Evidence for a reproductive sharing continuum in cooperatively breeding mammals and birds: consequences for comparative research

Extreme reproductive skew occurs when a dominant female/male monopolises reproduction within a group of multiple sexually mature females/males. It is sometimes considered an additional, restrictive criterion in the definition of cooperative breeding (i.e., when conspecifics provide parental care to other group members’ offspring). However, datasets that use this restrictive definition to classify species as cooperative breeders have two critical shortcomings. First, reproductive skew is systematically overestimated by including groups with a single sexually mature female/male when calculating the reproductive output of “dominant” females/males. Second, a lack of reporting on which species are classified based on limited data prevents accounting for uncertainty in classification. Considering these shortcomings, we show that extreme reproductive skew in multi-female and multi-male groups only occurs rarely in species previously classified as cooperative breeders using restrictive definitions (11 mammal species, 12 bird species). We further provide updated datasets on reproductive sharing in multi-female/male groups of cooperatively breeding mammals and birds that allow accounting for classification uncertainty. Our results demonstrate a reproductive sharing continuum even among those cooperatively breeding species argued to exhibit extreme reproductive skew. At the practical level, these findings call for significant changes in datasets that classify species by social systems. At the conceptual level, we suggest that reproductive skew should not be a defining criterion of cooperative breeding.