adulthood. The chimpanzee figures come from the number of risk years observed in each age class in Emery Thompson and colleagues’ (2007) supplementary table 2. For human hunter-gatherers the figures come from the female life table for Hadza foragers (Blurton Jones et al., 2002). As the percentages show, almost all of the chimpanzees that survive to adulthood then die during the childbearing years; only 1% do not. By contrast, 24% of the hunter-gatherer women die during the childbearing years; 76% do not.
Emery Thompson and colleagues (2007) demonstrated heterogeneity in chimpanzee fertility in their six-site sample by looking for associations between fertility rates and survival in females over the age of 25. They divided their observations into healthy and unhealthy years. An observation year for a given chimpanzee was considered healthy if she survived an additional 5 years or more, and unhealthy if she did not. Their figure 2 (Emery Thompson et al., 2007, p. 2152) shows that fertility in the thirties was about twice as high in females who would survive at least 5 more years than in those who would not. The finding indicates that mortality selection across the childbearing years culls the females with lower fertility. As the age classes shrink to almost nothing, they are increasingly biased to the less frail, more fertile females. Consequently, average fertility changes little even if the fertility of the survivors is declining relative to their own earlier rate.
We found similar heterogeneity in fertility in 19th century Utah women [the Utah Population Data Base (UPDB) (Bean et al., 1990)]. Although not hunter-gatherers, these women practiced natural fertility (Henry, 1961), so potential for continued childbearing is reflected by actual births. Individual records make it possible to investigate links between variation in fertility rate and age at last birth. Of 42,493 parous UPDB women born between 1849 and 1890, the 10,440 whose fertility ended before the age of 35 had fertility rates in the preceding years about half as high as the 2,695 women who would have last births after 45 (Hawkes and Smith, 2010). This parallels the chimpanzee variation with an important difference: all the women in the Utah sample, whatever their age of last birth, survived at least to the age of 50. The sample was restricted to women who lived at least to that age to avoid the confound of early last births due to early death (Hawkes and Smith, 2009). Subjects were also restricted to those married once and neither widowed nor divorced to reduce effects these characteristics may have on fertility.
Assuming that heterogeneity in fertility is similar in the hunter-gatherer women, this variation combined with the different survival schedules of humans and chimpanzees can explain the different shapes of the fertility schedules shown in Fig. 11.4. Most women, whatever their frailty, survive the childbearing years, whereas across those years mortality culls chim-