18. Sociality, Food Sharing, and the Evolution of Life Histories

Life history theory has focused on the life cycle tradeoffs faced by individuals who are constrained by the energy they can forage for themselves at each age. However, humans are deeply social and adults transfer food to children for many years, freeing them from this energy constraint but also bringing the risk that parental death could entail the death of all dependent offspring. Multiple simultaneously dependent offspring also bring a family life cycle squeeze in which dependency ratio doubles. Food sharing and alloparenting ameliorate both problems, providing life insurance and smoothing the life cycle squeeze, while permitting humans to rely on food resources that would be too uncertain for isolated individuals. Food sharing and intergenerational transfers in turn affect the way natural selection shapes life histories. We use microsimulations to study evolution of life histories. Births inherit the mother’s genome subject to mutations. Individuals live under different social arrangements and forage with productivity depending on population density. Natural selection on life histories occurs. We examine the way the size and relatedness of sharing group arrangements alter the evolution of life history traits through mutation and natural selection. We consider which social arrangements, with their corresponding evolved life histories, are most successful in a group competition where all face the same density constraint. There is a tradeoff between costs and benefits of sharing. We find that intermediate levels are most successful, unless childhood conditions strongly influence later life productivity.