percent per year (Goodall, 1986; Courtenay and Santow, 1989), about five times as high as among the four traditional human societies. Even gorillas, with much larger body sizes, do not live much longer than chimpanzees and have an adult mortality rate of about 5 percent per year (Harcourt and Fossey, 1981).
The most reliable estimates of adult mortality rates available for a pre-contact hunting and gathering group are derived from Aché research (Hill and Hurtado, 1996), because of the research focus on producing accurate measures of age and accounting for all adults that lived during the twentieth century. Figure 10-2 shows the age-specific mortality rate of Aché males and females. Adult mortality rates remain low and do not rise significantly until the seventh decade of life, where the rate climbs to 5 percent per year and reaches 15 percent per year by age 75. It should be mentioned that the data displayed in Figure 10-2 deviate somewhat from the age-specific mortality profile predicted by the Gompertz model (see Finch et al., 1990; Finch and Pike, 1996). According to that model, which is quite robust in predicting the mortality profiles of many animal populations (see references cited in Finch et al., 1990), human adult mortality rates are expected to double about every 8 years (ibid: 903). The slow rate of increase in mortality during early and middle adulthood estimated for the Aché may be due to small sample size. Alternatively, it may be that age-related increases in mor