Orangutan
Pongo

SOCIAL ORGANIZATION AND BEHAVIOR

The semi-solitary social organization and behavior of orangutans is highly unusual among the great apes. Adult males and independent adolescents of both sexes range alone while adult females range with their dependent and weaned offspring (te Boekhorst et al. 1990). As subadults, transient males and females may travel in very small groups together, but this does not extend into adulthood. Referred to as resident females, adult females and their infant and juvenile offspring live in discrete home ranges that overlap with other adult females (that may be their mothers or sisters) and within the larger home ranges of an adult male with which they preferentially mate (te Boekhorst et al. 1990; Rodman 1993; Singleton & van Schaik 2002). A resident male is one that has a home range that envelops the ranges of one to several adult females and he is considered the primary breeder for those females within his range. Non-resident males (both subadult and adult) and females range broadly either alone or in groups of a few individuals (Rijksen 1978; te Boekhorst et al. 1990; Mitani et al. 1991; Rodman 1993). The overlap of the home ranges of males and females means that orangutans of both sexes encounter each other while traveling and feeding, and may have brief social interactions (te Boekhorst et al. 1990; Mitani et al. 1991). Adult females that encounter one another can be aggressive, mutually avoid each other, or have seemingly affable relationships (Rijksen 1978; Galdikas 1984). Adult males also have overlap in their ranges and when they meet, encounters between them are usually characterized by agonistic displays. One type of orangutan social group is formed when resident adult males and adult females form a consortship group for a few days, weeks, or even months after copulation. It usually consists of the breeding pair and the adult female's infant or juvenile offspring (Utami et al. 2002). It is perhaps a better description of orangutan social organization and behavior to say that they are solitary but social rather than semi-solitary (Galdikas 1984).

Dispersal patterns of orangutans revolve, to some degree, around female philopatry. Females settle in home ranges that overlap with their mothers', though they do not interact with them more frequently than other adult females and there is little to no evidence of female bonding through affiliation, grooming, or agonistic support (Galdikas 1984; van Schaik & van Hooff 1996; Singleton & van Schaik 2002). There is no evidence that females exhibit territoriality. Males disperse over long distances from the home ranges of their mothers and have a seemingly nomadic phase until they can secure a home range, displacing the dominant, resident adult male (Delgado & van Schaik 2000). Male-male competition for mates is quite intense because of constraints on female reproductive potential and wide spatial distribution (Mitani 1989; Rodman 1993). Dominance hierarchies are formed and maintained between adult males that regularly encounter one another and the more dominant males are usually the largest and have the best body condition (van Schaik et al. 2004). Adult males are always dominant over subadult males (Fox 2002).

The social structure of orangutans is related, in some capacity, to their body size and diet. Because of their large size, they have high caloric needs and fruit is not uniformly available, spatially or temporally, in large enough amounts to permit permanent social groups (Delgado & van Schaik 2000). As a result, there is evidence for individual-based fission-fusion grouping where animals associate in parties on a fairly regular basis, but mean party size tends to remain very small (van Schaik 1999). The two types of social parties seen in orangutans are feeding aggregations and travel bands.

Orangutans gather and feed in large fruiting trees with abundant fruit crops (van Schaik & van Hooff 1996). These feeding aggregations include both resident (adult males and females) and non-resident (subadult males and females) orangutans, and individuals arrive and leave independently. Because of the large quantity of fruit available in a single location, competition for resources is decreased and individuals may benefit from social contact (e.g. gain access to mates or socialize infants) (Mitani et al. 1991; van Schaik 1999; van Schaik et al. 2004).

Orangutans also form travel bands, in which individuals coordinate travel between food sources for a few days at a time (Mitani et al. 1991; Delgado & van Schaik 2000). They are seen doing this only when there is a high density of fruit patches, during a mast fruiting for example, and travel bands usually only consist of a few individuals (Sugardjito et al. 1987; van Schaik & van Hooff 1996). Most commonly, these groups are mating consortships, comprised of one adult male and female traveling together and mating (van Schaik et al. 2004).

REPRODUCTION

Most information about reproductive parameters of orangutans comes from captive animals because of the difficulty of assessing reproductive capabilities in the wild (Knott 2001). Among female orangutans, menarche occurs between 5.8 and 11.1 years, and it occurs earlier in larger females with more body fat than in thinner females (Kaplan & Rogers 1994; Knott 2001). Menarche seemingly occurs earlier in captive orangutans than in wild orangutans, though the absence of a sexual swelling may affect estimates in the wild (Knott 2001). There is a period of adolescent infertility, as is seen in the other great apes, but it is unknown how long this lasts among female orangutans. There is some evidence that it persists for one to four years (Galdikas 1995; Knott 2001). The menstrual cycle lasts between 22 and 30 days and menopause has been recorded among captive orangutans at about 48 years old (Kaplan & Rogers 1994; Knott 2001). Gestation is about nine months and wild female orangutans usually give birth for the first time between ages 14 and 15, though they may be younger in captivity (Kaplan & Rogers 1994; Galdikas 1995). Orangutans exhibit the longest interbirth intervals of all great apes with about eight years between births (Galdikas 1995). Though there is no breeding season, female orangutans show significant seasonal differences in ovarian function during periods of food abundance and scarcity and are therefore less likely to conceive during times of ecological stress (Muller & Wrangham 2001). Because of this sensitivity to ecological conditions, females with overlapping home ranges exhibit rough reproductive synchrony (Galdikas 1995).

Male orangutans have a particularly interesting bimodal physical development, referred to as bimaturism or arrested development, which is influenced by the social context in which they live. Sexual maturity occurs between eight and 15 years in males, though they do not exhibit the secondary sexual characteristics or characteristic long-calls of a socially mature adult males (also called flanged males) until they are between 15 and 20 years (Rijksen 1978). Subadult, or unflanged, males (ages eight to 15) have fully descended testicles and are capable of reproducing but remain in a morphological state quite similar to adult females. When the proper social conditions arise, mainly in the absence of a resident male, they begin to develop the characteristic cheek pads, throat pouch, long fur, and behaviors of resident adult males (Rijksen 1978; Schürmann & van Hoof 1986). The shift from unflanged to flanged appearance can occur rapidly, within a few months. Along with the shift in physical appearance comes a shift in mating strategy. Unflanged males do not have a secure territory and are transient, roaming over wide areas and searching for receptive adult females. When they encounter a female in estrus, they force copulation and successfully impregnate her. This rape behavior is different than the strategy of a flanged male that uses long-calls to advertise his location and waits for receptive females to find him (Utami et al. 2002; Setchell 2003). Because females preferentially mate with flanged males, this process of sit-and-wait is effective for fully developed males. Females may also seek out flanged males for protection from sexual harassment by unflanged males (Fox 2002). Both male strategies are successful and frequently observed in and ex situ; in one study, about half of the infants born in one study group were sired by unflanged males (Utami et al. 2002). The mechanisms behind the evolution of this unusual characteristic of orangutan development are not yet fully understood.

PARENTAL CARE

Because of the semi-solitary nature of orangutans, adult males have no contact with infants and therefore no parental investment in the wild. The orangutan mother is the primary care provider and instrument of socialization for her offspring. The strongest, most salient social group among orangutans involves an adult female and her dependent offspring. Because an adult female is likely to have more than one offspring with her (an older adolescent and an infant), older siblings can be very important in socializing infants and juveniles (Munn & Fernandez 1997). From birth to about two years, an infant orangutan is completely dependent on its mother, always being carried during travel, depending on her for food, and sleeping in the same night nest as her (Rijksen 1978). For the first four months, the two never break contact and the infant is carried ventro-ventral. The amount of physical contact wanes over the following months such that by age one the infant is usually only in contact with its mother 25 percent of the time (Munn & Fernandez 1997). By two years of age, climbing and swinging skills are more markedly developed and a common travel form seen is "buddy travel" in which the young orangutan holds the hand of another animal while moving through the canopy (Munn & Fernandez 1997). The juvenile period lasts from about two to five years of age and the young orangutan begins to take short exploratory trips from its mother, but is always in visual contact with her, and play becomes increasingly important (with peers and with its mother) (Rijksen 1978). While a juvenile still sleep in the same nest as its mother, one form of play is to practice building nests, and by the end of this period of development, a juvenile may be sleeping in its own night nest (Rijksen 1978; Munn & Fernandez 1997). This is also the age range in which mothers allow less frequent nursing. By age four, orangutan juveniles are usually weaned, though they may still nurse during periods of stress until they are seven years old (Munn & Fernandez 1997). The adolescent stage starts about age five and lasts until age eight. While adolescents still have frequent contact with their mothers, they actively seek peers and play and travel with peer groups (Rijksen 1978). At this point in development, females are starting to show some sexual behavior, especially towards the resident male, and will begin to establish their own home ranges that overlap with, but do not encompass, their mothers' ranges (Galdikas 1995). The adolescent male orangutan remains socially immature, though sexually mature, and actively avoids contact with adult males. Once it leaves its mother, it begins its transient stage which lasts until it is able to displace a resident, flanged male, at which point it will develop the flanged appearance and will be considered socially adult.

COMMUNICATION

LISTEN TO VOCALIZATIONS

• Orangutan call repertoires  (For more information and sonograms, see: M.E. Hardus, A.R. Lameira, I. Singleton, H.C. Morrogh-Bernard, C.D. Knott, M. Ancrenaz, S.S. Utami Atmoko and S.A. Wich, "A description of the orangutan's vocal and sound repertoire, with a focus on geographic variation." In. Orangutans: Geographic Variation in Behavioral Ecology and Conservation, ed. by S.A Wich, S.S. Urami Atmoko, T.M. Setia, and C.P. van Schaik. Oxford University Press, 2009.; Universitåt Zurich; May 9, 2012)

The most important and easily distinguished vocal communication between orangutans is the long-call. This is only performed by flanged males, lasts for one to two minutes, and can be heard over several kilometers when conditions are right (Mitani 1985; Galdikas & Insley 1988). A flanged male uses this call primarily to alert other males (both subadult and adult) to his presence as well as advertise his presence and location to sexually receptive females (Galdikas & Insley 1988). There is some evidence that the long-call plays an important role in the suppression of development among adolescent and subadult males. When adolescent males hear long-calls, stress hormones are induced and rather than developing secondary sexual characteristics, development is arrested resulting in the unflanged appearance (Delgado & van Schaik 2000). While subadults flee from the sound of long-calls, sexually receptive females use it as a tracking device for flanged males and respond accordingly (Delgado & van Schaik 2000). Long-calls are made spontaneously by males, unlike the other characteristic call of adult males which is used under very specific conditions. The fast-call is elicited post-contact or post-conflict. For example, after hearing a long-call, crashing branches or trees (circumstantial evidence of another male in the vicinity), or seeing another male fleeing, a flanged male will respond with a fast call (Mitani 1985; Galdikas & Insley 1988). There is little information on other orangutan vocalizations; because of their semi-solitary lifestyle, vocal communication may not as important as it is for other apes.

Like the other great apes, orangutans are highly intelligent and closely related to humans and as such they have been the focus of language acquisition studies as researchers try to understand the origins of human language. Fewer attempts have been made to teach orangutans American Sign Language than chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). An orangutan named Chantek learned about 140 signs and was able to use them spontaneously, though most of his interest remained on signing about foods (Miles 1990).

SPECIAL NOTES

Sumatran orangutans of all ages and sex classes use and manufacture tools. While tool use was first seen and studied among captive orangutans, subsequent field work at Suaq Balimbing, Sumatra revealed that wild orangutans are also quite deft at using and making tools, but the behavior is rarely seen among Bornean orangutans (Commitante et al. 2003). In the wild, orangutans are seen using found objects as tools: leaves as "toilet paper," leafy branches as flyswatters, large leaves as umbrellas, and branches used in displays (Fox et al. 1999). They also manufacture tools: modifying sticks for collection of insects and to pry open large fruit and seed pods, and using stacks of leaves to hold spiny fruit as they open it (Fox et al. 1999). Captive orangutans are also capable of manufacturing tools for a variety of uses and situations: connecting short sticks to make one long stick to reach desired object, stacking boxes to make a ladder, using sticks to dig holes, making swings from ropes, and containing water by using objects as cups (Lethmate 1982). In some instances, orangutans score higher on tool-using tests in captivity than do chimpanzees by, for example, using one tool to make another. (Lethmate 1982).

Content last modified: June 13, 2005

Written by Kristina Cawthon Lang. Reviewed by Simon Husson.

Cite this page as:
Cawthon Lang KA. 2005 June 13. Primate Factsheets: Orangutan (Pongo) Behavior . <http://pin.primate.wisc.edu/factsheets/entry/orangutan/behav>. Accessed 2013 May 23.