Gibbon groups, generally made up of an adult male and female pair and up to four subadults and infants (Gittins 1982, Wilson 1975), live in territories which they actively defend from other gibbon groups. Though part of a gibbon group's territory is used exclusively by that group, there are usually some areas of overlap with the space used by other groups (Bramblett 1976). Estimates of the amount of overlap of this home range around a core territory vary between reports and species; Bramblett (1976) believes overlap in most white-handed gibbon groups to be about 10% (1976), Reichard and Summer (1997) found a 65% area of overlap in their white-handed group , Islam and Feeroz (1992) calculated a 9% overlap area for their hoolok gibbon group, and Gittins (1982) estimated an agile gibbon group's area of overlap with other groups' space to be 76%.
Reports of the size of these home ranges also vary, but are generally between 7 ha (Jiang & Wang 1999) and 120 ha (Wilson 1975), with black-crested gibbon home ranges reported to be as much as 200 ha (Jiang & Wang 1999). Compared to many other primate species, gibbons are spaced at a low population density; large areas are most likely needed to support each gibbon group because of their selective diets (Gittins 1982). Suitable habitat is usually saturated with gibbon territories (Brockleman et al. 1998).
Male gibbons take primary responsibility of defending the group's territory, though females engage in calling and may 'back up' a male in fights (Reichard % Sommer, 1997, Bramblett 1976). Gibbon territoriality is maintained at least in part by long bouts of singing, particularly duets by the adult pair and accompanying acrobatic displays, both of which allow gibbons to advertise their RHP (Bramblett 1976, Marshall & Marshall 1976). The overlapping areas of gibbon habitat are constantly in dispute, and 'battles' between groups are fairly common (Bramblett 1976). These battles usually involve male threats, displays - particularly acrobatic acts and swinging through the trees, and vocalizations -- mostly a specific battle vocalization (Bramblett 1976). Males may also engage in chasing, and ritualized and real fights; though rare, serious injuries and even death can result from these territorial fights (Palombit 1993). This territorial defense is costly; males spend approximately 5% of their awake time calling, and 5% engaged in display or battle (Bramblett 1976). The main driving force behind territory maintenance behavior is generally believed to be gibbons' intolerance for members of their own sex (Jiang & Wang 1999).
Territorial social spacing is believed to have developed in gibbons primarily because of their food resource distribution (Reichard & Sommer 1997). Gbbons rely greatly upon small, fairly-evenly distributed, predictable, and inconspicuous food sources (Gittins 1982). Because these sources are widely-spread and inconspicuous, gibbons would have to spend a great amount of time searching for food if in an unfamiliar area; remaining in a familiar range would bring a significant advantage of reduced foraging time (Gittins 1982). These food sources are not too abundant to be indefensible, and since suitable habitat is usually saturated (Brockleman et al. 1998), it would be a great advantage for a group to have food resources upon which they could depend assess.
Further, gibbons can afford territorial maintenance behavior because of low body weight and low predation pressure. Though gibbons may occasionally be threatened by pythons (Reichard 1998), their predation pressure is quite low, so they have little risk of attracting predators by their conspicuous displays. Gibbons also have a small body size and do not require constant food intake like many large animals do; they can thus energetically afford to maintain a territory. Hoolock gibbons were observed to spend only 39% of their (awake) day eating, for example, which means that they can afford to invest some of that left-over time in maintaining their territory (Islam & Feeroz 1992).
The substantial areas of overlap between groups may be due less to food resource access than to facilitate group interactions and to allow gibbons to sum up their neighbors. Intergroup encounters made up 9% of Reichard and Sommer (1997)'s study group, which was a much higher amount of time than should have been necessary for strict resource maintenance. Reichard and Sommer point to the prevalence of EPCs and group changes, and suggest that these group encounters (made possible by the overlapping ranges) may allow gibbons to weigh their reproductive options with their opposite-sex neighbors.