Department of Anthropology, Stanford University
Friday 2:45-3:00, Ballroom B
An organism's life history is comprised of its age or stage-specific vital rates and trade-offs that bind them. Fitness in structured populations is given by the renewal equation and perturbation analysis of the renewal equation can be used to measure the force of selection on vital rates. While perturbation analysis provides information about the direction of selection, it is not informative about constraints that are central to life history theory. We derive a simple means of measuring the constraints on structured life histories. Assuming stabilizing selection, we use fitness elasticities to measure constraints on selection on the life cycle. The ratio of the fitness elasticities of two traits measures the slope of the constraint curve that binds them at equilibrium. We apply this method to understanding the differing constraints between human and chimpanzee life histories, focusing specifically on the trade-off between adult fertility and pre-reproductive survivorship. The shapes of the constraint curves linking fertility and pre-reproductive survival are quite different between the two species. Human curves show a pronounced U-shape, while the chimpanzee curves are more monotonic and shallow. This result suggests substantial costs associated with both very early and late reproduction in humans. Overall, we find that a unit of investment in fertility among humans must yield minimally five times the benefit of a unit of investment in infant survival. The switch from investment in infant survivorship to investment in further fertility is strongly influenced by the capacity of mothers' continued investment to contribute to the survival of offspring.