Hylobates muelleri
Borneo gibbon

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

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

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.

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

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.

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.

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.

Gray gibbons are very agile. They travel mostly in trees by swinging from branch to branch via brachiation. This mode of locomotion involves extending their long arms over their head in order to hook their hands onto branches. Gray gibbons move quickly with long leaps and swings. They are able to cover 3 meters in a single swing and around 850 meters per day. Gray gibbons walk upright with hands over head for balance when walking on the ground, although they do not cover long distances in this fashion. Gray gibbons are not good swimmers and avoid open water.

Gray gibbons are usually seen in groups of 3 or 4. Solitary individuals are also common. These individuals are adults who have been forced to leave the family and have not yet established their own territory.

Gray gibbons are active for 8 to 10 hours per day. These animals are diurnal, rising at dawn and settling for the night before sunset. Males tend to become active earlier and for longer periods than females. Gray gibbons spend most of their day foraging in the main canopy of the forest.

Gray gibbons are social creatures but do not spend as much time in social interaction as do some other primate species. Grooming and social play take up less than 5 percent of the daily activities. The lack of interaction may be due to the small number of social partners available.

Male and female adults are, more or less, social equals. In one study, males were found to groom females more often and play with young more frequently. Too few studies have been conducted to determine if this behavior is common to all groups of H. muelleri.

Individuals of H. muelleri are very territorial. About 75 percent of the home range, average size 34.2 ha., is defended. Defense involves regular morning songs and calling at and chasing intruders. Gray gibbons rarely resort to physical violence when defending territory.

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.

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

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

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

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

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

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.

Monogamy is found in only 3 percent of mammals and is therefore a topic of debate concerning gibbons. One idea is that rather than monogamy being a product of the female needing defense by the male, monogamy results from other ecological factors, such as food availability and range size. More evidence needs to be collected to test this hypothesis. Another idea concerning the maintenance of monogamy concerns females and their range defense songs. It has been suggested that female response to other songs forces males into monogamy.

Much research has been done on the song of gibbons due to the important role it has in determining gibbon taxonomy. Research has shown that H. muelleri is able to identify other species of gibbons through song.