Introduction

Across group-living mammals, there is a continuum of reproductive skew from societies in which a single pair is responsible for all reproduction (singular breeders) to those in which all adults reproduce (plural breeders) (Keller & Reeve 1994). Factors influencing the degree of skew include extrinsic, ecological factors, such as saturation of available breeding sites, and intrinsic factors, such as the energetic costs of reproduction that limit the success of breeding attempts by subordinates (Vehrencamp 1983; Creel & Creel 1991; other chapters in this volume). Across this continuum, there is also variation in the degree to which members of the social group assist in the rearing of offspring of other members of the social group (i.e., communal care). Communal care that occurs at the singular-breeding end of the spectrum usually takes the form of nonbreeding helpers at the nest or den that defend and provision the offspring. Most chapters in this book focus on this kind of care. Communal care that occurs among plural-breeding species most frequently involves behaviors such as communal defense or provisioning of offspring, often by other breeding individuals within the group. In this chapter, we will examine the occurrence of communal care in plural-breeding mammals.