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White-headed langurs have sexual contacts with one another year-round, but there are distinct reproductive peak seasons. Copulations peak between July and August, while the peak season for parturitions is between December and January. The mean age of the female’s first parturition is at 5.4 years, with the youngest at 4.8 years and the oldest at 6.2 years. We observed 16 female langurs giving birth to 78 pups, and accurately registered 61 inter-birth intervals of adult females. The average inter-birth interval is 784.3 days or 25.8 months. That is to say, each adult female white-headed langur is able to reproduce 0.93 infants in a year. The length of the suckling period of white-headed langurs is 456 ± 28 days, which is equivalent to 15 ± 0.9 months. Two pieces of data registered from field observations show that the pregnancy period of a female white-headed langur is 138–139 days.

In 2012, we worked in the bush for 357 days and recorded the all-day activities of a white-headed langur family with high-definition video cameras, in both “Sampling All Occurrences of Some Behaviors” method and “Focal-Animal” method. Activities of white-headed langurs follow a constant 24 h diurnal cyclical pattern. In this chapter, we present a detailed summary of their diurnal activities. White-headed langurs periodically adjust the timing of their activities and rests according to seasonal cycles. Their inherent circadian clock keeps biological activities synchronized with the seasonal photoperiod changes in the Northern Hemisphere. We also researched on the “roles” of individuals in a family troop with respect to different sexes and ages and found that the duty of defending the family rested primarily on the resident alpha male, while sub-adult male and adolescent male served as assistance to their father. All females acted in roles of reproducing or nursing pups. Yellow-haired pups and infants spent a lot of time playing with family members.

In Nonguan Mountain area, there are 220 hills scattered in the dense jungle. Due to the restriction of the landform, we tracked the white-headed langur troops on fixed spots, along with line transect surveys, to estimate their numbers. From April till December 2015, we surveyed white-headed langur distribution in the Nongguan Mountains and divided it into three districts. According to our survey, we estimated a total of 798 (216 + 425 + 157) individuals. Since 1996, the langur population has been growing rapidly. The sex ratio within phase 1 and 2 was 1:1, but the phase 3 sex ratio after maturity was strongly biased toward females, with a ♀:♂ ratio of 22:3 (overall mean 7.3:1). Old white-headed langurs accounted for 6.5% of the population, adults at reproductive ages accounted for 32.3%, and the pups, juveniles, and adolescents made up 61.3%. Age distribution as such indicates that the age structure of the FJC white-headed langur population is balanced. Meanwhile, the net rate of annual population increase of six families during the last 20 years ranged from 4.0% to 54.5%, with a mean value of 40%.

We used the Vortex Population Viability Analysis (PVA) model to simulate the changes of the white-headed langur population over the past 20 years and verified it with field observation data from 1997 to 2016. We found that the population numbers calculated by the two methods are very close, with a fitting degree of R2 = 0.99792. According to our field investigation, the number of white-headed langurs in the Nongguan Mountains has increased from 105 in 1997 to 797 in 2015, while VORTEX 9.99c, after 500 iterations of simulations, shows that in the 19 years from 1996 to 2016, the population of white-headed langurs would grow from 105 to 858, with an extinction probability of 0% and a survival probability of 100%. Taking into account the effective breeding population size, age distribution, breeding strategy, mortality, population diffusion, environmental capacity, and other factors of the population in 2016, a positive future for the population is predicted through the PVA model. The effective breeding population size will increase with time, and the inbreeding rate will gradually be reduced. When the natural habitat of the whole Nongguan Mountain area of 13.2 km2 is supposed to sustain a maximum of 1800 white-headed langurs, it is expected to reach carrying capacity in 2027 and, if current conditions persist, remain stable in the future. From the perspective of growth trend, 2003–2021 is the key period for the rapid recovery and growth of this population and thus the golden period that saved the population from extinction.

The restoration of the white-headed langur population in the Nongguan Mountains has undergone several phases. In February 1997, only six family troops and three all-male troops survived the anthropogenic calamities (Chap. 4), with an aggregate of 105 white-headed langurs. In 2002, the population increased to 12 family troops and seven all-male troops, with a population density of 42.5 langurs/km2. From 2006 to 2015, the population increased to above 800, and the all-male troops started to migrate and disperse all over the Nongguan Mountains. We divide the lifespan of a white-headed langur into six age stages: infant (≤0.5 years old), yellow-haired pup (0.5–1.3 years old), juvenile (1.3–2.5 years old), adolescent (2.5–4 years old), sub-adult (4–5 years old), adult (>5 years old), and old (♂ >13 years; ♀ >25 years). The social structure of the white-headed langurs could be summarized into four basic systems: ante-family troop, family troop, transitory troop, and all-male troop. The white-headed langur does not show obvious sexual dimorphism, but the canine teeth of adult males are noticeably larger than in females. During the family troop phase, the white-headed langurs are strictly polygynous, but in their lifetime, they are polygamous.

Infants of many primate species have extensive interactions with group members other than their mothers, which can affect an infant’s fitness. Patterns of nonmaternal infant handling vary according to the mother’s response as well as the number, sex, age, and dominance ranks of social partners. The primary goal of this study was to identify the basic pattern and explore the function of nonmaternal infant handling behavior in wild white-headed langurs (Trachypithecus leucocephalus) at the Nongguan Hills in Guangxi, Southwest China. We report nonmaternal interactions in the first 3 mo after birth for 15 infants born during six birth seasons in a 67-mo field study. The nonmaternal infant handling we observed was generally in the form of caregiving, and mothers permitted most handling attempts. Infants were handled by nonmothers from the first day after birth. Infants spent 20–30% of the daytime associated with nonmothers during the first month of life, and time in contact with nonmothers decreased with infant age. Rates of nonmaternal infant handling varied significantly with the sex and age of handlers. Juvenile and subadult females handled infants significantly more than expected from their proportional representations in the group, whereas adult females did so less frequently than expected. Older male infants and juvenile males rarely handled infants, and adult males never did so. Infant handling behavior showed no correlation with the dominance rank of handlers relative to the mothers. Our data, although limited in some analyses, are in line with the predictions of the learning-to-mother hypothesis, but do not support the female competition or alliance formation hypotheses.

There are few detailed descriptions of parturition behavior in wild primates. Here we report for the first time birth-related behaviors in a highly endangered colobine monkey endemic to southwest China, the white-headed langur (Trachypithecus leucocephalus). We video recorded three complete birth events in three parturient females living in one reproductive group. The births occurred during the night at one of the group’s sleeping sites. One parturient was primiparous, while the other two were multiparous. All three delivered close to other group members. Common behaviors among the parturients included adopting a raised-rump posture during contractions, manual inspection of the anogenital area followed by hand-licking, manual self-assistance as the infant emerged, holding and licking the infant immediately after the birth, and ingesting the placenta. However, we found considerable individual variation in birth-related behaviors. The primiparous female had the longest prepartum phase and more contractions than the two multiparous females. All three mothers ingested the placenta, but the primipare took much longer than the two multiparous mothers to deliver the placenta, start to ingest it, and consume it. Other group members had little interaction with the parturients during the prepartum phase, but showed intense interest in the neonates. The newly born infants were held by nonmaternal adult females of the group within 5–9 h of their birth. Our case studies stress the importance of the social context at parturition in group-living primates, and suggest that parity of parturients may influence birth-related behavior.

Infanticide by males is common in mammalian species such as primates in which lactation lasts much longer than gestation. It frequently occurs in one-male groups following male takeovers and is likely a male reproductive strategy. Reported female countertactics include abrupt weaning of infants, dispersal, or paternity confusion. Here, we estimated costs of female countertactics in terms of weaning ages and interbirth intervals. We observed a population of white-headed leaf monkeys (Trachypithecus leucocephalus) in Nongguan Nature Reserve, China (1995–2006) mainly composed of one-male groups. Takeovers (N = 11) coincided with the peak conception period. Detailed data are presented for five takeovers (34 females, 29 infants, and 47 group-years) leading to six infant disappearances (42.9% of infant mortality). All presumed infanticides were in accordance with the sexual selection hypothesis. Following a takeover, females without infants or with old infants stayed with the new males, incurring no or low costs (via abrupt weaning). Females with young infants dispersing with the old males also experienced low costs. High costs (due to infant loss) were incurred by pregnant females and those with young infants who stayed with the new males indicating that paternity was not confused. Costs in terms of long interbirth intervals were also high for females leaving with the old males to later join the new males, despite infant survival. Female countertactics reflected female philopatry mediated by infant age. Presumably due to the seasonal timing of takeovers, most countertactics seemed successful given that 32.3% of females apparently incurred no costs and 41.2% incurred only low costs.

Understanding the reproductive parameters of endangered primate species is vital for evaluating the status of populations and developing adequate conservation measures. This study provides the first detailed analysis of the reproductive parameters of wild white-headed langurs (Trachypithecus leucocephalus), based on demographic data collected over an 8-year period in the Nongguan Karst Hills in Chongzuo County, Guangxi, China. From 1998 to 2002, a total of 133 live births were recorded in the population based on systematic censuses. Births occurred throughout the year, but the temporal pattern was highly correlated with seasonal variation in temperature and rainfall, with the birth peak coinciding with the dry and cold months of November–March. The average birthrate was 0.47±0.13 births per female per year and mortality for infants younger than 20 months was 15.8%. From 1998 to 2006, 14 females gave birth to 41 infants in four focal groups. The average age at first birth for female langurs was 5–6 years (n=5) and the interbirth interval (IBI) was 23.2±5.2 months (median=24.5 months, n=27). Infants are weaned at 19–21 months of age. The IBI for females with infant loss before weaning was significantly shorter than those for females whose infants survived. It appears that birth seasonality in the white-headed langurs is influenced by seasonal changes in food availability. The timing of conceptions was found to coincide with peak food availability. The reproductive parameters for white-headed langurs reported here are quite similar to those reported for other colobine species. One major difference is our observation of lower infant mortality in Trachypithecus. Am. J. Primatol. 71:558–566, 2009. © 2009 Wiley-Liss, Inc.

Numerous hypotheses have been developed to explain sex allocation. In male-dispersing, female cooperatively breeding species, the local resource competition model predicts male-biased birth sex ratio, the local resource enhancement model predicts female-biased birth sex ratio, and the population adjustment model predicts that biased birth sex ratio should not be favored if the two sexes are equally costly to rear. The male quality model predicts that, in polygynous species, females in better physical condition will either produce more sons than daughters or invest more heavily in sons than in daughters. White-headed langurs are a female philopatry and female cooperatively breeding species. During a 11-yr study, a total of 133 births were recorded, among which birth sex ratio (M:F = 73:49) was significantly male-biased. This is consistent with the prediction of the local resource competition model. On the other hand, if mothers balanced their investment between the two sexes, according to Fisher's population adjustment model, males should be the less-costly-to-rear sex. However, we found no sex difference for infant mortality (12.3% in males and 12.2% in females), and sons induced slightly longer interbirth interval (son: 26.4 ± 1.1 mo, daughter: 24.1 ± 0.6 mo) and lactational period (son: 20.9 ± 1.0 mo, daughters: 19.6 ± 0.5 mo) for their mothers. Thus, the population adjustment model was not supported by this study. The local resource enhancement model was not supported because birth sex ratio did not bias to females who provided more reproductive assistance. On the individual level, probit regression showed no relation between birth sex ratio and group size. Because the group size was considered to be negatively related to female physical condition, our study did not support the male-quality model. We suggested several possibilities to explain these results.

The number of males per group is the most variable aspect of primate social organization and is often related to the monopolizability of females, which is mainly determined by the number of females per group and their reproductive synchrony. Colobines show both inter-specific and intra-specific variations in the number of males per group. Compared with other colobine species, little is known about the social organization of white-headed langur (Trachypithecus leucocephalus), despite its endangered status and unusual limestone habitat. As a part of a long-term study of the white-headed langurs in the Nongguan Karst Hills, Guangxi, China, we quantitatively investigated their social organization by analyzing census data from 1998 to 2003. The population censuses revealed that the predominant social organization of bisexual groups was the one-male group, similar to a previous report on this species and many other Asian colobines. In such groups, one adult male associated with 5.1 adult females, 0.1 sub-adult males, 2.6 juveniles and 2.9 infants on average, with a mean group size of 11.7 individuals. In addition, three multi-male groups were recorded, consisting of 2-3 adult males, 1-5 adult females, 0-2 sub-adult males, 0-7 juveniles and 0-2 infants. They did not contain more adult females than the one-male groups and were unstable in group membership. The langurs outside bisexual groups were organized into small nonreproductive groups or lived as solitaries. The nonreproductive groups averaged 1.3 adult males, 1.3 sub-adult males and 2.6 juveniles. Juvenile females were present in such groups on 52.4% of all occasions. As predicted by the monopolization model, the prevalence of the one-male pattern in this species may mainly be attributed to the small number of females in the group. The possible reasons for the occurrence of multi-male groups and the presence of juvenile females in nonreproductive groups are also discussed.

Infanticide was first observed in langurs nearly 50 years ago, and this rare phenomenon has been inferred to have either an evolutionarily adaptive function or to be a pathological and non-functional behavior. In this study, we report 5 male takeover events in one-male groups of white-headed langurs in the Nongguan Karst Hills, Guangxi, China from 1998 to 2006. We recorded 13 attacks on 9 infants by extra-group males or new resident males. During the male takeovers, all of the infants younger than 6 months (with an average age of 3.6 months [N = 11]) in the groups disappeared. The infant death rate during the 4.2 months after takeover by a new male was significantly higher than the infant death rate calculated for most of the year. Older infants that were still nursing (with an average age of 14.1 months [N = 7]) were often attacked and seriously wounded by the extra-group males or new resident males, but all of them survived. The interbirth intervals of females whose infants were assumed to be killed by males were significantly reduced relative to those of females in groups with stable male tenure (mean = 10 months vs 25 months). Our data suggest that males kill unrelated and unweaned infants during the takeover period to decrease the time until the infants’ mothers resume fertility. Thus, infanticide would support sexual selection theory in white-headed langurs. The data also show that infanticidal behavior was directed toward the infants, especially those who were still nursing. Female dispersal may function as a counter-strategy to avoid infanticide.

A population census of white-headed langurs (Trachypithecus leucocephalus) was conducted using sleeping-site counts and the line-transect method from July to August 2002 at the Longrui Karst Hills, Guangxi, China. This area had been the largest habitat of the langurs across their range before the 1990s. While our survey revealed 146 old sleeping-sites, no white-headed langurs or fresh sleeping-sites were found in this area. Our study indicated that there had previously been a large population of langurs at Longrui Karst Hills, but now the langurs are possibly locally extinct in this area. If langurs still exist within the area, the population density must be very low. Interviews with local people confirmed that the number of white-headed langurs has been decreasing since the 1980s. Poaching was very common in the past and continues to be a problem at present, and it is likely that this has caused the decrease in the white-headed langur population at Longrui Karst Hills.