SOCIAL ORGANIZATION AND BEHAVIOR
Gray langur groups are extremely variable in both their size and composition,
especially between different habitats and between years (Mohnot & Srivastava
1992; Rajpurohit 1992). In general however, the social system can be both
polygynous and multi-male/multi-female (Borries et al. 1991). But, there is
evidence that multi-male groups are an atypical situation, and merely a
transition period following a takeover during the mating season and such groups
soon split into single-male and all-male groups (Mathur & Manohar 1990).
Further, multi-male groups usually turn into single-male troops around the
mating season (Newton 1987). Group sizes can range from 2 to 90 and sometimes
more than a hundred animals, although such agglomerations are rare and usually
groups are much smaller (reviewed in Newton 1988; Mohnot & Srivastava 1992;
Mathur 1996; Schülke 2001; Chhangani 2002a; see Vasudev et al. 2008).
There are three main types of group; uni-male bisexual groups (one adult
male, females, juveniles), multi-male bisexual groups (males and females of all
age/sex classes), and all-male groups (Rajpurohit 1992; Newton 1994; Chhangani
2002a). However, among study sites there is great variability, with some
populations mostly having only multi-male bisexual groups, while in others the
only type of bisexual group present is the uni-male group (Mohnot &
Srivastava 1992; reviewed by Rajpurohit 1992). All-male groups are typically
smaller than other types of gray langur groups and can contain adults,
subadults, and juveniles (Rajpurohit 1992). In some populations groups are
stable and uni-male the majority of the time (Sommer & Rajpurohit 1989).
Groups containing females are matrilineal (Mohnot & Srivastava 1992). If
provisioned by humans, group sizes are generally larger than those that are
unprovisioned (Mathur & Manohar 1986). There is long-term evidence that
female membership in groups is stable, as are home ranges, but with larger group
size, this is less true (Newton 1994; Koenig 2000).
Linear rank hierarchies are formed in both all-male groups as well as between
individuals of each sex in mixed-sex groups (Srivastava & Mohnot 1992;
Rajpurohit et al. 1995; Koenig 2000; Rajpurohit & Rajpurohhit 2005;
Rajpurohit 2008). In all-male groups, rank is determined mostly by displacement
of another animal, but also through chasing, fighting, copulatory success, and
harassment (Rajpurohit 2008). Occasionally, all-male groups will temporarily
split into sub-groups, probably for the purpose of looking for resources such as
females and food (Rajpurohit 1995). Within the female dominance hierarchy, the
females in the best physical condition were more likely to be higher-ranking
(Koenig 2000). In addition, the youngest sexually mature females are usually
the highest-ranking, and decline in rank as they age (Borries et al. 1991).
Photo: Sarah Hrdy
Relationships within a group between the adults vary by sex. Among
themselves, males may be peaceful, cooperative or agonistic, while males and
females are usually calm and cooperative with each other. Female-female
relationships are typically positive and they will feed, move, rest, groom one
another, embrace, and greet each other. In general, females usually groom males
more than they themselves are groomed by males (Ahsan & Khan 2006). Between
females, grooming is directed both up and down the dominance hierarchy but
higher ranking individuals groom others and receive grooming more than lower
ranking individuals (Borries et al. 1994). Within a group, aggressive and
submissive interactions are more often between high-ranking members (Rajpurohit
& Rajpurohit 2005). Post-conflict, gray langurs typically avoid each other
(Sommer et al. 2002).
Intergroup relationships are usually agonistic, and typically consist of
high-ranking males displaying, vocalizing, and fighting (Ahsan & Khan 2000).
Within a group, there are several types of aggression, including visual and
tactile threat gestures, displacement, charges and chases, and physical attacks
The method of replacement of a resident male in a uni-male group differs
between populations. In some groups it occurs quickly while in other groups it
is a drawn-out process (Rajpurohit et al. 2003). However, the speed and method
of group male reorganization may vary within a single population (Newton 1987).
The vast majority of male rank changes are associated with emigration and
immigration and usually, males leave troops as a result of aggression from
non-group males (Agoramoorthy 1994; Borries 2000). In one population, the
mechanism of male replacement usually started with an invasion of a uni-male
group by a multi-male group and the subsequent replacement of the resident male.
Subsequently, there is a period of multi-male organization ended by the
all-male group leaving a new male in the single-male group (Rajpurohit 1993).
It is estimated that the average time a male spends in a single-male group is 45
months (Newton 1987).
Female gray langurs typically stay in their natal group for the duration of
their life while males emigrate (Newton 1994; Borries 2000; Rajpurohit &
Rajpurohit 2006). Males emigrate from their natal groups before adulthood, but
the timing of this event varies among populations. For example, in one study
males emigrated at an average of 30.5 months of age while in a different study,
the average was around six years old (Borries 2000; Rajpurohit et al. 2006).
Gray langurs will attempt to revive sick individuals by sitting around them,
shaking them, jumping on the abdomen, and sitting on the ailing langur (Mathur
& Lobo 1987).
Because of the variability of the gray langur social systems, mating can be
both polygynous and polygamous (Borries et al. 1991). In one-male groups, the
resident male usually fathers almost all of the offspring in the group, while in
multi-male groups the alpha-male sires the most, followed by other group males
and even non-group males (Launhardt et al. 2001). Higher-raking females have
significantly higher reproductive success than lower-ranking individuals
(Borries et al. 1991).
Female gray langurs show no external signs of reproductive state, and will
mate during all reproductive states including when pregnant. This is perhaps to
confuse males about parenting and to prevent infanticide, which occurs often in
the species. Nevertheless, field data indicates that males are still able to
discern female reproductive condition through some unclear means (Ostner et al.
2006). Induced by the stress of having a new male in the group, pregnant
females will sometimes abort (Rajpurohit & Srivastava 1994). As mentioned
above, infanticide is common in gray langurs (Ostner et al. 2006).
For example, in one study, nearly a quarter of infants died through
infanticide (Agoramoorthy 1993). It appears that infanticide serves a sexual
selection function and allows greater potential for reproductive success in an
incoming male after a male takeover of a group although there is some recent
disagreement over this hypothesis (Ross 1993b; Rajpurohit et al. 2008). Infants
are often protected against males which are attacking them by resident group
adult males (Borries et al. 1999; Borries & Koenig 2000). Infanticidal
males are usually recent immigrants to the group and only attack infants that
are not their children (Borries & Koenig 2000).
Females usually solicit copulation and do so with solicitation behaviors such
as head-shuddering, lowering the tail, and presenting the anogenital region
(Sommer et al. 1992). Not all solicitations produce copulation however and even
during copulation mating pairs are often harassed by other group members (Newton
1987). Further, not all sexual behavior is between opposite sexes, and in one
study, around half of all sexual interactions of females were mounts with other
females (Sommer et al. 2006).
In some areas reproduction is year-round with peaks during warmer periods of
the year (Sommer et al. 1992). However, at other study sites, reproduction is
seasonal and some authors suggest that year-round reproduction is only found in
populations that are able to use human-related foods (Newton 1987). For
example, in southern Nepal at Ramnagar, mating and estrus is restricted to July
to October (births February to April) with females infertile outside of the
mating season (Ziegler et al. 2000). Reproduction is also seasonal in central
India, in the Maikal Hills, where reproduction occurs mostly between April and
August (Newton 1987).
The average reproductive cycle of gray langurs at Jodhpur, India was 24.1
days, with a gestation length around 200 days (Winkler et al. 1984; Sommer et
al. 1992). The age at first conception in females at this site is usually
around 35 months of age although in some other populations this age is higher,
and can be as high as around 6.7 years (Sommer et al. 1992; Borries et al. 2001;
Rajpurohit 2004). The interbirth interval is 16.7 months (Sommer et al. 1992).
However, because gray langurs are widespread, there is some variability between
study sites (see literature review in Sommer et al. 1992).
Births are usually singletons, but twinning is known, and the majority of
births occur at night (Winkler et al. 1989; Agoramoorthy 1992). At birth,
infants have thin, dark brown or black hair. The skin is pale, but darkens to
black by three months old (Sugiyama 1965). In one study, only around a third of
infants survived into their third year (Winkler et al. 1984).
At birth, the infant clings to the mother's chest and for most of the first
week of life, the infant is found on the mother's chest suckling or sleeping
(Sugiyama 1965). In the first two weeks of the infant's life, it will display
little locomotor behavior but will become increasingly adept after that. Play
behaviors also increase steadily after the first three weeks. Infant
vocalizations include squeaks and shrieks, usually to communicate stress, and
infants vocalize more in the first six weeks of life (Dolhinow & Murphy
1982). Quadrupedal locomotion is seen in an infant at around one month old,
although only in the second month of age is locomotion skillful. By the second
and third months of age they will walk, run and jump adeptly (Sugiyama 1965).
At six weeks of age, the infant gray langur is eating foods on its own. By 9 to
12 months old, infants are near their mother only around 20% of the time
(Dolhinow & Murphy 1982). Infant care is provided by other group females.
Upon reaching two years of age, females will attempt to provide allocare to
infants (Dolhinow & Krusko 1984). The infant is often transferred among
group females, and may nurse on several of them, but in general, if a gray
langur mother dies before an infant is 4 months old, the infant will generally
die (Sugiyama 1965; Dolhinow & Murphy 1982). After six months old,
locomotion is predominantly independent and carrying by the mother is rare
Infants are sometimes kidnapped by females from neighboring groups but are
sometimes retrieved by their mothers (Mohnot 1980).
Weaning starts at an average of 8.6 months old, and by 13 months of age,
infants are completely weaned (Rajpurohit & Mohnot 1991). As adolescent
gray langurs get closer to maturity, males become more peripheral in the group
and are eventually pushed out, while females increasingly spend time cultivating
social relationships and positioning themselves for entering the dominance
hierarchy in their natal group (Nikolei & Borries 1997).
Gray langurs have around 19 vocalization types including loud calls (also
termed whoops), harsh barks, cough barks, grunt barks, pant barks, grunts,
honks, rumbles, rumble screams, hiccups, and alarm calls (Bhaker et al. 2003;
2004). Loud calls (whoops) are only emitted by mature adult males and are often
uttered in conjunction with displays (Hohmann 1989; Bhaker et al. 2004). Such
calls are mostly heard in the morning, especially when leaving the sleeping site
and during other changes in group activity. Langurs utilize an inflatable
laryngeal sac during their emission (Hohmann 1989).
Harsh barks are also emitted by adult males, but subadult males also emit
them and they are heard usually when surprised by a predator (Hohmann 1989;
Bhaker et al. 2004). Cough barks are heard during group movement and are
emitted by adult and subadult males. Grunt barks are heard during group
movements and during agonistic interactions and are usually given by adult
males, but also sometimes by other group members (Hohmann 1989; Bhaker et al.
2004). Rumble screams may also be heard in agonistic situations. Pant barks
are heard in conjunction with loud calls and during inter-group interactions
while grunts are heard during many different types of situations, but especially
during agonism (Hohmann 1989; Bhaker et al. 2004). Adult males honk during
inter-group interactions. During approaches, embraces, or mounts, rumbles are
sometimes heard. Coughs pertain to many different types of situation and are
uttered by most group demographics, excepting infants and adult males, and are
usually heard and exchanged between more than one individual (Hohmann 1989;
Bhaker et al. 2004). All group members emit hiccups, especially when a
different group is sighted.
Finally, variable alarm calls are given by all demographics excepting small
infants, in response to predators, surprises and other groups (Hohmann 1989;
Bhaker et al. 2004). Gray langurs of varying ages may also use tonal contact
calls, including isolation peeps (juveniles, infants, females), warbles (all but
adult and subadult males), and squeals (mostly females) and shrieks (immature
individuals) (Hohmann 1989). Wailing is heard from weaning infants, and the
milk grumble is emitted by very young infants (Hohmann 1989). Adult males will
grind their teeth in agonistic situations, while adult females will chatter
their teeth during social grooming (Hohmann 1989).
Content last modified: October 28, 2008
Written by Kurt Gron.
Cite this page as:
Gron KJ. 2008 October 28. Primate Factsheets: Gray langur (Semnopithecus) Behavior . <http://pin.primate.wisc.edu/factsheets/entry/gray_langur/behav>. Accessed 2015 April 1.