A Life History Approach to Fertility Rates in Rural Gambia: Evidence for Trade-offs or Phenotypic Correlations?

Abstract

Life history theory predicts that a trade-off will occur betweeninvestment in current and future reproduction. We test this hypothesisin a rural Gambian population by determining whether women who haveinvested heavily in reproduction in the past have lower reproductiverates in the present. We find the opposite: women of high parity for agiven age have higher reproductive rates than those of lower parity. Wealso find no differences in fertility rate between women who beganreproducing early and those who began reproducing later, nor does thesex of the child at the start of the birth interval affect subsequentfertility rate. These results suggest that phenotypic correlations areprevalent in this population. Women of high quality are able toreproduce at a high rate throughout their reproductive careers, women oflow quality are only able to devote relatively little effort toreproduction throughout their lives. We have tried to control for thisheterogeneity among women by including variables for phenotypiccondition in our model. Condition was measured by adult height, weightand haemoglobin level. Weight (controlling for height) and haemoglobinwere positively correlated with fertility rate, which reinforces theview that phenotypic correlations are prevalent. Even controlling forthese variables, a positive correlation was still observed betweeninvestment in past and present reproduction. This suggests theheterogeneity between women that leads to these phenotypic correlationsis not adequately captured by these measures of female body condition.In this population, some of this unexplained heterogeneity may berelated to genetic variance in the ability to resist disease.