Proximity association in polygynous western black crested gibbons(Nomascus concolor jingdongensis):network structure and seasonality

We investigated the structure and seasonality of the proximity network in a group of polygynous western black crested gibbons(Nomascus concolor)using social network analysis.The spatial proximity changed seasonally and was affected by temperature and rainfall.Preferred proximity association was not distributed randomly among individuals.Kinship was one explanation for the social structure,as offspring preferred to maintain close proximity with their mothers.The proximity of infants to mothers decreased with age,and independent offspring had lower proximity to mothers than dependent ones.We found that the adult male had different proximity relationships with two different adult females.The frequency of proximity between the male and the infant-carrying female was significantly higher than that between the male and the female who had immigrated carrying one offspring of uncertain paternity into the group.Infanticide avoidance and/or predation protection for dependent infants might explain the proximity relationship differences.Temperature influenced group proximity association,with individual proximity increasing in the cold months and decreasing in the hot months.Group proximity decreased in months with higher anthropogenic disturbance.

Observation of intra-group and extra-group copulation and reproductive characters in free ranging groups of western black crested gibbon(Nomascus concolor jingdongensis)

Black crested gibbons(Nomascus concolor)are 1 of only 3 gibbon species that have been observed in long-term polygynous groups,but their mating behavior and reproductive characters have never been reported.Based on population monitoring over 7 years and direct observation for 26 months of the study groups in Wuliang Mountain,central Yunnan,we describe for the first time the copulation behavior and breeding pattern of free-ranging western black crested gibbons.The gestation period of black crested gibbons is estimated to be between 6 and 7 months.The average breeding interval is 3.5 years,with infant independence at approximately 2.5 years.We observed 2 intra-group copulations and 5 extra-group copulations.Copulations were initiated when a female gave a‘solicitation’gesture.When a male made any mating attempt,the female might refuse it.These results demonstrate direct female mate choice.Both male and female gibbons dispersed from their natal groups and sometimes replaced paired adults in other groups.We observed no evidence of infanticide during inter-group conflicts or after replacement of adults.Together with extra-group copulations,these phenomena indicate a flexible social organization and complex mating system.We also observed a male-biased sex ratio among offspring.More genetic work is necessary to describe the effects of inter-group copulation and the genetic diversity of this population.