Impact of group size on female reproductive success of free-ranging Rhinopithecus roxellana in the Qinling Mountains, China.

Group size influences female reproductive success through scramble/contest feeding competition, predation pressures and infanticide risks in primates. The Sichuan snub-nosed monkey (Rhinopithecus roxellana) is an endangered folivorous colobine species living in a multilevel society. From 2002 to 2008, we studied a free-ranging band of R. roxellana in the Qinling Mountains of China to examine the effect of group size on female reproductive success. During our observation period, the number of monkeys in the study band fluctuated from 61 to 108, and the number of one-male/multi-female groups within it varied from 7 to 10. A significant negative linear relationship was found between group size and birth rate, but group size was not significantly correlated with infant survival rate or interbirth interval. These results suggest that group size influences female reproductive success via within-group scramble competition in this folivorous species.