A life-history approach to fertility rates in rural Gambia: evidence for trade-offs or phenotypic correlations?

Life history theory predicts that a trade-off will occur between investment in current and future reproduction. We test this hypothesis in a rural Gambian population by determining whether women who have invested heavily in reproduction in the past have lower reproductive rates in the present. We find the opposite: women of high parity for a given age have higher reproductive rates than those of lower parity. We also find no differences in fertility rate between women who began reproducing early and those who began reproducing later, nor does the sex of the child at the start of the birth interval affect subsequent fertility rate. These results suggest that phenotypic correlations are prevalent in this population. Women of high quality are able to reproduce at a high rate throughout their reproductive careers, women of low quality are only able to devote relatively little effort to reproduction throughout their lives. We have tried to control for this heterogeneity among women by including variables for phenotypic condition in our model. Condition was measured by adult height, weight and haemoglobin level. Weight (controlling for height) and haemoglobin were positively correlated with fertility rate, which reinforces the view that phenotypic correlations are prevalent. Even controlling for these variables, a positive correlation was still observed between investment in past and present reproduction. This suggests the heterogeneity between women that leads to these phenotypic correlations is not adequately captured by these measures of female body condition. In this population, some of this unexplained heterogeneity may be related to genetic variance in the ability to resist disease.