The species as a whole occurs throughout Borneo except for the southwest (where H. albibarbis is found), from the north bank of the Kapuas River clockwise around the island to the east bank of the Barito River (Groves 2001).

Hylobates muelleri muelleri
This taxon is found in southeast Kalimantan, Indonesia, approximately south of the Mahakam River and west of the Barito River (Groves 2001).

Hylobates muelleri funereus
This taxon is found in Brunei, Indonesia (Kalimantan) and Malaysia (Sabah and Sarawak). It ranges from the northeast of Borneo (Sabah) south to the Mahakam River and perhaps west to the Baram District and the IV Division of Sarawak (Groves 2001).

Hylobates muelleri abbotti
This taxon is found in Indonesia (Kalimantan) and Malaysia (Sarawak). It occurs north of the Kapuas River and east as far east as the Saribas District of Sarawak (Groves 2001).

Gray gibbons, Hylobates muelleri, are found in throughout Borneo excluding the southwest region.

Biogeographic Regions: oriental (Native )

Other Geographic Terms: island endemic

Coloration of H. muelleri varies from gray to brown. The top of the head and the chest are darker than the rest of the body. Total body length ranges from 440 to 635 mm. Gray gibbons weigh between 4 and 8 kg. They have buttock pads, long canine teeth, and no tail. The basal part of the thumb extends from the wrist rather than the palm of the hand, allowing an extended range of movement.

Sexual dimorphism is not pronounced in H. muelleri. Males and females are similar in morphology.

Range mass: 4 to 8 kg.

Range length: 440 to 635 mm.

Other Physical Features: endothermic ; homoiothermic; bilateral symmetry

Sexual Dimorphism: sexes alike

Habitat and Ecology

Systems

• Terrestrial

Gray gibbons live in tropical evergreen and semi-evergreen rain forests.

Habitat Regions: tropical ; terrestrial

Terrestrial Biomes: forest ; rainforest

The majority of the diet consists of ripe, sugar-rich, fruit and figs. A smaller portion of the diet consists of leaves.

Plant Foods: leaves; fruit

Primary Diet: herbivore (Frugivore )

As frugivores, these animals may play some role in seed dispersal.

Ecosystem Impact: disperses seeds

Predation on these animals has not been reported. Avian predators and arboreal snakes are likely to be their most significant predators.

The call of gray gibbons has been studied in detail. Adult males sing long songs before sunrise. Females sing with males after sunrise and before 10:00 AM. Their duets average 15 minutes and occur on a daily basis.

Lone males sing longer songs than paired males, possibly to attract mates. Unpaired females rarely sing.

Grooming and social play are two forms of tactile communication used in this species. Mating is another.

Like other primates, these gibbons likely use gestures, facial expressions, and body postures to communicate.

Communication Channels: visual ; tactile ; acoustic

Other Communication Modes: duets

Perception Channels: visual ; acoustic

Although data are lacking for H. muelleri, other members of the genus Hylobates are known to have lived as long as 44 years in captivity, and 25 years in the wild. It is likely that this species is similar.

Range lifespan

Status: captivity: 57 (high) years.

Maximum longevity: 57 years (captivity) Observations: A wild born male was still alive in captivity when about 57 years of age (Richard Weigl 2005).

Gray gibbons are monogamous. The mated pair and their offspring occupy a defended home range.

Mating System: monogamous

The age of sexual maturation is 8 to 9 years.

Females have an estrous cycle of about 28 days. There are no visible sexual swellings, although the genitalia undergo color and turgidity changes that are thought to be associated with ovulation.

From the limited amount of data available, there appears to be no birth seasonality or birth peak in gray gibbons. The gestation period is 7 months. Usually a single young is born.

There is a limited amount of data on the mating of this species. Males initiate mating more frequently than females. If the female is willing to mate, she bends forward in acceptance. If the female is unwilling, she ignores the male or leaves the area.

Most gibbons produce offspring every 2 to 3 years. Nursing may last as long as two years. Because young typically stay with their parents until they reach maturity, it is difficult to say from what age they might be independent. It is reasonable to assume that H. muelleri is similar to other members of the genus with respect to these features.

Breeding interval: These animals likely produce a single offspring every 2 to 3 years.

Breeding season: These animals breed throughout the year.

Average number of offspring: 1.

Average gestation period: 7 months.

Range weaning age: 24 (high) months.

Range time to independence: 9 (high) years.

Range age at sexual or reproductive maturity (female): 8 to 9 years.

Range age at sexual or reproductive maturity (male): 8 to 9 years.

Key Reproductive Features: iteroparous ; year-round breeding ; gonochoric/gonochoristic/dioecious (sexes separate); fertilization ; viviparous

Average number of offspring: 1.

Although no specific information is available for gray gibbons, most gibbon females nurse and care for their young for about two years. Because young typically stay with their parents, older siblings may help in care of younger siblings. Males are also usually active in defending and grooming young.

Parental Investment: altricial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Male, Female); pre-weaning/fledging (Provisioning: Female, Protecting: Male, Female); pre-independence (Provisioning: Female, Protecting: Male, Female); extended period of juvenile learning

Year Assessed

Assessor/s

Reviewer/s

Justification

History

• 2000
  Lower Risk/near threatened
• 1996
  Lower Risk/near threatened

IUCN classifies H. muelleri as a species at lower risk of extinction. CITES places gray gibbons in their Appendix I category, which means the species is threatened with extinction. Gray gibbons are on the threatened species list due to logging practices on Borneo. Vast amounts of forest have been reduced to nothing. The future of this animal depends on the maintenance of its natural habitat, the forests of Borneo.

US Federal List: no special status

CITES: appendix i

IUCN Red List of Threatened Species: endangered

Population

Population Trend

Major Threats

Conservation Actions

Gray gibbons may compete with humans for certain food sources, such as fruits.

Hylobates muelleri is important in research due to its genetic and physiological similarity to humans.