Information comes from nine field studies of eight gibbon species (omitting H. concolor), with the most detailed on the siamang (in Malaya), lar, agile, and Kloss gibbons, the sketchiest on the hoolock (now remedied in a Bangladesh study), and the most important on the pileated, moloch, and Mueller's gibbons. A 1992 study of the siamang and lar gibbon in the very rich forests of north Sumatra, and a 1999 study of the hybrid (agile with Mueller's) gibbon in Kalimantan and concolor gibbons have been investigated in Yunnan and on Hainan Island by Chinese primatologists, but have yet to be fully published or reviewed.

Group size averages 3.8, including an adult pair and two young, but range from two to seven; there may often be three to four young. Only the concolor gibbon has been recorded as living in polygynous groups, with two to three adult females and young, and an average group size of 7.2 in Yunnan, although this requires confirmation. Infants are up to two to three years old before are wholly capable of independent travel; ju veniles up to five to six years; and subadults, physically adultlike, to eight years or so, when they leave the natal group.

Social interactions within groups are relatively infrequent, because the family group is so cohesive. Overt signals are rare, since the young watch and follow their parents. The only sounds heard, apart from the resounding group calls and the movement of branches and foliage, are squeals from an immature animal, usually the subadult, who has come too close to a parent, usually the male, and the bleats of an infant in distress as it is encouraged to move independently. Overt facial expressions are limited to open-mouth threats in aggressive/submissive interactions.

Only in the siamang does the male carry the infant during its second year of life, when weaned from the female (although it may still suckle at night as it sleeps with her). In this way, it learns first to recognize those animals on whom it is most dependent for its survival, the female and the male, and then the subadult with whom it plays while the adults groom. It interacts least with the juvenile. The adult female usually leads the group around the home range; hence, the need to shed the growing infant at the earliest opportunity. The juvenile follows the female, while the subadult lags behind in the rear. It is clear, however, that the adult male, from its central position, is influencing the direction of travel. The smaller gibbons separate more often, to forage on a broad front, as they move between the main food trees.

Grooming involves either adults and subadults during rest periods, or adults and young as they settle for the night (the juvenile tends to sleep with the male, the infant with the female). Play is the other main social activity, recorded in up to 4% of the active day in some studies (siamang, lar, pileated, and hoolock). While the infant (and juvenile) spend much time playing alone—swinging, jumping, manipulating tree parts—they do swing from, grapple with, and bite at adults and sub-adults, and sometimes the juvenile.

Song is a key diagnostic parameter for species and sexes. Gibbon family groups tend to sing daily to advertise their territory and strength of their pair bond. Male and female have distinctive parts; it is a true duet in most species, though most unusual among primates, and more common in birds. There is an introductory sequence, similar to the tuning up of an orchestra, which then leads into an organizing sequence for the great-call sequence of the female, often followed by a male coda; these two sequences then alternate for about three-minute intervals for the rest of the 15-minute bout. Group songs/duets may, of course, be much longer or shorter than this. In some species, male solos occur at dawn, and the duets follow after the first feed of the day. Kloss and moloch gibbon males in an area chorus before dawn, and females chorus after dawn; there is no duet.

The duet in all gibbon species serves to maintain mate and territory, especially to advertise availability and to attract a mate, to develop the pair bond as well as cement other bonds within the group, and to defend the mate and the territory. It seems that the female defends her mate and the male defends the forest space. These functions have been best clarified by playback experiments on Mueller's and agile gibbons

A male pileated gibbon (Hylobates pileatus) calling in Thailand. (Photo by Terry Whittaker/Photo Researchers, Inc. Reproduced by permission.)

in Borneo and on lar gibbons in Thailand. The resident pair has been shown to respond differently to the songs of neighbors than to those of strangers; the former they expect, the latter cause much agitation. The female reacts strongly to a strange female, as a threat to her pair-mate. Groups duet in response to a lone female calling, but silently approach a lone male that is calling.

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