Introduction

Cooperative breeders are often divided into two types: plural breeders, in which most or all adults within a group produce young of their own, and singular breeders, in which subordinate group members do not produce young (Brown 1987; Stacey & Koenig 1990). Singular breeders form the majority among communal breeders of most taxa. For example, Brown (1987) classified 89 percent of 111 communally breeding bird species as singular breeders. Singular breeding also predominates among communally breeding carnivores (the focus of this chapter), among which subordinates do not normally raise offspring in 57 percent of 28 social species (Creel & Macdonald 1995).

Why Suppress Subordinates?

Reproductive suppression of subordinates probably arises from competition for resources that limit breeding opportunities within a group (Brown 1974; Stacey 1979; Emlen 1991). If the number of young that can be raised within a group is limited to fewer than a single female can produce, then selection should favor the ability to prevent group mates from breeding. (For dwarf mongooses, evidence that resources limit reproduction comes from supplementally fed groups in which subordinate females were more likely to become pregnant: z = 1.89, P = 0.048). For males, competition for low-cost mating opportunities might favor the ability to suppress others even when resources do not limit reproduction.