Disassociation of social and sexual partner relationships in a gibbon population with stable one-male two-female groups.

Adult males living in a one-male multi-female social group are expected to try to monopolize copulations with resident females to increase reproductive fitness. Gibbons have traditionally been described as living in monogamous groups, with the sole resident adult male assumed to sire all of the group's offspring. Here, we used microsatellite analyses and behavioral observations to examine rates of extra-group paternity (EGP) over 16 years in a population of crested gibbons (Nomascus concolor) that form stable and long-term one-male two-female social units. Forty percent of offspring (N = 14) were sired by extra-group males. To understand this high level of EGP, we tested whether inbreeding avoidance was related to EGP. Females who engaged in EGP did not show larger pairwise relatedness with their resident male compared to females who did not engage in EGP. Nevertheless, the standardized heterozygosity of EGP offspring was significantly higher than for offspring sired by the group's resident male. These results provide partial support for the inbreeding avoidance hypothesis. It appears that resident male crested gibbons are unable to monopolize resident females' matings. Our results indicate that long-term social partners are often distinct from sexual partners in this population. Clearly, the breeding system of crested gibbons is more flexible than previously thought, indicating a need for integrating long-term behavioral data and genetic research to re-evaluate gibbon social and sexual relationships derived from concepts of monogamy and pair-bonding.

Influence of food availability and climate on behavior patterns of western black crested gibbons (Nomascus concolor) at Mt. Wuliang, Yunnan, China.

Food abundance and climatic factors can significantly affect the behavior of animals and constrain their activity budgets. The population of western black crested gibbons (Nomascus concolor) in Mt. Wuliang lives in montane forest and is close to the northern extreme of the distribution for gibbons (Hylobatidae). Their habitats show remarkable seasonal variation in terms of food availability, temperature, and rainfall. To understand behavioral adaptations of western black crested gibbons to different sets of ecological conditions, we examined relationships among food availability, mean temperature, rainfall, and behavior patterns by observing two groups for 1 year each. Our results revealed that activity budget was affected by food availability and mean temperature. The gibbons spent more time eating flowers when that resource was more available and spent less time moving when fruit was more available. The gibbons spent less time feeding and more time resting, and spent less feeding time on fruit and leaves when the mean temperature was lower. These results suggest that the gibbons displayed a pronounced preference for flowers as a food resource and adopted a time minimizer strategy when high-nutrient food items (i.e., fruit) were more available. In addition, the gibbons adopted an energy-conserving strategy during periods of low temperature. The flexibility of behavioral patterns in responding to food availability and temperature may potentially improve the gibbons' prospects of surviving and reproducing in a northern montane forest.

Significance of grooming behavior in two polygynous groups of western black crested gibbons: Implications for understanding social relationships among immigrant and resident group members.

In primates, grooming is considered among the most common behaviors for maintaining social bonds; however, to date, few studies have examined grooming behavior in gibbon species in detail. We used both a 5-min interval scan method and social network analysis to study grooming in two groups of polygynous western black-crested gibbon (Nomascus concolor) in Wuliang Mountain, Central Yunnan, China. Individuals in both groups spent little time in social grooming (1.45% and 1.97% of active time). We compared the two groups' grooming networks and found that the group that maintained a more stable social unit had a more complex grooming network while the group with new immigrants had a grooming network characterized by fewer grooming pairs. Females in both groups played important roles in the grooming network. A newly immigrant female spent the most time grooming others and chose the resident adult female as her main adult grooming partner. Other females from both groups chose the adult male as their primary grooming partner (except their offspring). A sub-adult male who had resided in his natal group for 2 years after maturing into an adult also groomed more and was at the center of the network. This male finally replaced the breeding male in his group 3 years after our data collection period ended. We hypothesize that the immigrant female and the resident young adult male engaged in more extensive grooming interactions as a behavioral strategy to gain tolerance from long-term residents. Our results suggest that female gibbons in polygynous groups actively cooperate in maintaining social relationships rather than co-exist through tolerance or avoidance. Our observations indicate that grooming networks in crested gibbons reflect individual dynamics and partly support the social cohesion hypothesis for primate grooming. In this regard, we suggest that changes in gibbon grooming networks can be used to predict social change.

Individuality in male songs of wild black crested gibbons (Nomascus concolor).

This is the first study of vocal individuality in male songs of black crested gibbons. The sound recordings were carried out at two field sites, Pinghe, Ailao Mountains, and Dazhaizi, Wuliang Mountains, both located in Yunnan province, China. A total of 127 coda phrases of 38 male songs bouts of eight individual male gibbons were analyzed. Stepwise discriminant function analysis was used to examine the acoustic individuality of the males. We found that individuality among neighbors was very pronounced. Moreover, individuality within a site (i.e. among neighbors) is higher than among individuals between sites. Our finding suggests that black crested gibbons may actively increase their degree of vocal individuality against that of their immediate neighbors by vocal adjustment.

Preliminary study on dietary selection in Shortridge's langurs ( Trachypithecus shortridgei) from China.

Understanding dietary selection and feeding strategies is important for the conservation and management of endangered primate species. Here, we conducted a preliminary study on the diet and feeding behavior of endangered Shortridge's langurs ( Trachypithecus shortridgei) within the Drung River Valley (Dulongjiang) in southwestern China. The study site lies at a high latitude (N27°47.5') and elevation (1 900 m a.s.l.) and is characterized by substantial annual rainfall (2 745.1 mm). From August 2012 to September 2013, we observed five groups of langurs and analyzed their overall food composition and dietary variation in spring and autumn. To understand their dietary adaptations to the distinctive habitat of the Drung River Valley, we also compared the diet of Shortridge's langurs to that of other Trachypithecus species inhabiting different environments. Results indicated that T. shortridgei fed on 52 plant species, 23 of which each accounted for ≥1% of their annual feeding time. Their primary dietary components included leaves (46.2%, young, mature, and petioles), fruits (28.7%, unripe 17.6%, ripe 11.1%), and mosses (10.2%). The langurs mainly consumed mature (34.2%) and young leaves (27.5%) in spring and ripe fruits (39.4%) and mature leaves (24.7%) in autumn. Two species of moss ( Macrothamnium macrocarpum and Scapania verrucosa, 21.2% of annual feeding time), which are usually found growing together on cliffs, played a relatively important role in the diet of T. shortridgei. The langurs mainly consumed ripe fruits of Saurauia napaulensis (7.1%) and Dendropanax burmanicus (7.1%), which were abundant at lower elevations. Trachypithecus species in temperate forests consumed more fruits and seeds but fewer leaves (similar mature leaves but fewer young leaves) than those species in tropical forests, which may be related to their availability and abundance. Compared to Trachypithecus species in temperate forests, the higher proportion of mosses and mature leaves but fewer young leaves in the annual diet of T. shortridgei are likely a response to the distinctive Drung River Valley habitat. Therefore, conservation of the main food plants of this threatened species could be vital for its survival and conservation management.

Ecology and social system of northern gibbons living in cold seasonal forests.

Gibbons in China represent the northernmost margin of present day gibbon species distribution (around N25°). Compared to tropical habitats, northern gibbon habitats are characterized by low temperatures and remarkable seasonal variation in fruit abundance. How gibbons adapt to their cold and seasonal habitats and what ecological factors affect their sociality are key questions for understanding their ecology and social system evolution, the elucidation of which will contribute to the conservation of these special populations/species. According to preliminary short-term studies, northern gibbons consume more leaves and use larger home ranges than tropical gibbons. Interestingly, some Nomascus groups consist of more than one adult female. However, these preliminary results are not well understood or incorporated into current socio-ecological theories regarding gibbon species. To better understand northern gibbons, our team has systematically studied three habituated groups of Nomascus concolor, three groups of N. nasutus, and two habituated groups of Hoolock tianxing since 2002. In this paper, we stress the challenges facing gibbons living in northern habitats and summarize their behavioral adaptations to their harsh environments. We also describe the northern gibbon social system and discuss the potential relationships between their ecology and sociality. Finally, we highlight future research questions related to northern gibbons in China.

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.