Modeling the effects of weaning age on length of female reproductive period: implications for the evolution of human life history.

OBJECTIVES: Human life history is unique among primates. Despite our extended lifespan, the length of the female reproductive period is shorter in humans than in our closest living relatives, chimpanzees. Here, we investigate whether this difference could be explained by another unique aspect of human life history-a young weaning age.
METHODS: Age-dependent female fertility is modeled with the Brass polynomial. We model female reproductive period length as single locus with multiple alleles. Selection acts on the length of the female reproductive period in an evolutionary agent-based simulation. We quantify the effect of weaning age on the optimal length of the female reproductive period under a range of adult mortality rates.
RESULTS: Females sacrifice a smaller proportion of their reproductive potential due to nursing by weaning their offspring at younger ages. As a consequence, the optimal length of the female reproductive period decreases as weaning age decreases, even when adult mortality is low.
CONCLUSIONS: Natural selection will favor mutations or strategies that can decrease weaning age without incurring fitness costs. In the presence of younger weaning ages, selection favors a shorter female reproductive period. To the extent that allocare can decrease weaning age without decreasing fitness, its ubiquity in human societies and near absence in other primate societies may explain why women have a shorter reproductive period. Furthermore, allocare may have provided human ancestors with an avenue to decreased weaning age-and, ultimately, a shorter female reproductive period-that was unavailable to their hominoid contemporaries.