SOCIAL ORGANIZATION AND BEHAVIOR
Primate behavioral ecologists have long debated the costs and benefits
of group living, but some of the factors that affect chimpanzee social
structure include decreased likelihood of predation, resource defense
and feeding efficiency, and higher copulatory success because of access
to mates (Sakura 1994; Boesch 1996). Chimpanzees live in a
fission-fusion social group consisting of a large community that
includes all individuals that regularly associate with one another (up
to a few hundred individuals) and smaller, temporary subgroups, or
parties. These subgroups are unpredictable and can be highly fluid,
changing members quickly or lasting a few days before rejoining the
community (Goodall 1986; Chapman et al. 1993; Boesch 1996). At Taï, the
average party size is between five and eight individuals (Boesch 1996).
Party size greatly increases when food availability increases, though,
and at Kibale, average party size is 10 but ranges from one to 47
individuals during periods of highest food availability (Mitani et al.
2002). Party size also increases when the estrous females are present
(Matsumoto-Oda et al. 1998; Mitani et al. 2002). Party composition is
variable, including unisexual and bisexual parties of adolescents or
adults, parties of adult females and infants, lone adult females and
their offspring, and mixed age and sex parties (Boesch 1996). The most
frequently observed party across all chimpanzee communities is the
mixed-sex party, though Taï males are the most inclined to associate
with females, and Gombe males are the least inclined to do so. At
Bossou, though, where the adult sex ratio is skewed, the high number of
females relative to males means the most common party composition there
is mother-infant (Boesch 1996).
Photo: Frans de Waal
There is a distinct linear dominance hierarchy in male chimpanzees, and
males are dominant over females (Goldberg & Wrangham 1997). Males
remain in their natal communities while females, in general, emigrate at
adolescence, between nine and 14 years old (Nishida et al. 2003). The
complete transition between groups may take up to two years, though, and
is characterized by vacillating between their natal group and new
community (Goodall 1986; Hasegawa 1989; Pusey 1990). Rates of female
transfer are much higher at Mahale and Taï than at Gombe and Bossou.
This may be attributed to the smaller population size and isolated
conditions at Gombe and Bossou; with fewer options, it is more
beneficial to remain in their own group and take behavioral precautions
to avoid inbreeding (Goodall 1986; Gagneux et al. 1999; Nishida et al.
2003). For female chimpanzees that do emigrate, though, they are not
likely to be related to other adult females in their new community and
the dominance hierarchy is linked to age (with younger immigrant females
ranking the lowest) and the status of their offspring (Nishida 1989).
Lactating females generally spend most of their time with their own
offspring, though they may be seen with other lactating females in
"nursery groups" (Pepper et al. 1999). Females become very sociable
during estrus, though, and are seen mostly in bisexual parties (Pepper
et al. 1999).
Given the female-biased dispersal pattern, male chimpanzees in a
community are more likely to be related to one another than females are
to each other, but matrilineal kinship does not always strongly
influence patterns of male chimpanzee social behavior. In studies at
Kibale, genetic analyses of males support the theory that cooperation is
of greater evolutionary significance than kinship affiliation (Goldberg
& Wrangham 1997; Mitani et al. 2000). Research at Gombe, on the other
hand, has consistently emphasized kinship as the most important
underlying factor of the strong social bonds (Goodall 1986). Close
relationships between males serve two purposes within chimpanzee
communities: inter-community interactions and intra-community politics
(Goldberg & Wrangham 1997). Some examples of inter-community
interactions include hostile attacks by groups of males and cooperative
boundary patrol parties. Intra-community interactions that are
dominated by male cooperation include securing and maintaining
dominance, mate guarding, and group hunting and meat sharing
(Goldberg & Wrangham 1997; Mitani et al. 2000).
First seen at Gombe in 1963, chimpanzee hunting behavior probably
evolved because of the direct benefits of a protein source in their
largely frugivorous diets, but it is more than nutritionally important;
meat is socially important as well (Mitani & Watts 2001). Meat is
social currency used to develop and maintain alliances between adult
males; it is usually shared reciprocally and non-randomly (Mitani &
Watts 2001). Hunting is cooperative in the sense that multiple males are involved in
cornering and capturing prey, though there is debate among researchers if this
is true cooperation (Videan pers. comm). Members of the hunting party are spread
out widely on the ground and in the trees (if hunting arboreal prey such
as colobus monkeys), and other members of the community often observe
and vocalize excitedly throughout the pursuit (Watts & Mitani 2002).
Hunting success increases with group size, and chimpanzees are more
successful where the canopy is broken and open. The male chimpanzees in
Kibale are the most successful hunters with an average success rate of
84%, though at other sites hunting parties have success rates above 50%
(Watts & Mitani 2002). The influence on red colobus
(Procolobus badius) populations because
of high success rates of chimpanzee hunters should not be ignored.
Chimpanzees are contributing to population declines of red colobus
monkeys in multiple sites across Africa, especially at Gombe (Struhsaker
1999).
REPRODUCTION
Males reach adolescence between nine and 15 years of age and are
capable of reproduction at 16 years or older. First
estrus is seen in females at 10 years of age and is characterized by
anogenital swelling. Menarche occurs a few months after the first
swelling and continues on a cycle of about 36 days (Goodall 1986).
There is a period of adolescent infertility in female chimpanzees that
usually coincides with permanent emigration from their natal groups
(Goodall 1986; Nishida et al. 2003). During the transition period,
females still exhibit sexual swellings that may serve as a passport to
gain males' tolerance in their new social communities (Boesch &
Boesch-Achermann 2000). Once established in their new communities,
young females cease cycling for two to four years but continue to
attract adult males and mate promiscuously. First parturition occurs,
on average, between 13 and 14 years and the interbirth interval is
between three and five years (Goodall 1986; Boesch & Boesch-Achermann
2000; Nishida et al. 2003). The lag time between menarche and first
parturition may have adaptive significance for emigrating females.
Having offspring before being accepted by the community could jeopardize
both the mother and infant (Boesch & Boesch-Achermann 2000).
Infanticide has been documented at Gombe, Mahale, and Kibale study sites and is
often attributed to sexual selection theory; males kill infants unlikely
to be their own, infanticide shortens interbirth intervals by inducing
cycling in females that lose infants, and infanticidal males thus
increase their chances of siring offspring. Infanticidal behavior by
females has also been observed, though there is some question as to
whether these were isolated incidences of pathological behavior, or if
it could be related to dominance rank in females (Goodall 1986; Pusey et
al. 1997).
Photo: Frans de Waal
Mating occurs throughout the year and there is no evidence of a birth
season, female chimpanzees do exhibit seasonality in the number of estrous
females within a group (Wallis 1995; Boesch &Boesch-Achermann 2000; Wallis 2002).
The number of estrous females is positively related to food abundance; because
of the energetic requirements of
ovulation
and mating, female chimpanzees are more likely to come into estrus during times
when food is readily available (Anderson et al. 2006). The majority of chimpanzee reproductive
behavior is promiscuous, with females mating with multiple males
opportunistically during estrus, though the majority of copulation
occurs during the 10-day period of maximal tumescence (Goodall 1986).
There are other types of reproductive strategies that are recognized as
well. Restrictive mating, where the dominant male restricts other males
from mating with estrous females in the community, consortship mating,
where an adult pair leave the community for several days to weeks, and
extra-group mating, where females leave their communities and mate
furtively with males from nearby communities (Goodall 1986; Gagneux et
al. 1999). Chimpanzee social and mating groups do not always overlap,
given the variety of reproductive situations. This may have evolved
because females have limited choice in mates after committing to a
community, and the dominance hierarchy of males often dictates which
males an estrous female will mate with. By having multiple strategies,
females can expand the pool of males from which they choose while not
losing the important support of the males in their communities (Gagneux
et al. 1999). Having multiple strategies also maximizes the chances of
males' reproductive success; they are able to vary, throughout their
lives, their mating strategies with depending on their position in the
dominance hierarchy.
PARENTAL CARE
In chimpanzees, the majority of parental care is the responsibility of
the mother and is critical to the survival and emotional health of
youngsters (Goodall 1986). Chimpanzee infants and juveniles benefit
from the close relationship with their mothers in terms of food, warmth,
protection, and the opportunity to learn skills. There is also some
evidence that a young chimpanzee achieves rank according to his or her
mother's status (Goodall 1986; Boesch & Boesch-Achermann 2000).
Chimpanzee newborns and mothers are in constant ventral-ventral contact
for the first 30 days of life. During the newborn period, chimpanzees
are helpless to survive without maternal support and though they have a
firm grasping reflex, it is not strong enough to support the infant for
more than a few seconds at a time. In fact, for the first two months of
life, chimpanzees are unable to support their own weight and depend
entirely on their mothers' support (Bard 1995). After five or six
months, chimpanzee infants ride dorsally on their mothers' backs.
During the first year of life, infant chimpanzees maintain almost
continual contact with their mothers. By the age of two, they begin to
travel and sit independently within five meters of their mothers and
this corresponds to a decrease in nursing and the onset of independent
eating and play behavior (Bard 1995; Coe 1990). Not until three years
of age do young chimpanzees venture more than five meters from their
mothers, and between ages four and six, the period of infancy ends with
weaning (Bard 1995).
During the juvenile period, from about six to nine years old,
chimpanzees remain close to their mothers but play independently and
have greater social interactions with other community members.
Adolescent females spend some of their time moving between groups and
are supported by their mothers during agonistic encounters while
adolescent males spend more time with adult males in social activities
such as boundary patrols and hunting parties (Bard 1995).
COMMUNICATION
Photo: Roy Fontaine
Visual and vocal communication are important to chimpanzee society. A
suite of facial expressions, postures, and sounds function as signals
during interactions between individuals and groups (Goodall 1986).
Chimpanzees have particularly expressive, hairless faces and facial
expressions play an important role in close-up communication between
individuals. For example, a "full closed grin" is in response to an
unexpected and frightening stimulus and evokes an instant fear response
in others. Other facial expressions include "lip flip," "pout,"
"sneer," and "compressed-lips face" (Goodall 1986). Body position and
stance also convey information to other individuals. Submissive
positions include extending the hand, crouching, and bobbing while
aggressive positions usually involve an individual trying to appear
larger than he is by swaggering bipedally, hunching his shoulders, and
waving his arms (Goodall 1986). Adult chimpanzees will also drum on the trunks
of trees, by beating their hands and feet against large trees, for a dramatic
display. Drumming can be heard in multiple contexts including while traveling,
during displays, encounters with other chimpanzee communities, and when arriving
at large food sources (Crockford &Boesch 2005).
LISTEN TO VOCALIZATIONS
Vocal communication also conveys a wide variety of emotional states and
intentions and often serves to affect the behaviors of those that hear
the calls. One important vocalization is the "pant-hoot," which
is the most commonly heard call of adult individuals and is used to
signify food enjoyment, social excitement, and sociability feelings
(Goodall 1986). One of the best ways to assess dominance rank is to
listen for "pant-grunts," which are directed towards dominant
individuals by submissive individuals (Goodall 1986; Pusey et al. 1997).
Distance calls are used to draw attention to danger or food sources for
other community members as well as establish location of other groups in
the area (Goodall 1986). It is given while feeding, during travel, and
when meeting chimpanzees from other parties or communities (Crockford &
Boesch 2005). The "pant-hoot" is a four part call which starts with soft
"hoos" and increasing in volume as the "hoos" build up to the climax of
the call, made up of screams and sometimes barks, followed by the ending,
in which the screams die down to soft "hoos" again (Crockford &Boesch 2005).
Another call heard under these circumstances is the "bark" which is modified
under certain circumstances as the "short bark," when hunting and the "tonal
bark," given in the presence of large snakes (Crockford &Boesch 2005).
SPECIAL NOTES
Highly distinctive behavioral differences between populations of
chimpanzees have been observed and documented. These behavioral
differences between communities include 39 different patterns of
tool-use, grooming, and courtship behaviors and are classified as
cultural differences (Whiten et al. 1999). Behaviors are classified as
culture if inter-generational transmission of behavior occurs through
social or observational learning to become a population level
characteristic. That is, these behaviors are not linked to genetic
differences among subpopulations nor are they related to ecological
differences between study sites. While some behaviors are species
typical, such as nest building, others are far from uniform across
chimpanzee populations. Termite or ant fishing, which may be the most
famous examples of chimpanzee tool use are seen only in some populations
while nut cracking behavior is seen only in West Africa (McGrew 1994;
Whiten et al. 1999).
Content last modified: April 13, 2006
Written by Kristina Cawthon Lang. Reviewed by Elaine Videan.
Cite this page as:
Cawthon Lang KA. 2006 April 13. Primate Factsheets: Chimpanzee (Pan troglodytes) Behavior . <http://pin.primate.wisc.edu/factsheets/entry/chimpanzee/behav>. Accessed 2017 February 4.
