Social Organization, Individual Dispersal And Reproductive Strategy Of Golden Snub-nosed Monkey (Rhinopithecus Roxellana)

Understanding the mechanism of individual shift between units within a primate social group is important for studying social dynamics of the primate. In old world monkeys, female philopatry is strongly linked to the polygynous species that charactering with matrilineal groups. During an 8-year long study of a troop of Sichuan snub-nosed monkeys in the Qinling Mountains of China, we observed 43 out of 72 (59.7 %) females transferred between One-Male Units (OMU) or disappeared from the troop indicating that it is both males and females that disperse in this species. After dispersal, the sub-adult females show much more tendency to give birth than adult females. The number of females immigrating into units with older males (being the males of the units for more than 3 years) is lower than units with newer males (being the males of the units for less than 3 years), indicating that males that recently took over the unit from older males are more attractive to the dispersing females. During 2001 to 2008, 16 new adult males appeared in the study troop which consisted of 6-8 OMUs. We observed 7 stable female units consist of unrelated females and their offspring. These female units were joined by a total of 13 males and have formed 13 OMUs over the 8 years. We observed 4 new OMUs immigrated to the troop during the study period, of which 2 merged with individuals from other unit and one emigrated from the troop later. The number of individuals in newly immigrated OMU was significantly less than that in resident OMU. Eight occasions of entire harem shifting have been observed between different resident males, and displayed in male replacement, inheriting, collapse and merging. Serious fighting between males were not observed and female seemed to initiate the transfer. Mate choice by females appears to have played a crucial role in the processes. Resident males are usually replaced before their daughters become sexually mature, or their daughter will be transferred. The results suggest Golden snub-nosed monkeys are organized in anon-matrilineal social system with less kin-bond. We observed 88 births from 47 females from 2001 to 2006. Two methods were used to calculate birthrate(Eisenberg et al., 1981). The first method giving 0.49±0.07 (mean±SD), and the second method giving 0.49±0.17 births per female per year in this troop. The mean interbirth interval (EBI) is 21.88±6.01 months (mean±SD). The mortality of infant born between 2002 and 2005 was 22.4%. The IBIs of females that had lost an infant before the age of 6 months were significantly shorter than that of females whose infants survived more than 6 months. A female usually gives birth once every two years if the previous offspring survives to a weaning age of 5-6 months, or will give birth in the next year if the previous young die before reaching an age of 6 months. Births were significantly concentrated from March to May of each year. The mean birth date was on April 14th; median was April 12th; the standard deviation is 13.98 days. Birth peak occurs 6-7 months after mating peak. From observations on 15 individuals that gave birth for the first time, we conclude that the wild female Golden snub-nosed monkeys in Qinling Mountains start giving birth at an age of 5 or 6 years old. We suggest the seasonal reproductive pattern is an adaptive response to the availability of seasonal food availability. Our results are consistent with the hypothesis that these reproductive characteristics are a result of adapting to the seasonality of mountain climate and food resources.To gain a higher benefit over cost, female primates will adjust their maternal investment strategy for offspring of different sexes, which mainly means enhanced birth for a certain sex. Male Quality Model (MQ) and Local Resource Competition Model (LRC) interpret the mechanism of this phenomenon as the response to the sexual selection pressure and resource competition pressure respectively. Based upon an four-year-long observation over the reproductive behavior of study group conducted by ad libitum sampling and all occurrence recording, this research found that there are more male newborn infants than female ones in the studied population, and its secondary sex ratio displays a bias ((?): (?)=1: 1.71). The research has revealed a linear relationship (PM = -0.4918MDI + 0.9329) between the social ranks of different social units and their male birth rates (t = -1.879; df= 24; P = 0.073<0.10; R~2 = 0.1331). The proportion of male newborn infants to all newborn ones in each social units increases with the decrease of social rank orders, and displays PM_H0.10). The result supports the LRC model and its inference Local Resource Enhancement Model (LRE) and Daughter Advantage Model (DA), which indicates that the competition pressure from food and mating opportunities is greater than sexual selection pressure and functions as the main influence factor for Sichuan golden snub-nosed monkeys' selection of maternal investment strategy.Spatial structure of study group was conducted by instantaneous and scan samplings and all occurrence sampling. We found that the fundamental unit of spatial structure in the monkey group was their social units. By using spatial distance as the judgment of social unit, we found that individual from the same social unit flock in the same tree or two adjacent trees with the high frequencies 60.6% and 29.1% respectively when they are being in no-locomotion The spatial distances between individuals range from 0 to 10m while their main distribution ranges form 0 to 5m, which displays a frequency downtrend with the growth of distance. However, it has also found that the spatial distances between individuals from different social units mainly range from 10 to 30m, with the peak between 20 and 25m, which are obviously longer than those between individuals from the same social unit. Thus, this researching result have consultable value could provide a method for the judgment of one-male unit for future surveys into wild Rhinopithecus roxellana. When the monkeys were resting and foraging, the spatial pattern of the group had the appearance of either a circle or an oval. The social units were assigned to one of four concentric circles (layers) that were used to distinguish their relative spatial orientation. The statistics showed that the higher-ranking social units always appeared in the two most central layers (df=1, (?)~2 =166.97, P<0.005). Lower ranking social units appeared most frequently in the outer most layer (df=1, (?)~2 =12.60, P<0.005)and middle ranking units usually located in the middle layers(df=1, (?)~2 =21.54, P<0.005). Higher-ranking units occupied larger and higher quality space. As the rank order of the social unit decreased, the quality and size of the space occupied also decreased. The characteristic shape of the group during movement was fluid.