Tests of reproductive skew theory: a review and prospectus

Abstract Reproductive skew refers to the unequal partitioning of reproduction by same-sex members of a social group. The concession, restraint and tug-of-war skew models attempt to predict which social and ecological factors influence skew, whilst assuming different power dynamics and making contrasting predictions. The factors underpinning within-species variation in skew remain unclear, despite decades of research. Individuals vary in both the benefit of leaving the group to breed (outside option) and in the contribution they make to group productivity if they stay. Variation is likely among several aspects of individual quality, so that individuals and species may show different associations between the subordinate's outside option and group productivity. Yet, no existing skew models allow for variation in this association, here named the quality-productivity coefficient (QPC). If high subordinate quality is associated with being a good helper, for instance if both help and quality improve with age, the QPC would be positive. If, instead, there is a trade-off or other contrast between breeding ability and helping effect, then the QPC will be negative. Here, we incorporate the QPC and derive new predictions for the concession, restraint and tug-of-war models, that sometimes changed their predictions. We review the empirical tests of the association of skew with relatedness, dominant’s competitive advantage, group productivity and subordinate’s outside option. From 16 studies available to calculate an effect size, relatedness correlated positively with skew; yet overall most of 45 studies reported no association. The association of skew with the outside option was negative or null, fitting the restraint and tug-of-war models. The mostly non-significant effect of group productivity supported the three models. Our review highlights future fruitful avenues for the study of cooperative breeding, especially more work to assess the association between subordinate’s outside option and subordinate’s effect on group productivity.