mates, they present original data on the pooled age-specific fertility and survival rates of baboon populations and describe the variations of reproduction and survival with abundance of food and dominance status.
The matrilineal dominance hierarchies that Altmann and Alberts describe give a tantalizing analog to the demographer’s staple explanatory category, or socioeconomic status. Among the Amboseli baboons, within each group daughters tend to inherit their mother’s rank. Rank matters. Low-ranking females have lower fertility. But occasionally, under favorable conditions members split off into a new group, with fewer members to outrank them. Possibly these social arrangements help regulate aggregate group fertility and population growth, with group fission supplying a mechanism for relaxation of constraints.
The account by Altmann and Alberts is a telling example of social behavior altering demographic parameters in another primate. Behavioral flexibility is not unique to humans. Reproductive effort includes both mating and parenting, and both components involve complex social strategizing in other primate species as well as our own. Investment in social relationships has demographic payoffs. It turns out that the patterns of response to food availability in the Amboseli baboons are similar to some of the human patterns described by Carol Worthman in Chapter 10.
Altmann and Alberts calculate quantities called elasticities that describe the sensitivity of population growth to small changes in age-specific rates of fertility and survival and that often serve as measures of susceptibility to forces of natural selection. Growth is much more sensitive to a proportional change in infant or child survival per year than to a proportional change in fertility per year. This finding might be taken to underline the significance of parenting behavior and investment in offspring quality over and above the maximization of offspring ever born. However, overall comparative effects are more complicated to calculate. From a baboon mother’s point of view, improving average infant survival by some factor requires improving survival a dozen times over, for a dozen or so infants, whereas raising fertility in a year by the same factor is a one-time investment. Relative returns remain to be assessed. The power of the approach is the ability to perform parallel calculations of demographic trade-offs for humans and other primates, taking advantage of nature’s natural experiments.
The terms of discourse of genetics are particularly attractive for organizing thinking about behaviors because they not only lead forward from genes to physiological mechanisms but also backwards from genes to their evolutionary origins. We ask not only what a gene produces in the body but also where it comes from out of prehistory. For human fertility behavior,