Maternal investment in sons and daughters in provisioned, free-ranging Japanese macaques (Macaca fuscata)

It has been suggested that the physical condition of a mother may affect her pattern of investment in her sons or daughters. In addition, when competition over local resources becomes intense, severe aggression among the philopatric sex leads to a higher mortality of the sex and a biased birth sex ratio toward the opposite sex in the low rank. Parental sex-biased investment has been studied in many animal species to test these hypotheses, but the results have been highly inconsistent. We examined maternal sex-biased investment in relation to dominance rank using data on infant growth, infant mortality, birth sex ratio, and delay of subsequent reproduction by rearing current offspring from a provisioned, free-ranging Japanese macaque (Macaca fuscata) troop at Takasakiyama, Japan. The results showed that there was no sex difference in infant body mass among offspring of high- and low-ranking females. Use of the logistic regression model to analyze infant mortality with several independent variables failed to show a statistically significant sex bias. Birth sex ratio did not differ significantly between high- and low-ranking females. Among high-ranking females, there was no significant difference in delivery rate in the next year between those that reared a son and those that reared a daughter. For low-ranking females, however, the delivery rate after rearing daughters was markedly lower than that after rearing sons. Thus, there was no evidence of statistically significant maternal male-biased investment. For low-ranking females, we found a delay in subsequent reproduction for mothers after rearing daughters and no sex difference in offspring mortality. These results suggest that the females increased their offspring's chances of survival, irrespective of sex, by postponing their subsequent delivery.