Maternal investment and birth sex ratio bias of the golden snubnosed monkey {\sl Rhinopithecus roxellana} in Qinling Mountains of China

Female primates will adjust their maternal investment strategies for their offspring based on sex, which mainly results in biased birth-sex ratios and enhanced maternal care for a certain sex. The Male Quality Model (MQ) and the Local Resource Competition Model (LRC) suggest that this phenomenon is a response to sexual selection and resource competition pressure, respectively. Based on a four-years observation of the reproductive behavior of a Sichuan golden snub-nosed monkey group, this research found more male than female newborn infants and its secondary sex ratio was biased towards males (♂:♀=1:1.71). The research revealed a linear relationship (PM = -0.4918MDI＋0.9329) between the social ranks of different social units and their male infant birth rates (t =-1.879, df=24, P=0.073, R~(2)=0.1331 ). The proportion of male newborn infants to all newborn infants in each social units increased with a decrease in social rank order (PMHcollect behavioral data on maternal care, and we found no significant difference in maternal care of sons and daughters after birth (t=0.390, P=0.72). We used Two-way ANOVA to analyze the effect of female rank and newborn infant sex on female interbirth intervals (IBI). Our results showed that female rank did not influence IBI (F_(2,21)=0.123;P=0.885); however, the sex of newborn infants did affect IBI (F_(1,21)=3.293;P=0.086). Our results supports the LRC model and its inference Local Resource Enhancement Model (LRE) and Daughter Advantage Model (DA), which indicates that food and mating opportunity competition had a greater effect than sexual selection pressure on Sichuan golden snub-nosed monkey maternal investment strategies. This reproductive strategy is a response to the environmental selection pressure, .and makes its life history more adaptive value.