The Sociality Continuum: Monopolized and Shared Breeding Systems Among
Cooperatively Breeding Birds


du Plessis, M. and Emlen, S.T. (eds.) (1999). "The sociality continuum: monopolized and shared breeding systems among cooperatively breeding birds", pp. 2857-2931 (Symposium No. 47), In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22nd International Ornithological Congress. Johannesburg: BirdLife South Africa.

Cooperatively breeding birds show considerable variability in patterns of within-group reproduction. Many species are singular breeders that live in social units in which all (or virtually all) reproduction is performed by a single breeding pair, and helpers are almost always nonbreeders. Other species are more egalitarian plural breeders in which reproductive roles are normally shared by many different group members, and helpers are frequently breeders themselves. However, dichotomizing cooperative breeders as either singular or plural breeders is an over-simplification; singular and plural breeding are not so much distinct categories as they are extremes along a continuum of reproductive suppression (what has been called the "eusociality continuum" by Sherman et al 1997). Although cooperative breeders can be portrayed as occupying particular positions along the continuum, this too, may be an over-simplification, because it ignores important intra-specific variation which may exist in the degree of breeding suppression (e.g., variable mating systems). Also, one needs to biologically differentiate between nuclear-family systems and those based on probabilistic parentage.

There has been considerable interest in intra-specific variation in the degree of breeding suppression and reproductive skew within cooperative groups. Empirical work has shown that patterns of within-group reproduction are influenced by a number of different factors, including subtle conflicts of interest between males and females, or breeders and helpers, and avoidance of incestuous matings. We are now in a position to construct rigorous theoretical frameworks for understanding the factors ultimately responsible for variation in reproductive skew both within and between natural populations. However, there still are very few empirical tests of the predictions that emanate from these theoretical models.

This symposium focuses on both intra- and inter-specific variation in the degree of reproductive suppression and skew in cooperatively breeding birds. Each of the participants attempts to place the results of their own studies within the broad theoretical context of skew theories. We hope that this symposium will alert ornithologists to recent developments in reproductive allocation (skew) theory, and stimulate colleagues studying cooperative breeders to collect data to test the new predictions. Such knowledge could create a better understanding of the evolutionary forces that shaped the range of breeding systems that exist among cooperatively breeding birds today.