Morne A. Du Plessis¹ & Stephen T. Emlen²

1Percy Fitzpatrick Institute of African Ornithology, University of Cape Town, Cape Town, South Africa, e-mail morne@botzoo.uct.ac.za; ²Section of Neurobiology and Behaviour, Cornell University, Ithaca, NY 14853-2702, USA, e-mail ste1@cornell.edu

Du Plessis, M.A & Emlen, S.T. 1999. The sociality continuum: Monopolised and shared breeding systems among cooperative-breeders. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban: 2857. 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, however, are more egalitarian plural breeders in which reproductive roles are normally shared by many different group members, and helpers are frequently breeders themselves. Dichotomising 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 of what can be called the continuum of reproductive suppression or the 'sociality continuum'. 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). We are now in position to construct a rigorous framework for understanding the factors ultimately responsible for the variation in reproductive suppression within natural populations. We currently still have little understanding of what accounts for inter-specific variation in the degree of reproductive suppression.