Low-ranking wives have more baby girls

More baby girls are born to low-ranking wives in polygynous marriages than higher-ranking wives and women in single-wife marriages, according to a new study in Rwanda. The findings are published online today in the Royal Society journal Biology Letters.

Most societies in the world are polygynous, which means men have several wives at the same time. This arrangement can put a strain on women, according to previous research that shows women in polygynous marriages to have lower fertility rates and suffer higher stress than women in monogamous marriages. Their children also have higher mortality and poor growth rates, and this is especially true for lower ranking wives.

Previous research has shown that lower-ranking wives receive fewer resources from their husbands and are in poorer health than higher ranking co-wives. In many populations, such as those in this study, survival of sons is known to be highly dependent on adult status and resources. If this were true, then lower raking wives should give birth to a higher proportion of girls, who are more likely to survive.

To test the theory, Thomas Pollet from the University of Groningen in the Netherlands and team looked at a sample of over 95,000 Rwandan mothers, who were in either monogamous or polygynous marriages. The latter were either ranked as the first wife, the second wife, or the lower-ranking co-wife in the marriage, to see whether rank affected the sex of children born to each woman.

Low-ranking wives not only had fewer children than other mothers, showing that they have lower fertility and are thus in poorer condition, but they also gave birth to significantly more girls. What's more the the greatest difference was between low-ranking wives, who had 106 daughters for every 100 sons and women in monogamous marriages, who only had 99 girls per 100 boys.

The results show that a woman's condition can bias the sex-ratio of her offspring, a topic which has been hotly debated by scientists for years.