SOCIAL ORGANIZATION AND BEHAVIOR
Very little is known about the behavioral ecology of white-cheeked
gibbons. Most information comes from the very small Chinese population
and details about the Laotian and Vietnamese populations are based on
anecdotal field observations (Geissmann et al. 2000). However, the
available data suggest that the life history characteristics of the
white-cheeked gibbon are similar to other gibbons (Geissmann et al.
2000). Historically, gibbons were thought to live only in monogamous social groups that include an adult
male and female and their immature offspring. Recent research has shed light on the social structure of gibbons, though, and while many form long-term pair bonds, extra-pair copulations may occur (Palombit 1994; Reichard 1995). A gibbon family is
territorial and defends its territory with regular morning songs
performed by the breeding male and female (Leighton 1987). Groups have,
on average, four individuals and usually include an
adult male and female, one infant and one juvenile, although the group may also
include one adolescent and one subadult (Leighton 1987). There is a
high degree of social and behavioral equality between adult males and
females and codominance is exhibited (Leighton 1987; Geissmann et al.
2000). Because of this egalitarian social atmosphere and no competition
between males for access to females, white-cheeked gibbons are not
sexually dimorphic.
Photo: Bertrand L. Deputte
Gibbons defend their territories both vocally and physically. The
primary means by which adult males and females defend their territory is
by ritualized morning duets. The morning duet occurs at dawn and is a
long, loud, energetically expensive song in which the female and male
sing individual notes in specific patterns. The duet lasts
between 10 and 13 minutes but is longer during the rainy season and
shorter during the dry season (Geissmann et al. 2000). This seasonal
variation in song bout may be related to energy expenditure and food
availability. During the rainy season, when fruit is widely available
and foraging time is shorter, more time and energy can be allocated for
singing whereas during the dry season, foraging time increases and more
energy is expended on finding food rather than territory defense
(Geissmann et al. 2000). The song is initiated by either sex but is
always ended by the adult male (Schilling 1984). Females begin singing
long notes of increasing frequency with note durations and intervals
between notes continuously decreasing and reaching a climax of short
barks that eventually trail off (Geissmann et al. 2000). Males sing
three different notes that include boom notes, staccato notes, and high
frequency modulated notes. During his part of the duet, a male
continuously cycles through these three notes and his song is
accompanied by locomotory displays including violent brachiation, branch
shaking, or prancing (Deputte 1982; Geissmann et al. 2000). The other
mechanisms by which gibbons defend their territories include aggression
and physical contests. Intergroup encounters may include chasing,
fighting, and loud-call displays as well (Leighton 1987). Reports of
facial scars, broken canines, and open wounds have been documented in
some but not all gibbon species and it is usually males that are
afflicted (Leighton 1987). Females are usually involved in call
displays, both morning duets and other calls during border disputes,
while adult, subadult, and adolescent males of a gibbon group physically
defend their territory from intruders or full takeovers (Leighton 1987).
Because of the monogamous social structure, both males and females leave
their natal groups when they reach
sexual maturity, and transfer is hastened by aggressive encounters between the subadult gibbon and its same-sexed parent (Leighton 1987;
Macdonald 2001). Gibbon parents often become
antagonistic towards their subadult offspring and evict them from the
family group to decrease the possibility of inbreeding (Leighton 1987).
Though young gibbons attempt to settle near their natal territories, the
availability of vacant or appropriate habitat is a limiting factor.
Females are more likely to remain close to their natal territories than
males, and vagabond young males seek out females. Once paired, a gibbon
couple then settles in an appropriate habitat and begins to defend it as
their own territory (Leighton 1987).
REPRODUCTION
Limited data are available on reproductive parameters in both wild and captive white-cheeked gibbon. Sexual maturity occurs
between six and eight years of age but captive white-cheeked gibbons
have bred as early as four years-old (Geissmann 1991). With sexual
maturity comes a change in pelage color of female white-cheeked gibbons
from their black juvenile coat to the characteristic yellow or buffy
coat of adults. Males remain black but develop the white cheek patches
as they reach sexual maturity (Geissmann et al. 2000). The menstrual
cycle of female gibbons lasts, on average, 21.6 days (between 12 and 27
days) and is accompanied by some genital swelling during estrus, though
it is not as pronounced as that seen in chimpanzees (Pan troglodytes)
and bonobos (Pan paniscus) (Nadler et al. 1993; Lukas et al. 2002).
During estrus, female gibbons entice males to mate with quick, jerking
upper-body movements known as bobbing (Lukas et al. 2002). First
parturition occurs between six and eight years of age and gibbons
exhibit an interbirth interval of two to three years. A mating pair of
gibbons will produce an average of five to six offspring over their
reproductive lifetimes (Leighton 1987; Geissmann 1991; Macdonald 2001).
There is no conclusive evidence for birth seasonality in white-cheeked
gibbons, though some species of gibbon exhibit mild birth peaks at
the end of the rainy season (Leighton 1987).
PARENTAL CARE
Though white-cheeked gibbons are monogamous and males have high
paternity certainty, the majority of parental care is the responsibility
of the female gibbon (Varsik 2001). Gestation lasts about seven months
(Lukas et al. 2002). Infancy lasts from birth to about two years, at
which time the infant is weaned and enters the juvenile period (Leighton
1987; Macdonald 2001). Juvenile gibbons move independently of their
mothers and this period lasts from two to four years (Leighton 1987).
The period of adolescence is from about four to six years of age in
which the young gibbon is not yet adult-sized but is moving and sleeping
independently from its mother and beginning to reach sexual maturity.
According to Leighton (1987), young gibbons were not observed to play on a daily basis, this may be due to the small social groups and lack of play partners.
COMMUNICATION
Photo: Bertrand L. Deputte
The territorial long call duets of white-cheeked gibbons serve an
important function by communicating location and area of occupancy to other
gibbon groups. The development of this duet begins during the juvenile
period when young gibbons practice the songs and displays that they will
one day perform as adults in their own territories (Deputte 1982).
Juvenile males and females mimic portions of an adult female's song
while she is performing her great call. As they mature, adolescent
females begin to hone their singing skills and eventually perfect the
song of the adult female white-cheeked gibbon. Young males will
eventually switch the type of call they give (usually coinciding with
sexual maturity) and begin to sing the chorus of an adult male as they
are gradually peripheralized from the family group (Deputte 1982;
Geissmann et al. 2001).
LISTEN TO VOCALIZATIONS
A form of visual communication in white-cheeked gibbons is the sexual
dichromism exhibited by adult males and females. Coat color is an
intraspecies cue that conveys information about age and reproductive
status, and it may also function in mate choice (Gerald 2003). The
differences in coat color and markings are important, for example, to
distinguish sexually mature males from subadult males. Perhaps this cue
is useful for dispersing females looking for a potential mate; they can
quickly identify if males are mature enough to be potential mates
(Gerald 2003).
SPECIAL NOTES
Other reasons that gibbons are separated from the great apes include the
differences in intelligence and tool-using behaviors. Unlike
chimpanzees (Pan troglodytes),
bonobos (Pan paniscus),
gorillas (Gorilla)
and orangutans (Pongo), gibbons do not use tools in the
wild and do not score as well on cognitive tests in captivity (Rumbaugh
& Washburn 2003). In captivity, gibbons have been recorded using
objects in their enclosures as tools, but short of dropping branches on
human observers that follow them, they have not been seen using tools in
the wild. On intelligence tests they do not generally perform as well as great
apes but generally do better than monkeys (Rumbaugh & Washburn
2003). In recent tests, though, gibbons moderately scored higher in a cognitive task
than chimpanzees and capuchins (Cebus) (Cuningham et al. in press).
Content last modified: July 3, 2005
Written by Kristina Cawthon Lang. Reviewed by Alan Mootnick.
Cite this page as:
Cawthon Lang KA. 2005 July 3. Primate Factsheets: White-cheeked gibbon (Nomascus leucogenys) Behavior . <http://pin.primate.wisc.edu/factsheets/entry/white-cheeked_gibbon/behav>. Accessed 2010 February 9.
