MODELS OF STATUS-CORRELATED BIAS IN OFFSPRING SEX-RATIO

We study a simple model describing the genealogic structure of a population in which the sex ratio of offspring varies with a parent's position in a social hierarchy. This model is based on species in which one sex disperses and in which rank in a stable social hierarchy for the other sex is "inherited" by the offspring of that sex. We find that biased production of offspring in such a social system has important implications for group-size regulation, for the genetic relationships among individuals at any given time and across generations, and for the disparity between the social environment experienced by a parent and its offspring. The general model predicts that group size is tightly regulated and that group size is unstable in groups with underproduction by high-ranking mothers. We then further examine the particular case of the model in which the nondispersing sex (females) is overproduced by higher-ranking parents. In this case, we find that females are unusually closely related; all females in a group have a common female ancestor 5-10 generations back. Moreover, daughters of mothers in the middle of the hierarchy experience the largest intergenerational disparity. In contrast to daughters of high- and low-ranking females, they occupy ranks much lower than their mothers'.