Hylobates pileatus
pileated gibbon

The distribution of Hylobates pileatus once extended from south Thailand to west of the Mekong in Cambodia, but is now found only in southeast Thailand, extreme southwestern Laos, and northwestern Cambodia. Other than one zone of sympatry with Hylobates lar in Kao Yai National Park, Thailand, the current range of H. pileatus excludes all other gibbon species. (Brockelman and Srikosamatara, 1993; Geissman, 1991a; Geissman, 1995; Groves, 1972; Nowak and Walker, 1999)

Hylobates pileatus can be found in tropical deciduous monsoon forests, dense evergreens, and tall moist montane forests throughout southeast Asia. They prefer old-growth forests with dense evergreen cover and avoid areas with high disturbance, patchy cover, large stands of exotic trees, and trees standing taller than the primary canopy. (Marshall, Jr., et al., 1972; Nowak and Walker, 1999; Phoonjampa, et al., 2011; Wolfheim, 1983)

Pileated gibbons have slender torsos, long arms, no tail, and ischial callosities. Head-and-body length ranges from 450 to 640 mm, and weight ranges between 4 to 8 kg, with an average of 5.4 kg for females and 5.5 kg for males. Both sexes are born covered in light buff-colored fur, with black spots on the top of their head and chest appearing at 10 to 12 months of age. These black spots grow continuously until sexual maturity. Adult females have a large black patch from the top of the head to the groin on the ventral surface, which forms an inverted triangle. The rest of the body remains light gray or buff. The hair above the female’s ears is white, and is long enough to hang over the temples in characteristic “Dagwood tufts” by 7 years of age. Sub-adult and young-adult females have a white brow band that decreases with age, pregnancy, or decreasing physical condition. Adult males are almost completely black with a narrow white face ring and crown ring as well as white hands, feet, and a genital tuft. Male hands and feet also have a fringe of hair halfway up their sides. Some males have lightly grizzled silver hairs on their legs and lower back. As with most other species in the genus Hylobates, the hair on the ulnar side of the forearm grows in the direction of the elbow, while hair on the radial side grows in the direction of the wrist. Infant pileated gibbons have pink skin, which darkens with age and sun exposure, turning a light gray by adolescence and, eventually, a dark charcoal gray by the time they reach sexual maturity. Females often have adult pelage by 4 years, while males have adult pelage by 6.5 years. Both males and females have hairless facial areas and very dense fur elsewhere on their bodies. Neither sex has a laryngeal sac. (Francis, 2008; Geissman, 1995; Groves, 1972; MacDonald, 2009; Marshall, Jr., et al., 1972; Mootnick, 2006; Nowak and Walker, 1999)

The teeth of Hylobates pileatus have a distinct cingulum (i.e., a shelf-like ridge around the outside of an upper molar) on the lingual side of the upper cheekteeth, a more lateral metaconid coupled with a more lingual hypoconid, and a comparatively large third molar. The skull morphology consists of a bowed zygomatic arch, downward pointing foramen magnum, and thick orbital rim. The dental formula of H. pileatus is 2/2, 1/1, 2/2, 3/3 = 32 and is common to all Hylobatidae. (Groves, 1972)

All gibbons, including pileated gibbons, are monogamous and form small family groups consisting of a mating pair and young offspring. These groups occupy and defend a constant home range. At any one time, a single family group usually consists of one mating pair and two offspring of staggered ages, which eventually leave as additional offspring are born. (Brockelman, et al., 2008; Nowak and Walker, 1999)

The testes of male pileated gibbons descend late in the juvenile period, as is common with most species of Hylobates. Pileated gibbon males have the shortest bacula of all Hylobates. Females reach sexual maturity at around 7.5 years of age, and males reach sexual maturity between 5 and 8 years of age. Captive pileated gibbons have reached sexual maturity earlier in both sexes, which may be due to stress, increased resource abundance, or a number of other factors. (Geissman, 1991b; Groves, 1972; Humphrey and Bain, 1990; King and Mitchell, 1987; Marshall, Jr., et al., 1972)

The estrous cycle of female pileated gibbons averages 27 to 30 days with a 4 to 5 day menstruation. Average gestation lasts 6 to 7.5 months. Like most other gibbons they produce one offspring per reproductive cycle, and most young are weaned between 1 and 2 years old. Infants stay with the mother until about age 2, when they begin to move around independently. There is no known breeding season for members of the genus Hylobates. (Geissman, 1991b; King and Mitchell, 1987; Nowak and Walker, 1999; Srikosamatara, 1984)

Pileated gibbon are weaned between 1 and 2 years after birth, and infants remain with their mother until about 2 years old. As with other gibbons, juvenile pileated gibbons remain with their family group and feed within the group's home range until sexually mature. Parents increase antagonism towards sub-adult offspring and prohibit them from mating in an effort to drive them from the group and territory. The age at which a sub-adult is driven out may depend on the size of the family group and resource availability. Little is known of paternal care in pileated gibbons. (Chivers, 1979; Geissman, 1991b; Srikosamatara, 1984)

The longest-lived Hylobates pileatus in captivity survived 31 years. Little other information exists on the lifespan of H. pileatus. (Humphrey and Bain, 1990)

Pileated gibbons, like all other gibbons, are almost completely arboreal and travel mostly through brachiation, the movement of swinging from branch to branch with the arms extended above the head. This form of locomotion, which can be very rapid, usually occurs in the upper and mid-canopies. Brachiation allows for large leaps of up to 9 m or more, with neither hand touching a branch. When on the ground, pileated gibbons are bipedal and hold their arms extended above the head for balance. Pileated gibbons are diurnal and travel between 400 and 1300 m (averaging 833 m) in a single day with their family group. Sleep sites are usually centralized within the groups territory and generally consist of tall trees with few lower branches and vines. The same tree is rarely used more than once and almost never used on two consecutive nights. Characteristics common to sleeping trees most likely help reduce risk of predation, as well as decrease interactions with potential competitors. Pileated gibbons often sleep in trees that are close to the last feeding trees of that day. Intra-group individuals choose trees 10 to 15 m apart, with infants sleeping with the mother and younger offspring sleeping in trees closer to the mother’s tree. (Nowak and Walker, 1999; Phoonjampa, et al., 2010; Srikosamatara, 1984)

Hylobates pileatus spends about 8.2 hours per day resting (37%), with the remainder of the day consisting of feeding (26%), travelling (25%), grooming (5%), calling (4%), and playing (3%). The remaining 15.8 hours of the day are spent sleeping. Gibbons are social groomers, and intra-group rank determines who grooms whom, but little of the pileated gibbon's day is dedicated to grooming. (Chivers, 1979; Srikosamatara, 1984)

Up to three groups of Hylobates pileatus have been reported in 1 km^2 of newer growth forests and up to 6.5 groups per square kilometer can be found in old growth forests. Groups of H. pileatus in Thailand’s Khao Soi Dao Wildlife Sanctuary have home ranges of approximately .36 km^2 with .16 to .27 km^2 of defended territory used exclusively by the group. (Nowak and Walker, 1999; Phoonjampa, et al., 2011; Srikosamatara, 1984)

Hylobates pileatus couples announce their territories via a unique duet song. Females begin with a “great call,” and males joins midway through the great call with a series of shorter calls. The duet concludes with a single phrase of the males shorter call. The calling female also begins a locomotor display of brachiation and branch breaking during the song, sometimes accompanied by the male. The song pattern and tones of phrases used are unique to H. pileatus and are used as a conspecific identifier. Song bouts usually occur in the morning, with more recently established groups singing more often than older groups. Pileated gibbons sing least on rainy, cloudy, and windy days. (Brockelman and Srikosamatara, 1993; Geissman, 1995; Marshall, Jr., et al., 1972; Nowak and Walker, 1999)

Pileated gibbons are mostly frugivorous, with 45% of their diet consisting of soft-skinned or hard-rinded fruit and 26% consisting of figs. Figs are found in large patches and can be fed on for a longer period of time than other fruits, which are more dispersed. Thus, travel and search time significantly decrease when feeding on figs as opposed to other fruits. An additional 13% of the pileated gibbon’s diet is from young leaves and 2% is from unopened leaf shoots. THey also consume insects (15% of total diet), eggs, and small vertebrates. Pileated gibbons budget more time for fruit consumption during morning and evening hours, while leaves and insects are consumed more in the middle of the day. This pattern of behavior may be to immediately restore energy lost during the previous night's sleep and prepare for energy loss the following night, as fruits are high in carbohydrates. Pileated gibbons prefer to eat while sitting on branches in the middle and upper canopies. Pileated gibbons require open water during during the dry season. (Humphrey and Bain, 1990; Nowak and Walker, 1999; Srikosamatara, 1984)

There is no information regard major predators of Hylobates pileatus. However, large carnivorous species such as clouded leopards and pythons do exist throughout this species range. (Phoonjampa, et al., 2010)

As a frugivorous species that travels long distances within its home range, Hylobates pileatus is an probably important seed disperser for various tree species within its home range. Dispersal of seeds away from parent trees reduces seed predation and competition between emerging conspecifics. This increases both the probability of survival for the resulting young trees and local plant diversity. Individual seeds that pass through the pileated gibbon’s digestive tract are also more likely to germinate than those that do not. In addition to seed dispersal, 15% of H. pileatus's diet consists of insects, which may help reduce the abundance of insect pests. There is limited information on parasites specific to H. pileatus, however they are potential carriers of chagas disease (also known as American trypanosomiasis) which is caused by protists Trypanosoma cruzi and hepatitis B. They are also host to a number of different endoparasites, including flatworms and roundworms. (MacDonald, 2009; McConkey and Chivers, 2007; Srikosamatara, 1984)

Hylobates pileatus is hunted by humans for food and captured for the pet trade throughout their range. Analysis of the hepatitis B virus that occurs in wild H. pileatus populations can help with further understanding human-associated hepatitis B, its possible origins, and its host-virus interactions in terms of infection and disease. (Aiba, et al., 2003; Burton and Burton, 1988; Sall, et al., 2005; Wolfheim, 1983)

Pileated gibbons are naturally susceptible to a hepatitis B virus similar to that which affects humans. It has been postulated that this disease could be transferred to humans, although there have been no such reports thus far. (Aiba, et al., 2003; Sall, et al., 2005)

All gibbons are threatened by hunting and habitat destruction. Hylobates pileatus is listed as an endangered on the IUCN's Red List of Threatened Species, with an estimated population of 65,000 individuals in Thailand and Cambodia, combined. Slash-and-burn agriculture in Thailand has destroyed forest habitats, and hunting pileated gibbons for sustenance is still practiced. In Cambodia, deforestation continues to reduce available land habitable by H. pileatus. The habitat of H. pileatus is not only becoming reduced in overall area, but is also becoming fragmented, separating populations ecologically and reproductively. These small, fragmented patches of land have low prospects for population maintenance. Although many areas currently inhabited by H. pileatus are protected, enforcement of conservation based laws is weak and numbers continue to decrease as a result. (Humphrey and Bain, 1990; Nowak and Walker, 1999; Phoonjampa, et al., 2011)