Relatedness Asymmetry and Reproductive Sharing in Animal Societies

We have shown (Reeve and Keller 1995) that reproduction i social groups is generally shared more equally in sister-sister groups than in mother-daughter associations, and we proposed a model based on the relatedness asymmetry between group members to account for this pattern. Emlen (1996) endorses our model but raises the possibilities that the model might not apply to several types of vertebrate familial societies, that the well-documented dominance of parents over offspring confounds the prediction of our model, and that greater skews in mother-daughter associations may reflect greater avoidance of incestuous mating in these associations. Below we consider these points in turn. Our model examines the consequences of females having asymmetric relatedness to each other's offspring (as will arise in mother-daughter associations) in the simple case in which females have a single mate. However, as Emlen points out, extrapair fertilization is common in vertebrates, with the effect of decreasing the relatedness among siblings. If a fraction f of the brood is sired by another male (unrelated to the parental male or female), the average relatedness between siblings is