Mortality curves

Caughley (1966, 1977) and others found that mortality curves (qx) for female mammals, such as Orkney voles (Microtus arvalis) (Leslie et al. 1955), toque monkeys (Macaca sinica) (Dittus 1977), buffalo (Syncerus caffer) (Sinclair 1977), Himalayan thar (Hemitragus

Age in years

Figure 4.10 Mortality curves for male and female impala {Aepyceros melampus). Data from Jarman and Jarman (1973); analysis from Ralls et al. (1980).

Age in years

Figure 4.10 Mortality curves for male and female impala {Aepyceros melampus). Data from Jarman and Jarman (1973); analysis from Ralls et al. (1980).

jemlahicus), domestic sheep (Ovis aries), Dall mountain sheep, and elk (Cervus elaphus) all follow a U-shaped pattern (Fig. 4.10). The U-shape is the result of the fact that juvenile phases usually have high mortality, but they are coupled with adult phases that have low mortality. In the final phases of life mortality increases (the senescent phase). Ralls et al. (1980) suggested that in polygynous species, although females show the U-shaped pattern, males have a spike of mortality in sub-adult to young adult age classes. Impala (Aepyceros melampus) and toque monkeys, for example, illustrate this pattern. These higher male mortality rates are due to male-male competition for mates, and the tendency for males to leave the natal group in many species (Ralls et al. 1980).

Interestingly, a similar pattern can be found for the United States human population (Fig. 4.11). United States Vital Statistics for 1986 (Anonymous 1988) have a U shape for females, with high mortality in the first year of life, followed by a low rate of mortality until age 15. Thereafter the mortality increases throughout life. Males show higher mortality rates from the age of one onwards. More striking, however, is that the largest separation in mortality rates between the sexes occurs from the ages of 15 to 30. These age classes are exactly those discussed by Ralls et al. (1980), in which male mammals suffer greater mortality in the sub-adult to early-adult age classes. Do human males, like other mammals, suffer these higher mortality rates due to male-male competition for females and/or due to dispersal away from the parental home, as Ralls et al. (1980) have suggested? Or are there other reasons, such as the suggestion that human male brains mature at a slower rate than those of females (Thompson et al. 2000)?

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