Siamang
Symphalangus syndactylus

SOCIAL ORGANIZATION AND BEHAVIOR

In general, most siamangs live in monogamous pairs accompanied by up to 6 immature individuals, with usual group membership between 2 and 6 individuals (Kawabe 1970; MacKinnon & MacKinnon 1980; Raemaekers & Chivers 1980; Norikoshi 1986; Palombit 1992; 1996; Lappan 2005). However, groups with multiple adult males are known, and such groups can be sexually polyandrous and can last several years (Lappan 2005). While monogamy was considered the rule and siamangs were considered to mate for life, it now appears that the pair-bonds are much more dynamic than originally thought and extra-group copulations are known in the species (Palombit 1992; 1994a). Further, changes in the mating pair can be precipitated by several means, including replacement of one of the pair by an extra-group siamang, mate desertion, co-dispersal, and father-son mating replacements (Palombit 1994b).

Both male and female siamangs cooperatively maintain their territory through several means; ranging near the territorial edges (patrolling), calling at (singing) and confronting (chasing and interacting with) intruders, in addition to loud calling in the morning (Chivers 1974; Orgeldinger 1991; Palombit 1996b). The territory is actively defended from other siamang groups and from solitary individuals (Palombit 1996b). In captivity, males participate more in non-vocal territorial defense than do females (Orgeldinger 1997). Singing or duetting occurs infrequently, only about once every 4-5 days and is usually heard in the morning although not before dawn (Kawabe 1970; Gittins 1980; Palombit 1996b). Such calling can be heard more than 2 km (1.2 mi) away and can last up to twenty minutes but the discrete function of this calling is unknown (Kawabe 1970; Lamprecht 1970; Palombit 1996b). Inter-group interactions usually occur along the periphery of a specific siamang group's territory, and can last up to several hours. Usually these interactions do not result in injuries, but consist of vocalizations, displays and chasing (Palombit 1996b).

For the most part, members of a siamang group usually spend the day within 10 m (32.8 ft) of other group members, and rarely more than 30 m (98.4 ft) apart (Chivers 1974). Agonistic gestures occur infrequently, but consist of open-mouth threats, grimace and lip-smacking displays (Palombit 1996a). Grooming is the most common form of physical social interaction, but most bouts last less than 5 minutes. In addition, grooming is usually between mature individuals, and mostly happens later in the day (Chivers 1974). When resting or relaxing, siamang pairs will huddle or maintain physical contact (Palombit 1996a).

In the wild, gibbons (including siamangs) exhibit a lack of dominance over one another (Gittins & Raemaekers 1980). In captivity, there is no dominant sex, with co-dominance occurring sometimes, while in other cases the male was dominant to the female and the female dominant to the male (Orgeldinger 1991). Within multi-male siamang groups, there can be low levels of aggression and affiliative behaviors between males can occur (Lappan 2007).

Both males and females emigrate, however there is evidence that females typically disperse long distances and males, while variable in their dispersal distances, emigrate only short distances from their natal groups (Chivers & Raemaekers 1980; Palombit 1996b; Lappan 2007). Emigration from the natal group usually occurs at sexual maturity (around 8-10 years old), after which the young adults leave, resulting from increased peripheralization and agonism from the resident adults (Chivers 1974; Palombit 1996b).

REPRODUCTION

The siamang reproductive system is characterized by monogamy in most cases, however in uncommon multi-male groups polyandrous mating occurs (Palombit 1996b; Lappan 2005). Also, mating with individuals outside of the pair-bond occurs, usually with adult or sub-adult members of a neighboring group (Palombit 1994a; 1996b). Siamang females exhibit externally visible changes in the sex skin changing from black to red and white although it is unclear if the color changes are associated with ovulation or menstruation (Chivers 1974). Copulation in captivity usually takes place in the context of bonding or resting behaviors (Orgeldinger 1996).

The usual copulatory posture consists of a male approaching behind a female and mounting, copulating in a dorso-ventral position (Chivers 1978; Orgeldinger 1991). However in the wild, other copulatory positions have been observed, including suspended and face-to-face copulation (Aldrich-Blake pers. comm. cited in Chivers 1978; Chivers & Raemaekers 1980). Ventro-ventral mating postures have been seen in captivity as well (Orgeldinger 1991). Copulations occur at any time of the day (Chivers 1974).

There may be seasonality of mating in wild siamangs. Conception occurs mostly between May and July during flowering-fruiting peaks, with births between December and February, however copulations have been observed outside of this peak, and infants have been seen in August and September (Koyama 1971; Chivers 1978; Chivers & Raemaekers 1980). Potential seasonality could be due to ecological conditions at birth which are conducive for lactation as well as food availability during pregnancy (Chivers 1978). During a breeding period, sexual activity occurs about once per day on alternating days (Chivers 1974).

The age of sexual maturity in siamangs is estimated at between 8 and 9 years of age and interbirth intervals are commonly greater than three years in the wild (Geissmann 1991; Palombit 1992).

Captive autosexual behavior has been observed (Mootnick & Baker 1994).

PARENTAL CARE

At birth in captivity, siamang infants weigh on average 536.9 g (18.9 oz) but can range between 390 and 685 g (13.8 and 24.2 oz) (Geissmann & Orgeldinger 1995). Neonates are grey-pink with only meager hairs, but by one week of age the body is covered with hair except for the head. By the second week, the head has fur as well (Soo-Hoo 1966; Chivers & Chivers 1975; Palombit 1992). The birth of twins does occur but is rare, and in the event of twinning, the infants tend to develop somewhat faster than single births (Dal Pra & Geissmann 1994). In captivity, the majority of births take place at night (Orgeldinger 1991).

In the wild, immediately after birth, all of the infant's time is spent either clinging to the mother in a ventral position or being carried by the mother (Lappan 2005; Liebal 2007). After three months of age however, the amount of maternal carrying time decreases steadily (Lappan 2005). In both the wild and in captivity, as the infant develops, contact between the mother and the infant slowly decreases while contact between the father and the infant increases (Chivers 1974; Alberts 1987). In captivity at around one year of age, the infant seeks paternal contact more than maternal contact (Alberts 1987). In the wild this is seen as well at around one year of age, where the mother becomes intolerant of the infant, and the father assumes care (Chivers 1974; Gittins & Raemaekers 1980). In captivity and in the wild, the father does most of the carrying at about one year of age or slightly later, and other group members, especially siblings, sometimes also carry the infant (however, when observed in captivity, this may have been the result of the infants in question being twins) (Chivers 1974; Dielentheis et al. 1991; Dal Pra & Geissmann 1994; Lappan 2005). In some cases, captive parental care of the infant is almost evenly divided between both parents, and the amount of wild male care of infants varies substantially, even within the same study population, and especially when multi-male groups are compared to single-male groups (Ferenc & Wielich 2000; Lappan 2005). In the wild, it is under paternal care that the infant learns to move independently, feed, and socially interact. Under the care of the male in the second year of life, carrying decreases significantly as independent travel increases (Chivers 1974). By 15 months of age, a large proportion of independent travel is seen (Lappan 2005). At three years of age, only occasional parental help in travel is offered (Chivers 1974).

In captivity, around 9 weeks of age, the infant gains some independence from the mother, but still maintains contact with her, with a complete lack of contact first seen at around 16 weeks. Solid foods are first consumed around 12 weeks old. One-armed suspension starts at 24 weeks of age, and the first brachiation is seen at 35 weeks. Bipedal locomotion starts at 43 weeks. Sibling play starts at 29 weeks, and receiving grooming from siblings starts at 13 weeks. Grooming of others starts at 45 weeks, and the infants will first call at 32 weeks (means compiled from several captive studies by Dal Pra & Geissmann 1994:335). In the wild however, social play develops only during the 2nd year of life and usually occurs during group rest (Chivers 1974). In addition, most infants in the wild do not consume solids until 6 months of age (Lappan 2005).

In the wild, weaning starts as early as 3 months, but is normally not complete until about a year of age, and sometimes longer (Chivers & Raemaekers 1980; Lappan 2005).

Infanticide by males is reported in captivity (Orgeldinger 1991). In addition, in captivity when a nearly one year old infant died, both parents significantly increased their social behavior, including copulations (Orgeldinger 1996). Gestation can be between 189 and 239 days but is most likely around 210 days (Geissmann 1991).

COMMUNICATION

In captivity, a total of 20 communicatory gestures are observed; 8 visual and 12 relating to touch. Touch gestures are more commonly seen than visual gestures, and in general, gestures are associated with play (37.4%), grooming (23.2%), agonistic interactions (11.7%), and other interactions (<10%) (Liebal et al. 2004; Liebal 2007). There is inter-individual variation in the production of gestures and in one study, no individual performed all of them, and most performed only about half of the repertoire (Liebal 2007). In addition to communicatory gestures, the species exhibits 7 communicatory actions and 4 facial expressions (Liebal et al. 2004). Communication actions are usually seen during play (94.6% of instances) and facial expressions are usually seen during grooming (35.3% of instances) (Liebal et al. 2004).

Siamang calls can be heard over a distance of over 1.5-2 km (0.9-1.2 mi) and are usually performed at the highest (emergent) levels of the canopy (Kawabe 1970; Chivers 1974; Gittins & Raemaekers 1980). The characteristic throat sac functions in calling to influence the character of, to resonate and to amplify vocalizations (Tembrock 1974; Chivers unpub. data cited in Barkell 1988). Singing occurs usually in the morning, about once every 4-5 days in the wild (Palombit 1996b). Sometimes termed a "duet," these calling bouts last 10-20 minutes and are mostly performed by the mating pair, and sometimes other members of the group (Palombit 1996b; Geissmann 2000). Group calls can be divided into three categories of vocalization; "booms," "barks," and "screams" (Chivers 1974). In addition, contained within the "duet," is the so-called female-only "great call" which is the culmination of the "duet" and comprises all types of vocalizations, including "barks" and "booms" (Palombit 1996b; Geissmann 2000). Calling bouts are stereotyped and are also sometimes accompanied by locomotion, however the function of such bouts is incompletely known (Palombit 1996b). Possible purposes for calling could be to maintain or strengthen the pair-bond and for territorial defense (Lamprecht 1970; Geissmann 1999). In general, the more elaborate communicatory bouts characterize interactions between neighboring groups (Chivers 1996). Similar in structure to "duets" are alarm call bouts, which serve a different function and provoke responses from neighboring siamang groups as well as being heard in potentially threatening situations (Chivers 1974; Geissmann 2000). At a sign of danger, group members will sometimes emit soft grunts, and if the threat persists, this will expand into barking by other members of the group at the same time (Chivers 1974).

In the wild, facial expressions, gestures, and calls are few in number in normal intra-group interactions (Chivers 1974). Infants will squeal or bleat when distressed, especially when the infant is isolated or trying to follow other group members (Chivers 1974). Other intra-group vocalizations include the "squeal," which is used to show submission during aggressive interactions, feeding "glunks" which may be communicatory and "gurgles" which are heard from immature individuals while playing (Chivers 1974; 1976).

In captivity, siamangs will use their hands in conjunction with vocalizations, presumably to modify the sound production in some way (Badraun et al 1998).

The siamang possesses a gland on its sternum which might function in olfactory communication (Geissmann 1987; 1993).

Content last modified: January 29, 2008

Written by Kurt Gron.

Cite this page as:
Gron KJ. 2008 January 29. Primate Factsheets: Siamang (Symphalangus syndactylus) Behavior. <http://pin.primate.wisc.edu/factsheets/entry/siamang/behav>. Accessed 2008 May 11.