Hylobates moloch (Audebert, 1798)

Taxonomy and Naming

The silvery gibbon (Hylobates moloch), also called the Javan gibbon, is a primate belonging to the gibbon family Hylobatidae.

Distribution

It is endemic to the Indonesian island of Java, where it lives in undisturbed rainforests at altitudes up to 2,450 m (8,040 ft).

Conservation Status

It has been classified as Endangered on the IUCN Red List since 2008, with the estimated wild population containing fewer than 2,500 mature individuals.

Fur Coloration

Its fur is bluish-grey, with a dark grey or black cap on its head.

Skeletal Morphology

Like all gibbons, the silvery gibbon has no external tail, scapulae positioned on the dorsal side, and reduced flexibility in the lumbar region.

Limb Morphology

It has long, curved fingers and forelimbs that are very long relative to its hind limbs.

Weight

On average, adult silvery gibbons weigh 8 kg (18 lb).

Activity Pattern and Locomotion

The silvery gibbon is diurnal and arboreal, climbing trees skillfully and moving through forests via brachiation.

Brachiation Adaptations

This movement is enabled by its mobile wrist joints, full rotation of the upper arm, and the ability to lock elbows when suspended.

Diet

Its diet is made up of fruits, leaves, and flowers.

Reproduction Cycle

On average, females give birth to a single young every three years, after a seven-month gestation period.

Offspring Development

Offspring are nursed for around 18 months, and stay with their family group until they reach full maturity at 8 to 10 years old.

Social Structure

Like all gibbon species, the silvery gibbon lives in pairs and marks out a territory that the pair defends vigorously.

Territory Size

Their territories are relatively small, covering around 42 acres (17 hectares).

Territorial Vocalization

Females sing to mark their territory several times a day, and if strangers are detected, males scream to scare the intruders away.

Vocalization Gender Patterns

Most solo song bouts and scream bouts are produced by females.

Female Vocal Timing

Female vocal bouts happen after 05:00, with peak vocal activity around 06:00.

Male Vocal Traits

In contrast, male vocal bouts, which mostly occur before 05:00, are structured as choruses with a minimum of three participants.

Male Chorus Frequency

These choruses occur roughly every 8.5 days.

Chorus Duration Comparison

They start earlier and last longer than female solo song bouts.

Male Song Timing

Most male songs begin while it is still dark, between 03:55 and 04:40.

Male Aggression

Males are typically very aggressive towards other gibbons outside their pair.