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).
Pongo pygmaeus
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.
Pongo pygmaeus
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).
Pongo pygmaeus
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
Pongo pygmaeus
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 2018 April 23.