SOCIAL ORGANIZATION AND BEHAVIOR
Hoolock gibbon group size averages around 3 individuals, and typically ranges from around 2-6 individuals
however solitary individuals are sometimes seen (McCann 1933; Tilson 1979; Gittins & Tilson 1984;
Siddiqi 1986; Alfred & Sati 1990; Choudhury 1990; 1991; Mukherjee et al. 1991-1992; Alfred 1992;
Feeroz & Islam 1992; Islam & Feeroz 1992; Gupta 1994; Choudhury 1996; Ahsan 2001; Chetry 2002;
Das et al. 2003; Gupta et al. 2005; Kakati 2006; Das et al. 2006). Groups with more than one adult male or more
than one breeding adult female have been known to occur, as have all-male groups (Choudhury 1991;
Mukherjee et al. 1991-1992; Ahsan 1995; Choudhury 1996). The majority of groups consist of a serially monogamous
mated pair of adults and usually one or several immature individuals (Tilson 1979; Gittins & Tilson 1984;
Alfred 1992; Choudhury 1996; Ahsan 2001). Over the course of the day, all group members on average
are no more than 12.5 m (41.0 ft) from one another. During travel, the adult of either sex can lead
the group but travel is usually single-file (Mukherjee 1986; Islam & Feeroz 1992; Ahsan 2001).
Territories are defended through territorial disputes usually led by the group adult male. Such
disputes normally occur in areas of overlap with the home ranges of other hoolock gibbon groups (Islam & Feeroz 1992).
Territorial boundaries are maintained through vocalization bouts, the visiting of all parts of the
territory often, disputes with neighboring groups, and the progressive expulsion of offspring from
the territory as they age (Ahsan 2001). Intergroup encounters occur often and usually consist of
vocalization and counter-vocalization, with males chasing one another, but typically not violently
(Gittins & Tilson 1984; Gupta et al. 2005). Aside from vocalizing during territorial disputes,
females usually do not participate otherwise and remain peripheral (Gittins & Tilson 1984). Such
overlap zones between home ranges have been called territorial boundaries between hoolock groups
(Tilson 1979). Vocalizations are likely important in the maintenance of territorial boundaries and
Solitary individuals do not maintain discrete territories and have been seen moving between the
established territories of other hoolock groups (Tilson 1979). Intergroup encounters can exceed
one hour in length (Gittins & Tilson 1984).
Grooming is often seen during group social activities and serves in the maintenance of social bonds,
reducing aggression, and for hygienic purposes. Grooming frequency increases between February and
May when the weather is warmer and is usually started and performed by the adult male of the group
(Ahsan 2001). The typical grooming bout involves one individual grooming the head and back of the
other (Islam & Feeroz 1992). However, grooming frequency is usually quite low overall (Mukherjee 1986).
Self-grooming is also seen often (Alfred 1992). Play usually occurs between immature individuals but
occasionally one of the adults will play with immature members of the group (Alfred 1992;
Sati & Alfred 2001). Play is one of the most common social behaviors (Sati & Alfred 2001).
The formation of a new hoolock gibbon group can occur in several ways; the pairing of a brother and sister,
the pairing of a father and daughter, emigration and subsequent re-mating, or dispersed subadult pair
formation (Ahsan 2001).
Hoolock gibbons of both sexes emigrate from their natal groups when they become mature adults (Gupta et al. 2005).
Hoolock gibbons are typically monogamous, living in mating pairs with offspring (Siddiqi 1986;
Choudhury 1991; Gupta et al. 2005). Mating occurs during the summer rainy season with births during
the winter, usually between September and January (McCann 1933; Alfred 1992; Tilson 1979;
Choudhury 1990; Alfred 1992; Ahsan 2001). Regardless, in some studies copulation occurs nearly year-round
and summer births have been seen (Alfred 1992; Ahsan 2001). Copulation usually occurs in the
morning, lasts on average around half a minute and is usually in a ventro-ventral position
(Islam & Feeroz 1992; Ahsan 2001; Sati & Alfred 2001). Newborns are almost always seen only between
November-January (Alfred & Sati 1990).
Gestation is between 6 and 8 months (Sati & Alfred 2001; Ahsan 2001). The inter-birth interval is
estimated at around two years, although some authors place it higher, around 3-4 years
(Alfred & Sati 1990; Ahsan 1994 cited in Ahsan 2001). The female menstrual cycle lasts on average
27.8 days (Matthews 1946). Sexual maturity in females is reached around 5-8 years of age, and is
shown by the fading of the black coat to the paler pelage of adult females (Tilson 1979; Islam & Feeroz 1992).
Births are singletons (Choudhury 1990). Neonatal infants are almost completely white, pale-grey
whitish-yellow, or whitish-yellow with black hands, feet, and face (Choudhury 1991; Ahsan 2004; Mootnick 2006).
Body length of the infant is normally between 20 and 35 cm (7.9 and 13.8 in) (Alfred 1992).
By a year old, the pelage darkens to mostly brown/black and the coloration becomes progressively
darker until 24 months of age, when they are completely black with distinct eyebrows (Ahsan 2004).
This process occurs in both sexes and later, females experience yet another color change when they
attain the adult female pelage coloration around 5-8 years old, at sexual maturity (Tilson 1979; Islam & Feeroz 1992).
For the first two months of life in the wild, infant hoolock gibbons predominantly cling to the ventrum of
their mother, including the chest and belly (Alfred 1992; Islam & Feeroz 1992; Ahsan 2004).
The infant hoolock gibbon first ventures away from its mother at around 6 months of age and by 17 months
of age, a degree of independence is seen, with the infant playing with others, but always within
several meters of the mother (Ahsan 2004). By 21 months old leaping and jumping is first seen and
by 22 months old such locomotor behaviors are more adept. Suckling can occur through 26 months of age,
as does sleeping with the mother (Ahsan 2004). In fact, the infant will persist in sleeping with
the mother until the birth of a new infant (Sati & Alfred 2001).
Infanticide by an adult male has been observed in hoolock gibbons (Alfred & Sati 1991).
Photo: Alan Mootnick
The vocalizations of hoolock gibbons are not sexually dimorphic, meaning that there are no sex-specific
vocalizations, a fact that sets them apart from all other species of gibbons (Haimoff 1985;
Choudhury 1989; 1991; Geissmann 1995). Calls can usually be heard up to a kilometer away (Choudhury 1989).
Types of call include female growling, hoos, great calls, semi-great calls, repair and aborted calls,
organizing sequences, alarm calls, cries, whimpers and squeals (Ahsan 2001). Infants have
characteristic distress calls (Alfred 1992). Other vocalizations consist of growls and exhalation sounds,
and probably function in intra-group communication (Alfred 1992). Males may call alone before sunrise
(Gupta et al. 2005).
Usually, calls are uttered during loud and complex call bouts or duets, which make up almost 90% of
all vocalization and occur mostly in the morning, usually lasting up to 20 minutes (Gittins & Tilson 1984;
Alfred & Sati 1986; Choudhury 1989; 1991; Alfred 1992; Ahsan 2001; Gupta et al. 2005). There are
three main call bout types; the great call, organizing sequences, and alarm calls (Ahsan 2001).
Sometimes juveniles of either sex may join in a call bout (Choudhury 1989). Singing does not occur
every day, but when it does occur, usually occurs from high areas such as tall, large, peripheral
trees on hilly terrain and usually near or on overlap of home ranges (Choudhury 1991; Ahsan 2001;
Hasan et al. 2007). The majority of calls are produced near the edges of the territory or outside
of it (Islam & Feeroz 1992). Further, it is possible for call bouts to occur more than once a day
(Gittins & Tilson 1984; Choudhury 1989). Call bouts can be elicited in a number of ways. These
include the singing of a nearby group at the territorial edge, during a territorial conflict, in food
competition, the singing of a neighbor group not in an overlapping territory, vocalizations of other
animals (especially birds), and train whistles (Ahsan 2001). Once calling commences, call bouts are
often responded to by other hoolock gibbon groups throughout the forest (Alfred 1992). Sometimes, neighboring
groups only start calling after the adjacent group ceases (Gittins & Tilson 1984). Call bouts serve
to initiate territorial disputes which ensue if the calling does not keep neighboring groups apart
(Islam & Feeroz 1992). During a bout, calling may be continuous or discontinuous (Islam & Feeroz 1992).
As a call bout continues, the female great call vocalizations accelerate, with the male joining
halfway through the great call (Geissmann 1995). Temperature and weather also affect the frequency
and occurrence of hoolock gibbon call bouts (Ahsan 2001).
Functions of calling in hoolock gibbons include the maintenance of the pair-bond, reinforcement of
social ties, mate attraction and defense, mate solicitation, and in territoriality reinforcement
and defense (Islam & Feeroz 1992; Ahsan 2001).
Hoolock gibbons possess skin glands which may have some function in olfactory communication (Geissmann & Hulftegger 1992).
Content last modified: August 13, 2008
Written by Kurt Gron. Reviewed by Alan Mootnick.
Cite this page as:
Gron KJ. 2008 August 13. Primate Factsheets: Hoolock gibbon (Hoolock) Behavior . <http://pin.primate.wisc.edu/factsheets/entry/hoolock_gibbon/behav>. Accessed 2015 March 30.