Bateman's principle in cooperatively breeding vertebrates: The effects of non-breeding alloparents on variability in female and male reproductive success

The sex-specific slopes of Bateman's gradients have important implications for understanding animal mating systems, including patterns of sexual selection and reproductive competition. Intersexual differences in the fitness benefits derived from mating with multiple partners are expected to yield distinct patterns of reproductive success for males and females, with variance in direct fitness predicted to be greater among males. These analyses assume that typically all adults are reproductive and that failure to produce offspring is non-adaptive. Among some species of cooperatively breeding birds and mammals, however, non-breeding adult alloparents are common and may comprise the majority of individuals in social groups. The presence of a large number of non-breeding adults, particularly when coupled with greater social suppression of reproduction among females, may alter the relative variance in direct fitness between the sexes, thereby generating an apparent contradiction to Bateman's Paradigm. To explore quantitatively the effects of non-breeding alloparents on variance in reproductive success, we used genetic estimates of parentage and reproductive success drawn from the literature to calculate the relative variability in direct fitness for females and males in alloparental and other societies of birds and mammals. Our analyses indicate that in mammals and, to a lesser extent, in birds, variability in direct fitness is greater among females in species characterized by the presence of non-breeding alloparents. These data suggest that social interactions, including social suppression of reproduction, are powerful determinants of individual direct fitness that may modify sex-specific patterns of reproductive variance from those described by Bateman.