PILEATED GIBBON

Hylobates pileatus

Geographic Distribution and Habitat
Also known as capped or crowned gibbon, pileated gibbons are native to eastern Thailand, western Cambodia, and southwest Lao People’s Democratic Republic. They live in tropical deciduous monsoon forests, dense evergreen stands, and tall moist forest regions. They prefer tall trees and they are often found in the mid- to upper canopy.

Pileated gibbon range, IUCN 2020

Size, Weight, and Lifespan
The head-to-body length for both male and female pileated gibbons ranges from 1.5 to 2 ft (450 to 640 mm). Their weight ranges from between 8 to 18 lb (3.5 to 8 kg).

While there is limited information regarding the lifespan of the pileated gibbon, the longest-lived individual in captivity survived for 31 years. 

Appearance
Pileated gibbons are small apes with rounded heads, shaggy coats, long torsos and forearms, and no tails. Their buttocks have ischial callosities, or a thickened layer of skin, that allows them to sit upright and balance on tree branches without discomfort.

Despite both genders being similar in size and weight, sexual dimorphism is visible in their coloration: males have short black coats, with white hands, feet, and brow band, while females range in color from pale yellow to silvery grey with a black head and belly. This color difference is call sexual dichromatism. Both genders typically have a white circular streak around the crown and sides of their head, hence the name “pileated,” which means “capped.” Their faces are bare and have dark pigmentation.

They have an opposable thumb and an opposable big toe that allow them to grasp and carry things with both hands and feet.

What Does It Mean?

Ischial callosities:
A thickened piece of skin found on the buttocks of animals.

Sexual dimorphism:
Distinct difference in size or appearance between the sexes of an animal in addition to difference between the reproductive organs themselves.

Visit the Glossary for more definitions

Diet
The diet of pileated gibbons is predominantly frugivorous, which means that they eat mostly fruit. Soft skinned or hard-rinded fruit comprises 75% of their diets, while the rest includes leaves, insects, eggs, and small vertebrates like birds. Figs are an important part of their diet mainly because they are found in large patches and can be fed on for longer periods of time, thus decreasing travel and search time.

Behavior and Lifestyle
Being mainly arboreal, pileated gibbons are masters of brachiation, swinging from branch to branch with their exceptionally long forearms, and can bridge gaps between trees that are as wide as 30 to 50 ft (9 to 15 m) and reach speeds of up to 35 mph (56 km/h). The physiology of their ball-joint wrists greatly reduces the amount of energy they need to expend in their upper arms and torsos when in locomotion. While they are extremely agile in trees, they are unable to swim and will try to avoid the water.

When on the ground, pileated gibbons walk bipedally and throw their hands above their heads for balance, reminiscent of a tightrope walker. Scientists have studied their movements for clues to determine what evolutionary causes may have led to humans walking upright.

Daily Life and Group Dynamics 
Pileated gibbons live in pairs or family groups comprised of an adult male and female and their offspring. Their relationships are monogamous, with life-long partnerships.

The daily life of a pileated gibbon consists of resting (37%), feeding (26%), travelling (25%), grooming (5%), calling/singing (4%), and playing (3%).

Unlike other apes, they do not make nests for sleeping and, instead, prefer to sleep alone in an upright position in a tree, facilitated by their ischial callosities.

Fun Facts

Pileated gibbons are monogamous, which typically only occurs in about 3% of mammals.

Every morning, pileated gibbons sing spectacular duet songs together that warn other groups to stay away from their territories.

Communication
Communication is predominantly accomplished via visual and tactile cues; however, couples and families sing harmonious and intricate duets and choruses together. Gibbons have large inflatable throat pouches, called a gular sac, that amplifies their vocalizations. They are far less likely to sing on rainy and cloudy days.

Upon awakening each morning, a family loudly announces its presence in the forest by using animated gestures and by belting out hooting calls. The chorus is typically initiated by an adult female and lasts about 30 mins. It warns other groups to stay out of their territory and away from their food source. A typical family has a home range of about 74 to 124 acres (30 to 50 hectares) of rainforest land.

Social grooming, which is done quite extensively among the pileated gibbon, is an important tactile method of communication used to strengthen and reinforce bonds between individuals.

Reproduction and Family
Pileated gibbon families occupy and defend a home range and rarely leave this range for a new one.

Sexual maturation is reached between the ages of 5 and 8 in males and at around that age of 7 in females. Captive pileated gibbons are documented to reach puberty much earlier. Increased levels of stress or resource abundance could be contributing factors to this early onset. Offspring will stay with their family group until they reach maturation.

Females are in estrus, or are sexually receptive, for a period of 27 to 30 days, with a 4- to 5-day menstrual cycle.

The average gestation period lasts for about 6 to 7.5 months, resulting in the birth of a single offspring. Infants are weaned between 1 and 2 years of age and follow their mother until age 2, at which point they start moving around independently. Compared to other species, the pileated gibbon does not have a favored breeding season.  

Ecological Role
As with most primate species, the pileated gibbon helps to disperse seeds throughout the forest through consumption and excretion. Seeds that pass through their digestive tract are more likely to germinate than others.

Since their diets include a fair number of insects, they may also help to reduce the insect pest population.

Conservation Status and Threats
The pileated gibbon is listed as Endangered on the International Union for the Conservation of Nature’s Red List of Threatened Species (IUCN, 2015). Hunting for bushmeat and habitat fragmentation and degradation are major threats to the pileated gibbon. Logging, agriculture, hydroelectric development, and new human habitation development contribute to the destruction of their homes.

While there is limited information regarding common predators of the pileated gibbon, large carnivorous species like the leopard and python live throughout their home range.

Pileated gibbons are carriers of the Chagas disease, which is caused by the hepatitis B virus, and are hosts to a number of different endoparasites, including flatworms and roundworms. As a result, they are used in biomedical studies in an effort to learn about human-associated hepatitis B. 

Conservation Efforts
The pileated gibbon is protected from trade in Appendix I of Convention of International Trade in Endangered Species of Wild Fauna and Flora (CITES). 

Due to the loss and fragmentation of the pileated gibbon habitat, an International Studbook was established in 1990, under the World Association of Zoos and Aquariums (WAZA) umbrella, and developed conservation breeding programs that operate at the regional level by the Association of Zoos and Aquariums (AZA), European Association of Zoos and Aquaria (EAZA), and Japanese Association of Zoos and Aquariums (JAZA).

In Cambodia, protected areas for pileated gibbons were developed in contiguous places like Samkos, Aural Wildlife Sanctuaries, and the Central Cardamom Protected Forest. These areas consist of nearly 7,000 groups.

The Tab Lan National Park, Pang Sida National Park, Ta Phraya National Park, Khao Ang Ru Nai Sanctuary, and Khao Soi Dao Sanctuary are the largest protected forest areas in the Thailand region.

References:

  • http://www.waza.org/en/zoo/choose-a-species/mammals/primates/apes/hylobates-pileatus
  • http://animaldiversity.org/accounts/Hylobates_pileatus/#D3EFBDB8-B380-4CC4-8734-CBBF74585337
  • http://www.iucnredlist.org/details/10552/0
  • http://www.arkive.org/pileated-gibbon/hylobates-pileatus/
  • https://www.nationalgeographic.com/animals/mammals/group/gibbons/
  • http://www.newworldencyclopedia.org/entry/Gibbon
  • http://www.enchantedlearning.com/subjects/apes/gibbon/
  • http://www.theprimata.com/hylobates_pileatus/

Written by Nina Shangari, January 2018