THE EVOLUTION OF COOPERATIVE BREEDING BY DELAYED RECIPROCITY AND QUEUING FOR FAVORABLE SOCIAL POSITIONS.

Abstract

Cooperative breeding, in which some members of a social group help others to reproduce, recurs as a normal feature of social organization in a variety of birds and mammals. In many cases, these helpers do not themselves reproduce while assisting in the care of others' young. How such apparently altruistic helping could evolve has been a central question in the recent development of sociobiology. The hypotheses advanced so far fit into three general categories (Brown, 1978): (1) immediate, direct benefits for helpers, such as immediate improvement in survival or chances for breeding; (2) indirect benefits as a result of selectively helping genealogical relatives; and (3) delayed, direct benefits as a result of eventually acquiring a favorable position for breeding. This paper focuses on the third category of explanation. The aim is to develop quantitative conditions under which delayed benefits can provide a sufficient explanation for the evolution of helping and to consider some of the evolutionary problems of delayed benefits. It is important to emphasize that the three kinds of hypotheses above are not mutually exclusive. All three effects might contribute to the evolution of cooperative breeding in a particular species or population. Indeed, it is possible that none of the three alone could provide a sufficient explanation for a particular case of cooperative breeding, whereas a combination of two or all three could. Nevertheless, a first step is to test the adequiacy of each hypothesis separately. To make these tests, we need to know the conditions under which each hypothesis can provide a sufficient explanation for the evolution of cooperative breeding. These conditions are comparatively well formulated for the first two hypotheses. For instance, immediate, direct benefits can provide a sufficient explanation for cooperative breeding provided that each individual in a group realizes an immediate gain in fitness, either through production of young or increased survival, in comparison to its fitness when breeding alone. Immediate, direct benefits to group members play an important part in the evolution of cooperative breeding in some species (Vehrencamp, 1978). In many species, however, helpers do not reproduce, or at least have very low chances for reproduction, so that immediate, direct benefits cannot provide a complete explanation for the evolution of helping in these cases. The indirect benefits from kin selection are also relatively well understood. Although these benefits are clearly important in the many species of cooperative breeders in which helpers join their natal groups, kin selection has nevertheless remained a controversial explanation for the evolution of cooperative breeding (Brown, 1978; Brown and Brown, 1981; Emlen, 1978, 1981; Gaston, 1978b; Ligon and Ligon, 1978a, 1978b, 1982; Koenig and Pitelka, 1981; Woolfenden, 1981). To determine whether or not indirect benefits can pro-