessof whether the intrinsic mortality rate of subordinates increasesor decreases with rank. The prediction does not necessarilyhold, however, where the costs of helping are higher for lowerranked individuals: a situation that may be common in vertebrates.The model makes two further testable predictions: that the helpingeffort of an individual of given rank should be lower in largergroups, and the reproductive success of dominants should begreater where group members are more closely related. Empiricalevidence for these predictions is discussed. We argue that theeffects of rank on stable helping effort may explain why attemptsto correlate individual helping effort with relatedness in cooperativelybreeding species have met with limited success.