Infertility and forager demography.

Abstract

A model of the effects of infectious sterility on reproductive histories of women was presented to criticize the recent treatment of comparative forager population statistics, in which Blurton Jones et al. suggest that Hadza fertility is higher than ]Kung fertility because the Hadza have more food and a lighter workload. Infectious infertility in the ]Kung is a better explanation of the difference. Mean length of the reproductive span is a better indicator of the presence of infectious infertility than the primary sterility rate. A Markov chain model of reproduction had 4 states: at risk, pregnant, lactating, and sterile. The probability that a woman becomes sterile before she has a birth is in fair agreement with Howell's suggestion that 10% of ]Kung women never had a live birth. This agreement suggests that there was little exposure to sterility before the onset of full fecundity in this population. It is likely that the sexual partners of young ]Kung women were age mates rather than older men who would be more likely to transmit infection. The ]Kung share their land in Botswana with the Herero, who suffered severe infertility before antibiotics became available in the 1960s. In older Herero cohorts, primary sterility was only 15%, while sterility at every parity above zero was 25%. The cumulative effect of infertility in the Herero was severe: completed family size in older cohorts was less than 3 and the mean age at last birth was 27. The Herero example shows that the overall effects of sterility on reproduction and the level of primary sterility are not simply related.