Birth sex ratios and social rank in rhesus monkey mothers

Abstract

The ability to control the sex of her offspring could be of survival value to a mother^1,2, and prompts questions about mechanisms of sex determination. Trivers and Willard have suggested that mothers potentially able to invest much in offspring should bear those kinds of infant that repay high levels of maternal investment most effectively². One hypothesis concerning matrilineal primate species states that a dominant mother would leave more descendants through a daughter rather than a son if that daughter inherited her high rank^3–5. On the other hand, a low ranking mother of such a species would leave more descendants through a son, for he is likely to emigrate at puberty and not necessarily inherit his mother's low rank⁶. Unfortunately, long-term data on primate birth sex ratios are in short supply. Here we present data collected over 20 years on a captive colony of rhesus monkeys which support the above hypothesis. High ranking rhesus monkey mothers in our captive colony⁷ were more likely to give birth to daughters than sons, while the remaining mothers were more likely to bear sons than daughters. Finally, we suggest a mechanism for adjusting birth sex ratios in macaques.