different. Fig. 11.4 displays the average age-specific fertilities for three hunter-gatherer populations and the conservative age-specific fertility schedule synthesized from six wild chimpanzee populations by M. Emery Thompson and others (2007). Human populations can differ widely in fertility levels, but among them—hunter-gatherers included—the change in the rate of babies born to women of each age has a familiar peaked shape. “[I]n all populations where reliable records have been kept, fertility is zero until about age 15, rises smoothly to a single peak, and falls smoothly to zero by age 45–50” (Coale and Demeny, 1983, p. 27). The fertility schedule for wild chimpanzees is flat-topped instead. The rate reached before the age of 20 continues with little change for two more decades.
The percentages running along the horizontal axis in Fig. 11.4 show the relative size of each age class compared to the first age class of
FIGURE 11.4 Age-specific fertility rates (ASFR) for humans and chimpanzees. Humans (open circles) are represented by the average of three hunter-gatherer populations: !Kung Bushmen of Botswana (Hill and Hurtado, 1996), Ache of Paraguay (Hill and Hurtado, 1996), and Hadza of Tanzania (Blurton Jones et al., 2002). Estimates for chimpanzees in the wild (closed squares) come from the conservative fertility schedule synthesized from six study sites by Emery Thompson et al. (2007). The bumps reflect small sample size (627 risk years in the initial chimpanzee adult age class declining to 8 risk years in the 45- to 49-year interval (Emery Thompson et al., 2007, supplementary table 2). The percentages along the horizontal axis indicate the proportion of those reaching adulthood that survive to the age class. The top row of percentages comprises estimates for chimpanzees from the number of risk years in each age class (Emery Thompson et al., 2007, supplementary table 2). They are just slightly lower than the model in Fig. 11.1 from the life table (Hill et al., 2001). The bottom row comprises human estimates from the female life table for Hadza hunter-gatherers (Blurton Jones et al., 2002).