Reproductive Skew in Cooperatively Breeding Birds:
An Overview of the Issues

Emlen, S.T. (1999). "Reproductive skew in cooperatively breeding birds: an overview of the issues", pp. 2922-2931, In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22nd International Ornithological Congress. Johannesburg: BirdLife South Africa.

The term reproductive skew refers to the distribution of reproduction among individuals living within social groups. Higher skew societies are those in which reproduction is monopolized by one or a few dominant individuals; low skew societies are those in which reproduction is distributed more equitably among all group members. Two general theoretical frameworks exist for explaining variation in reproductive allocation: optimal skew (concessions) theory and incomplete control theory. The two frameworks represent very different views of conflict resolution within societies. I discuss several misconceptions surrounding optimal skew models. These include 1) the semantic error that because dominants are said to "concede" reproduction to subordinates, no overt conflict over breeding is expected, 2) the view that many cooperatively breeding birds do not fit the conditions for optimal skew models to apply because helpers do not always increase the nesting success or survival of the breeders they assist, and 3) the idea that inbreeding avoidance represents a third, and mutually exclusive, theoretical framework for understanding reproductive skew. I then describe three areas in which the predictions of optimal skew and incomplete control models differ, areas that I believe should become foci for future empirical testing of the theories. These involve: 1) the role of ecological constraints, 2) the importance of genetic relatedness, and 3) the expected patterns and levels of aggression. I argue that nuclear family (singular breeding) species are poor candidates, but that extended family (plural breeding) species are promising candidates, for differentiating between the different skew theories. Finally, I list five areas where optimal skew models need to be expanded by incorporating additional factors that will make them more applicable to vertebrate organisms.