Introduction

Behavioral biologists and comparative psychologists have long been interested in species that display a social system labeled “cooperative breeding”, in which members of the social group assist in rearing young that are not their own offspring. The definitional hallmarks of cooperative breeding concentrate on the distinctive attributes of these care-giving individuals, and they include (1) delayed dispersal from the natal group, (2) reproductive suppression, and (3) care for others' offspring. The individuals engaging in care of young may be nonbreeding adults or subadults (usually called helpers, auxiliaries, or alloparents), or they may be reproductive adults sharing in the care of young with other breeders in the social group. In the former, all three of the previously mentioned characteristics are seen, whereas in the latter, typically only the third characteristic is required for groups to be considered cooperative breeders. The types of care given to young by alloparents is remarkably diverse and includes patterns classified as both depreciable parental care (defined by Clutton-Brock 1991 as those patterns that change as a function of the number of offspring that are cared for; e.g., provisioning with food) and nondepreciable ones (those patterns that do not vary with the number of offspring that are cared for; e.g., burrow defense).

Cooperative breeding has received considerable attention from biologists, especially subsequent to the publication of Hamilton's landmark paper on the evolution of sociality (Hamilton 1964).