Bushman birth spacing: Direct tests of some simple predictions

Abstract

Blurton Jones and Sibly (1978) developed a model of costs (weight of food and baby carried while foraging) of !Kung women's reproduction under ecological-economic constraints that were described by Lee (1972). Predictions are drawn from this model and tested on Howell's (1979) data from reproductive histories of 172 individual women. Women were rated on a scale of dependence on bush or cattlepost foods. The ratings were compared with what is known about the places at which the women gave birth to their children. Agreement was good and the population was divided into two groups for study: 65 women most dependent on bush foods and 70 women substantially dependent on agricultural produce. Thirty-seven women of uncertain or intermediate status were omitted. As predicted by the model, among the women who were dependent on bush foods: (1) first interbirth intervals (IBI) were shorter than were later IBI; (2) the survivorship of children in first IBIs was not strongly related to length of IBI, with shorter IBI giving as good survivorship as longer IBI; (3) IBIs lengthen as the number of surviving children increases, until the fourth child; (4) after the fourth child IBIs do not differ significantly although they tend to be shorter (contrary to the prediction, a null hypothesis whose statistical support is consequently poor); (5) for IBIs after the first IBI, mortality increased markedly as IBI decreased; (6) mortality was even more closely related to backloads entailed by each IBI, as calculated by Blurton Jones and Sibly (1978); (7) mortality was less closely related to backloads calculated from an alternative version of the model from which weight of food was excluded; (8) as reported by Howell (1979), a new pregnancy followed rapidly after the death of the preceeding child but (9) as predicted, a new pregnancy did not follow so fast after the death of older children; (10) for the cattlepost women IBIs were shorter than for women dependent mainly on bush foods; and (11) there was no significant relationship between IBI and mortality for the cattlepost women, and mortality at short IBI was lower than in bush women. The assumption that the benefit accruing from more births will be balanced against the costs (costs to survivorship assumed to result from the work entailed by caring for each child) was successful in giving many predictions confirmed by the data. The significance and limitations of this “optimization” study are discussed.