Social Spacing
The Classical Territory
Gibbon families live in territories, which the adults typically defend and maintain for life. Gibbons have classical territories, meaning that mating, feeding, and raising young all occur in this defended space. Each gibbon family group has a home range of approximately 34 hectares, seventy-five percent of which is defended and is considered its territory (Leighton 1986). A gibbon maintains and marks its territory through calling and encounters with other groups (Leighton 1986). A male usually sings his solo around sunrise to establish his presence on the territory (Leighton 1986). Females initiate duets with their mates in the morning as well. These duets not only let other females know that she is present in the territory, but also make it clear to neighbors that she is paired (Leighton 1986).
Gibbons are able to maintain territories primarily because their food sources are defensible. The fruit and other food sources are plentiful (but not overly abundant) and fairly evenly distributed, making it easy for a gibbon group to defend a particular area as its own. These food sources are also considered long-lasting, which makes defending a territory around them beneficial (Leighton 1986). Often territories are right size to provide just enough food for a pair and its offspring, which is another reason gibbons live in single family groups (Raemaekers and Raemaekers 1985; Leighton 1986). Territories large enough to contain more individuals and more food sources would be too large to defend. Another factor contributing to the gibbon territory is female intrasexual aggression. Females have an aversion to each other and disperse themselves in order to avoid contact. The males often follow and defend the area around the female, creating a territory based on the female distribution (Reichard and Sommer 1997).
Intergroup Encounters
Gibbon home ranges can overlap as much as 64 percent with neighboring gibbon home ranges; as a result, aggressive intergroup encounters are frequent (Reichard and Sommer 1997). Most encounters occur during foraging along each group’s territorial border. Thus, most disputes occur in or around food trees and occur most often in the morning hours when gibbons forage the most (Reichard and Sommer 1997). Groups usually initiate encounters when they sense the presence of other groups within 100-150 meters (Reichard and Sommer 1997).
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An intergroup dispute begins with a territorial call produced by both the male and female, after which the male usually leads the group toward the rival group (Islam and Feeroz 1992). The male performs most of the territorial defense; the female will call or accompany her mate for a while during a dispute, but then she typically returns to the territory (Raemaekers and Raemaekers 1985). Older offspring will also help maintain the territory through defense (Leighton 1986). The male may perform visual displays, including spreading his legs, fluffing his fur, and hanging from and jumping on branches (Islam and Feeroz 1992). A male demonstrates his dominance through territorial calling, solos, and his ability to swing, jump, and move swiftly through the trees (Reichard and Sommer 1997). Chasing occurs in 61 percent of encounters, while physical contact occurs in only 8 to 9 percent (Reichard and Sommer 1997). Most of the aggression shown during an encounter occurs between individuals of the same sex (Islam and Feeroz 1992). There are no stable dominance relationships between neighboring males, because every male typically dominates and submits to every other male after encounters at one time or another (Reichard and Sommer 1997). |
Photograph courtesy of David Pearce <http://www.primates.com/welcome.htm>
In a study by Raemaekers and Raemaekers (1985), a playback of duet calls of neighboring gibbons triggered pairs to respond territorially, often approaching the area of the playback, especially if the playback occurred at the center of the territory. A playback of neighboring pair duets can also result in verbal responses from other pairs. Raemaekers and Raemaekers (1984) found that a playback of normal pair duets resulted in no verbal response from other gibbon pairs, while a playback of an ‘ooaa duet’ triggered pairs to spontaneously perform the ooaa duets as well. They, then, concluded that the ooaa duet performs a specific role in territoriality and aggression between gibbon groups that the normal duet does not perform (Raemaekers and Raemaekers 1984).
Female-defense
Male aggression is often interpreted as female-defense in the gibbon (Reichard and Sommer 1997). While food sources are abundant in multiple areas and can be shared, a male’s mate cannot be shared and copulations with her are not available very often (Reichard and Sommer 1997). Thus, the male defends what is most crucial to his reproductive success. By defending his mate, a male helps preserve his reproductive security, while defending his food source would leave his female open to mate with other males. Since the female is in estrus only every two to three years, the male must ensure that when she is available to mate that he is the one who contributes his sperm.
Group encounters can also be viewed as ways to examine other “reproductive options” for paired gibbons (Reichard and Sommer 1997). A female especially is able to observe the fitness of her own mate during the encounter, while comparing his fitness to that of the neighboring male (Reichard and Sommer 1997). This comes into play later when discussing monogamy and the occurrences of extra-pair copulations.