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Gray langur
Semnopithecus

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).

Semnopithecus entellus
Semnopithecus entellus
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 (Bogges 1976).

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).

REPRODUCTION

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).

Semnopithecus entellus
Semnopithecus entellus

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).

PARENTAL CARE

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 (Sugiyama 1965).

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).

COMMUNICATION

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 2014 August 28.