Towards a unified theory of cooperative breeding: the role of ecology and life history re-examined

We present quantitative models that unify several adaptive hypotheses for t he evolution of cooperative breeding in a single framework: the ecological constraints hypothesis, the life-history hypothesis and the benefits-of-phi lopatry hypothesis. Our goal is to explain interspecific variation in the o ccurrence of cooperative breeding in terms of interspecific variation in li fe-history traits and ecological conditions. We analyse two models, accordi ng to whether or not helpers can inherit their parents' territory. Major re sults are (i) territory inheritance always promotes cooperative breeding; ( ii) if territories are not inherited, neither ecological constraints nor va riation in life-history traits predict interspecific variation in cooperati ve breeding; and (iii) if territories are inherited, the mechanism of densi ty regulation is crucial in determining which factors promote cooperative b reeding. If density dependence acts on the probability to obtain a free ter ritory or on the survival of dispersers, variation in ecological constraint s cannot explain variation in cooperative breeding. Lower adult mortality f avours helping, not because it reduces the availability of free territories , but because it enhances the direct benefits of helpers. If density depend ence acts on fecundity, lower probability of obtaining a free territory and lower survival of dispersers promote cooperative breeding. In this case, l ower adult mortality works against the evolution of helping. We suggest tha t the difference between birds and social insects in the covariance between cooperative breeding and life-history traits is due to different mechanism s of density regulation that operate in these taxa, and we explain how natu ral selection on habitat choice might have caused these different mechanism s to operate.