Geographic Distribution and Habitat
The pileated gibbon (Hylobates pileatus), also known as the capped or crowned gibbon, is a native of the southeast Asian countries of Thailand, Cambodia, and (a small portion of) Laos. They live in lush seasonal evergreen and mixed deciduous-evergreen forests, up to elevations of about 4,900 feet (1,500 m). They prefer old-growth forests with dense cover, avoiding forests that have been disturbed and have too many gaps in the canopy. The Cardamom Mountains of southwestern Cambodia are a particularly important region for pileated gibbons, and it is estimated that 20,000 pileated gibbons make their home in this mountain range. In Thailand, most pileated gibbons live in the four largest forest areas in the country. Their range in Laos is limited, so fewer pileated gibbons live there.
Size, Weight, and Lifespan
Pileated gibbons weigh between 9 and 18 pounds (4–8 kg), with males just slightly larger than females. Their head and body length ranges from 18 to 25 inches (45–64 cm). They have been known to live to the age of 31 years in captivity.
Pileated gibbons, like other gibbons, are relatively small-bodied apes with extremely long arms and legs. Like all apes, pileated gibbons lack a tail, but they have both opposable thumbs and opposable big toes to help them grasp branches and manipulate objects. They display obvious sexual dimorphism—or physical differences between the sexes—in the form of their color patterns. Females are a buffy gray over most of the body, with a patch of black extending from the top of their head to their groin. They sometimes have a white browband across their forehead, which fades away with age and pregnancy. Males, by contrast, are ink-black all over, except for a white ring around their faces, white hands and feet, and a white genital tuft. Both sexes have tufts of hair extending from their temples around the back of their head, giving them the appearance of a monk or perhaps a mad scientist. This is where their name comes from: “pileated” means “capped.”
All pileated gibbons are born covered in buff-colored hair. When they are about 10 months old, they develop black spots on their head and chest. These spots grow as they age until they develop their adult coloring.
Pileated gibbons are true frugivores—fruit eaters. Figs compose approximately 25% of their diet, and other fruits make up 45%. Young leaves and insects each make up about 15% of their diet. Eggs and even small vertebrates are also eaten opportunistically. Fruits are normally eaten in the morning and evening, while insects are usually eaten during midday. This may be because fruits are high in carbohydrates, so they are used to break the night’s fast in the morning and to load up on long-lasting energy for the night.
Behavior and Lifestyle
Pileated gibbons are very well adapted to a life spent fully in the trees. They move about primarily via brachiation—swinging from branches with their long arms. Watching a gibbon in motion is a bit like watching an acrobat at the circus, except what the acrobat trains for years to do, gibbons can do innately! They can reach tremendous speeds—up to 35 mph (56 km/h)—by brachiating and making very long leaps, up to 30 feet (9 m) or more, completely airborne. Pileated gibbons very rarely find themselves on the ground, but when they do, they walk on two legs, holding their arms above their head somewhat awkwardly for balance—after all, they’re not adapted for ground life. Pileated gibbons have been described as shy, elusive, and aloof.
About one-third of a pileated gibbon’s waking hours are spent resting. A quarter of their day is spent eating and another quarter traveling. They travel long distances within their home range every day. Their remaining time during the day is spent grooming and playing. They sleep close to 16 hours per day! They sleep at night in tall trees that have few lower branches or vines, to help avoid nighttime predators. They almost never sleep in the same tree twice consecutively. Each member of the group usually chooses their own tree, except for a mother and her infant, but the trees are usually close together. Pileated gibbons have high “sleep efficiency”—that is, they are actually asleep almost the entire time that they are in their sleeping site—although a full moon and high winds can make for lower-quality sleep.
“Singing in the rain”? Not for pileated gibbons! Though they are known for their unique duets, pileated gibbons are less likely to sing on cloudy, windy, and rainy days.
Not only do hybrid pileated/white-handed gibbons often look different than “pure” gibbons, their songs are also a unique blend of each species’ songs.
Pileated gibbons live in small family groups consisting of a mated pair and their offspring. Their average group size is four individuals—two parents and two offspring of staggered ages. As they have more offspring, their oldest matures and leaves to form their own family. Grooming is an important method of bonding with others, and their rank within the family determines who grooms whom.
Pileated gibbons are known for their distinctive vocalizations, particularly their duets. The female of the group begins the call with a “great call,” and the male chimes in midway with a series of short calls. This is accompanied by the female’s display of swinging and branch breaking. This song is unique—no other species has a song quite like it. These songs are usually performed in the morning, and scientists have noted that newly formed groups sing more than longer-established groups.
Pileated gibbons are monogamous, selecting one mate and typically staying with that mate for life—a rarity in the animal world. After a female copulates with her mate, which happens year-round, she is pregnant for 6 to 7.5 months, after which she gives birth to a single baby. Little is known about parental care among pileated gibbons, except that the mother nurses her baby for about a year, and takes care of it for another year. When the offspring reaches the age of about two, they are more independent and begin to move around more on their own.
Females reach sexual maturity at about 7.5 years of age, while male sexual maturity may come sometime between 5 and 8 years of age. They stay with their family groups until they are sexually mature, at which point the parents may “convince” them to leave by displaying antagonistic behaviors and preventing them from mating. The exact age at which they are booted out of the family unit may depend on resource availability.
In a small area of their total range, in Kao Yai National Park of Thailand, pileated gibbons live alongside and even hybridize with the white-handed gibbon (H. lar). Scientists believe that historically, they had much more overlap in their ranges. Some individuals resemble a cross between the two species, and they are likely the result of direct crossbreeding. Even among apparently “pure” pileated gibbons, genetic analysis has shown that many individuals in this region actually have some slightly mixed ancestry with white-handed gibbons, even though the two species split around 3–4 million years ago. It is believed that hybridization is not a new phenomenon, as the species have likely had some amount of hybridization for a very long time. It is unknown whether hybrid individuals suffer reduced rates of reproduction. However, because the two species only overlap geographically in a small area, hybridization is rare among the species as a whole. Besides the white-handed gibbon, pileated gibbons do not live alongside any other gibbons.
As for predators, there is no conclusive evidence of what may prey upon pileated gibbons, but leopards and pythons live in the same range and may prey on them. As frugivores, and especially ones that travel long distances during the day, pileated gibbons are important seed dispersers. Seeds that pass through pileated gibbons’ digestive systems are even more likely to germinate than those that do not.
The International Union for Conservation of Nature lists the pileated gibbon as Endangered (IUCN, 2015), appearing on the IUCN’s Red List of Threatened Species. It is believed that pileated gibbons have experienced a population loss of more than 50% over the last 45 years, which represents three pileated gibbon generations. This loss has mainly been caused by habitat loss and hunting, issues that have not slowed or ceased. It is predicted that over the next 45 years, another 50% of pileated gibbons will be lost. Currently, there are an estimated 35,000 pileated gibbons in Cambodia and 12,000 in Thailand, with a significantly smaller population in Laos. Thus, their total population is estimated to be roughly 50,000 individuals.
In Thailand, pileated gibbons live entirely within protected areas and, fortunately, slash-and-burn agriculture resulting in habitat loss has largely waned in recent years. That said, historic habitat loss in Thailand is profound and pileated gibbons are still at risk in Thailand and elsewhere from subsistence hunting and illegal collection for the pet trade.
In Cambodia, where most pileated gibbons live, the situation is much more dire. Habitat destruction is rampant and presents the main threat against pileated gibbons. Most populations in Cambodia do not live in protected areas, and their habitats are at great risk of loss to agriculture, logging, hydroelectric conversion, and human settlements. Outright habitat loss is not the only threat, either: habitat fragmentation—the splitting of habitat into isolated “islands”—is also a significant threat to pileated gibbons. Populations within these “islands” cannot easily interact or reproduce, potentially leading to reduced genetic diversity within the species.
Pileated gibbons are listed in Appendix I of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival. Pileated gibbon habitat is largely protected in Thailand due to protected areas such as Tab Lan, Pang Sida, and Ta Phraya National Parks. Community involvement and education is the main form of conservation action taking place in Thailand. A conservation initiative aimed to reduce opportunistic hunting took place in the country in 2009. Before the initiative, the gibbon population in Thailand declined by 70% between 1979 and 2008, largely due to bushmeat hunting. By 2012, the gibbon population had increased by 20%, and increased even more by 2016.
In Cambodia, where habitat loss is the major threat, the highest priority conservation action is protecting land and, importantly, strengthening the administration of protected lands. There are three important protected areas for pileated gibbons in the country: the Samkos and Aural Wildlife Sanctuaries and the Central Cardamom Protected Forest. Efforts by the Cambodian government in cooperation with Conservation International have resulted in the development of a management plan for the protected areas and the deployment of ranger units to combat illegal poaching and logging.
The nonprofit organization Wildlife Alliance is also working with the Cambodian government to rescue live, illegally collected pileated gibbons. Five- to six-thousand animals of many species are rescued by the organization annually. Most animals are immediately released upon confiscation. If a pileated gibbon needs medical care before release, it is housed in a temporary rehabilitation program before being released under IUCN protocol. For pileated gibbons that cannot be released—such as those that have grown too used to humans and don’t have the skills needed to survive in the wild—they are incorporated into the organization’s captive breeding program. In the program, mothers raise their own offspring, who are eventually released into the wild themselves. A monitoring study of pileated gibbons that have been released into the wild found that individuals that have been raised by their mother with minimal human intervention were more likely to be successfully released. Individuals that were confiscated from the wildlife trade, often hand-reared by humans, were less likely to successfully return to the wild.
- Leroux, N., R. Bunthoeum, N. Marx. 2019. The Reintroduction of Captive-born Pileated Gibbons (Hylobates pileatus) into the Angkor Protected Forest, Siem Reap, Cambodia. Primate Conservation 33.
- Markviriya, D., N. Asensio, W. Brockelman, et al. 2022. Genetic analysis of hybridization between white-handed (Hylobates lar) and pileated (Hylobates pileatus) gibbons in a contact zone in Khao Yai National Park, Thailand. Primates 63: 51–63. https://doi.org/10.1007/s10329-021-00958-y
- Matsudaira, K., U. Reichard, T. Ishida, S. Malaivijitnond. 2022. Introgression and mating patterns between white-handed gibbons (Hylobates lar) and pileated gibbons (Hylobates pileatus) in a natural hybrid zone. PLoS ONE 17(3). https://doi.org/10.1371/journal.pone.0264519
- Reyes, K. R., U. Patel., C.Nunn, et al. 2021. Gibbon sleep quantified: the influence of lunar phase and meteorological variables on activity in Hylobates moloch and Hylobates pileatus. Primates 62: 749–759. https://doi.org/10.1007/s10329-021-00920-y
Written by K. Clare Quinlan, April 2023