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Lar gibbon
Hylobates lar

SOCIAL ORGANIZATION AND BEHAVIOR

Lar gibbons usually live in serial monogamous pairs with up to 4 offspring in each group (Palombit 1996; Bartlett 2007; Reichard 2003; Reichard & Barelli 2008). While group living in serial monogamous pairs is the norm, it is not the rule, and at some sites there are also multi-male groups as well as groups with multiple adult females (Reichard 2003; Barelli et al. 2008b; Reichard & Barelli 2008). Long-term data indicate that females may live in several different types of groups (e.g. pair or multi-male) over their lifetimes (Reichard & Barelli 2008). At one location, multi-male groups comprised over 20% of groups present (Reichard & Barelli 2008). Extra-pair copulations have been seen on a number of occasions (Reichard 1995; Reichard & Sommer 1997; Reichard 2003). In fact, at Khao Yai, most females are polyandrous as opposed to monogamous, even females in a single-male group (Barelli et al. 2008b; Reichard & Barelli 2008). In other words, social relationships are flexible (Reichard 1995; Reichard & Barelli 2008). In the case of individual pair-bonds, changes can result from desertion (often for another mate), replacement of one of a pair by a peripheral individual, disappearance or the death of one of the adult lar gibbons (Palombit 1994; Brockelman et al. 1998). Changes in pair composition can be common (Palombit 1994).

Within-group behavior is variable throughout the year, from almost a fifth of the activity budget to only a small percentage, with more social activity occurring at times of increased ripe fruit abundance. The three predominant types of within-group social interaction include grooming, play (wrestling, chasing, and slapping and biting), and social contact, with grooming the most common. Within-group aggression is rare and in general, immature lar gibbons play more than adults (Bartlett 2003). There are some indications that allogrooming serves more a hygienic than social function in lar gibbons and tends to be reciprocal between individuals (Ellefson 1974; Reichard & Sommer 1994).

Encounters between different groups of lar gibbons can range between agonistic (physical altercations) and friendly (between-group playing or grooming) interactions (Reichard & Sommer 1997; Bartlett 2003). Most interactions between groups are agonistic, but can be purely vocal and even neutral in which both groups coming into contact barely react to one another (Bartlett 2003). Groups may also travel, feed or rest together when they come into contact (Reichard & Sommer 1997).

Males mainly participate in most territorial disputes, but females are sometimes approached by males during intergroup interactions (Reichard & Sommer 1997; Bartlett 2009). Disputes generally occur near the boundaries of the home range when two groups are in visual contact with one another and typically last around an hour (Bartlett 2003). Most inter-group interactions are accompanied by vocalizations (Reichard & Sommer 1997). The variability of the nature of interactions between neighboring groups may partly be the result of variable social and kin relationships between members of neighboring groups (Bartlett 2003). However, inter-group interactions can be quite violent, and there is evidence that wounds incurred in territorial aggression have resulted in the death of combatants (Palombit 1993). There is seasonal variation in the occurrence of inter-group encounters and territoriality may function in resource defense (Bartlett 2009).

Hylobates lar
Photo: Alan Mootnick

There is no personal leadership of a lar gibbon group, although adult females lead group progressions most often (Barelli et al. 2008a).

In one long-term study, males dispersed from their natal groups at around 10 years of age and usually attain a territory by replacing one of their resident adults. Dispersal distances are usually around one or two territories from the natal territory; averaging in one population 0.71km (0.44 mi) (Brockelman et al. 1998).

REPRODUCTION

While lar gibbons usually live in serial monogamous pairs, their reproductive system is complex and can be polyandrous, sometimes including flexible sexual relationships which often occur outside of the usual pair bond (Reichard 1995; Reichard & Barelli 2008). Polygamous mating has also been seen (Bartlett 2007). This is primarily due to the changing relationships among lar gibbons over their lifetimes, often including successive pairings with different mates and inclusion in different social group types (Reichard & Barelli 2008). However, in the case of extra-pair copulation by females, frequency of copulation remains much higher with the pair-mate than with other males (Barelli et al. 2008b). Mating occurs in every month of the year, but most conceptions occur during the dry season (March) with a peak in births during the late rainy season, in October (Barelli et al. 2008b; Savini et al. 2008).

Lar gibbon females exhibit swelling, protrusion and color change of the sex-skin beginning several days before ovulation and ending after ovulation, usually lasting around 7-11 days (Dahl & Nadler 1992; Barelli et al. 2007). Pregnant females also show sexual swelling, but at random times during pregnancy (Barelli et al. 2007).

Female solicitation postures include placing herself in front of a male and looking back at him, presenting her genitals to a male (Ellefson 1974; Barelli et al. 2008b). Copulation is dorso-ventral, with the male behind the female (Ellefson 1974). Females may refuse copulation by moving away from the male, vocalizing, or agonistically refusing the advances of the male (Barelli et al. 2008b).

On average, females reproduce for the first time at about 11 years of age in the wild, much later than in captivity (Reichard & Barelli 2008). The average reproductive cycle is 21.1 days long (Barelli et al. 2007). At a minimum, the interbirth interval is 3 years, and averages 41 months (Brockelman et al. 1998; Reichard & Barelli 2008). Gestation in wild lar gibbons is around six months (Barelli et al. 2007).

Male-male mounting has been observed among wild male lar gibbons (Edwards & Todd 1991).

PARENTAL CARE

In general, observations of lar gibbon births and infant development are extremely limited. At birth, lar gibbons weigh on average 383.4g (13.5oz) and are nearly naked. They are able to vocalize soon after birth (Crandall 1964; Ellefson 1974; Geissmann & Orgeldinger 1995). At one study site in Thailand, births showed a peak at the rainy season/dry season transition in September and October (Reichard & Barelli 2008). In the wild, infants are carried by clinging to their mother's ventrum (Ellefson 1974). One infant in the wild did little more than cling to the female for the first several weeks of life (Ellefson 1974). One wild individual first ingested solid foods in the fourth month of life, timing also seen in captivity (Ellefson 1974; Roberts 1983). Also around this time, one wild infant started moving a short distance from its mother (Ellefson 1974). While uncommon, a measure of parental care of an infant was seen in captivity including predominantly play behaviors (Clemens et al. 2008). The ability to brachiate was first seen in a captive infant around 9 months of age (Roberts 1983).

Infants are weaned around two years of age (Ellefson 1974; Reichard & Barelli 2008).

Infant mortality is low, under 10% (6.3%) in the first year of life (Reichard & Barelli 2008).

COMMUNICATION

Hylobates lar
Photo: Marilyn Cole

Lar gibbons have seven basic vocalization note types. These include the "wa", "hoo", "leaning wa", "oo", "sharp wow", "waoo" and "other" type of vocalization (Raemaekers et al. 1984; Clarke et al. 2006). These basic types are usually assembled into longer phrases (Clarke et al. 2006). There are a number of types of call bout emitted by lar gibbons in different contexts. These include normal duets (structured vocalizations given by the mated pair usually during the mid-morning or afternoon), "ooaa" duets (similar to normal duets but rarer), calls emitted when disturbed (e.g. when predators are seen, in times of alarm, during territorial disputes or conflict), adult male solos (often given early in the day, near dawn or before), subadult male solos, adult female solos (given by mated females when isolated), and contact calls (Raemaekers et al. 1984; Raemaekers & Raemaekers 1985). Pair duets are very loud, and can be heard as far as several kilometers away (Reichard 1998). Subadult males have been known to interact with one another through solo calling as well (Raemaekers & Raemaekers 1984a). Once uttered by a pair, "ooaa" duets are infectious through the forest, often initiating similar vocalizations from other lar gibbons within earshot (Raemaekers & Raemaekers 1984b).

Most calling occurs in the morning (Yimkao & Srikosamatara 2006). Vocalization is heard in almost all intergroup encounters, including male solos, duets, and female solos (Reichard & Sommer 1997). Duet calls probably serve to reinforce the bond between the mated pair, but also to exclude other gibbon individuals (Raemaekers et al. 1984). Duets also function in distant vocal communication with other lar gibbon groups and in territorial defense (Raemaekers & Raemaekers 1984b). The duet culminates in the female's great call and is given from anywhere in the territory (Raemaekers et al. 1984). The great call of an individual female is distinct and unique from the great calls given by other females (Yimkao & Srikosamatara 2006). In one study, vocal exchanges between males were the only form of interaction in 20% of inter-group encounters (Bartlett 2003). In addition, vocal behavior in general functions in the complex sexual competition within the species (Clarke et al. 2006). With habitat disturbance due to logging, calling frequency decreases (Johns 1985).

Female sexual swellings visibly communicate information about reproductive condition (Barelli et al. 2007). Male display behavior to increase conspicuousness includes one-arm dangles, locomotion back and forth around a particular pathway, and the breaking of branches (Ellefson 1974). Grimaces (exaggerated smiles) indicate subordination while open-mouth threats indicate an agonistic threat. Branch-jerks occur when a dominant gibbon shakes a branch and elicits submission from another individual (Ellefson 1974).

Content last modified: November 17, 2010

Written by Kurt Gron. Reviewed by Alan Mootnick.

Cite this page as:
Gron KJ. 2010 November 17. Primate Factsheets: Lar gibbon (Hylobates lar) Behavior . <http://pin.primate.wisc.edu/factsheets/entry/lar_gibbon/behav>. Accessed 2014 September 17.