Hylobates are comprised of the gibbons and siamang. They are small (10 – 30 pounds) arboreal apes native to the subtropical and tropical Asian rainforests stretching from China to Indonesia. According to Stanford, et al., “These apes form monogamous pair bonds and like most monogamous species, they show very little sexual dimorphic expression with relation to body size. Their territories are defended through various species – specific vocal arrangements as they are the most vocal non human primates.” The disparity in species – specific duetting suggests that the functionality of duetting varies across the genus (Geissman and Orgeldinger). Hylobates, on average, have a lifespan of approximately 25 years in the wild and 40 in captivity. Adult size is reached around age 6 and sexual maturity at age 9. Females give birth to 1 -2 offspring every 2 years.
The anatomical design of Hylobates is well suited for life in the trees and for brachiation. According to Burnie and Wilson, “Brachiation is a method of locomotion utilizing arm swinging and hanging coupled with the use of the body as a pendulum. They release their grip with one hand at the highest point of the swing, while focusing on the next hand-hold, which may be 9 feet away. Hylobates have arms that are approximately 40 percent longer than their legs and a median wingspan of 5 feet. The size of the thumb is reduced and it is more proximally positioned which enhances gripping.”  Each Hylobates species is recognized as endangered. Deforestation and hunting by humans is the main threat to their survival.

H. lar

Hylobates lar (White-handed Gibbon)
• Length: 16.5 – 23 inches
• Weight: 10 – 17 pounds
• Status: Endangered
H. lar can be found throughout Thailand, Malaysia, and Indonesia. This ape is extremely active during daylight hours, using the night to rest among the canopies. Like most gibbons this species forms pair bonds that typically last for life. Their day is begun with a pair bond duet consisting of the female loudly hooting to a climax with the male responding in more simple tones. These calls are used to reinforce their territorial claim and to possibly reinforce the pair bond. Their high metabolic output requires a diet fueled by fruits (which accounts for approximately 50 percent of their caloric intake), leaves, insects, and flowers.
As previously noted all gibbons form monogamous pair bonds. According to studies by Brockelman, et al. conducted at Khao Yai National Park in central Thailand, “Sexually mature males and females diffuse from their natal group at the age of 10. Reasons for dispersal vary but the three most consistent seem to be: the avoidance of inbreeding; the obtaining of a mate; the obtaining of resources and territory. These gibbons will usually travel a distance of 2200 feet from his/her natal territory in an attempt to obtain a mate and establish a home territory. Groups and territories are most likely formed by: an ousting of, or replacement of the vacancy left by the death of one member of an established group by an incoming adult; the banishment of the entire group by a newly formed pair; or the possible inheritance of a territory from the natal group. Non-reproducing subadults, who invest in their natal group via defense, grooming, and juvenile/social play are sometimes tolerated. Juvenile/social play is a very important factor in group bonding and consists of very acrobatic chasing, brachiation, wrestling, and mock biting. It is begun in the first year of life and wild gibbons experience approximately 1 hour each day.”

H. concolor

Hylobates concolor (Crested Gibbon)
• Length: 18 – 25 inches
• Weight: 10 – 20 pounds
• Status: Critically Endangered
Unlike most gibbons this species has a crest of hair running along its crown. Offspring are born yellow with adult males becoming black and females brown or gray.

H. leucogenys

Hylobates leucogenys (White-cheeked Gibbon)
• Length: 18 – 25 inches
• Weight: 10 – 20 pounds
• Status: Unconfirmed
Until very recently this gibbon was considered a subspecies of the crested gibbon. They are distinguished by geography: The Crested Gibbon is found in northeast Vietnam while the White-cheeked gibbon is found in southwest Vietnam.

H. moloch

Hylobates moloch (Silvery Gibbon)
• Length: 18 – 25 inches
• Weight: 10 – 15 pounds
• Status: Critically endangered
Their silver hair is accented by their pale eyebrows, cheeks, and beards. The male and female of this species do not participate in duet calls.

H. syndactylus (Siamang)

Hylobates syndactylus
• Length: 35 inches
• Weight: 22 – 33 pounds
• Status: Endangered
Unlike most species of Hylobates, the female siamang is the dominant member of the pair. The siamang is the largest of the lesser apes and can be found throughout Indonesia and Malaysia. Siamangs are the loudest and most vocally complex of all the Hylobates. Each member of the pair uses their “elastic” throat sacs to resonate calls for means of defense and territorial claim. Siamang pairs spend a great deal of their day in very close proximity to one another. While either foraging, brachiating, resting, or eating, these apes are usually within 1 – 30 feet of each other. Their diet consists mainly of leaves, followed by fruits, blossoms, and grubs.
Studies of the Protective and territorial (P/T) behavior of siamangs indicate that they defend their territory by intense morning group calls, regular boundary patrols, and calling at and chasing away an intruder. According to Orgeldinger, “Animals in zoos (especially apes) are known to consider humans as possible members of their own species and possible threats. P/T behavior is divided into 6 categories: (1) vigilance – intense boundary watching; (2) display; (3) threat postures [mock biting, head nodding, and bipedal displays]; (4) alarm; (5) duetting; and (6) attack behavior. Males are most often found in front of the female, are much more vigorous in their P/T behaviors, and while females will engage other females, males are always the main combatants. In captivity, siamangs spend approximately 4.7% of their daily activity duetting (those housed with conspecifics spent 7.5%), compared to those of wild siamangs at approximately 1%. The disparity in time is usually accounted for by the many more disturbances encountered in the zoological setting. These duets average 15.3 minutes in duration, consist of 6 ‘great call sequences,’ and are sung at a greater tempo than wild siamangs. All duets for both wild and captive siamangs are most powerful after the first feeding of the day and rarely are heard throughout the late afternoon or evening.”
Geissman and Thomas’ studies of duet singing and pair bonding in siamangs suggest that there is a high correlation between the two. They conclude that, “Duetting was much more frequent and powerful from pairs that groomed most often, spent the most time in proximity to each other, and behaved contemporaneously. If duetting has to be learned each time a new pair forms, the possibility of partner desertion would be minimized in an effort to control the time invested in learning the song. The complexity of the siamang duet in its sex – specific repertoire, number of different vocal interactions, and the strict rules that govern the sequence of and the intervals between these interactions are possibly the most sophisticated opus of any land vertebrate next to man.”