Social Spacing and Organization
Courtesy of Ecovolunteer/
Gibbon Sanctuary
(permission pending)
Social Organization
Gibbons live in small nuclear families consisting of a mated couple and offspring. The number of individuals in a group average 3-4 but can range from 2-11 individuals. Offspring can range from infants to adolescents to sub-adults. Usually upon reaching maturity sub-adults are forced out of the group by the same-sex parent. They must live in the periphery between group territories or try to start their own territory either by overtaking an existing group or by finding a territory recently vacated through death (Dispersal). Some sub-adults are allowed to remain within a group if they are contributing to the family in some positive manner (Social Cooperation). Group size is directly dependent on food resources. Groups using small, evenly distributed resources remain small to minimize intragroup competition and energetic traveling costs. Groups seeking large, unevenly, or clumped resources benefit from large groups to have intergroup defense (high abundance minimizes intragroup competition). Another benefit to a larger group size is added protection to help the father in defending the territory (Fuentes, 2000). New research is leaning more towards higher rates of non-nuclear families resulting from aggressive takeovers by one male against another (aggression).
Gibbon families try to avoid contact with each other. Overlapping home ranges allow for different families to share some food resources. Through vocalizations, however, temporal separation is achieved. With little contact with one another, no dominance hierarchy is thought to exist in the wild. However, in tightly confined or cramped areas (especially in captivity) a dominance system has been observed. Esser et al. terms this a "dominance organization consisting of constraint, coordination, and control." Esser and colleagues studied gibbons on Hall's Island in Bermuda (see below). These confined quarters caused conflict where social adaptations were inevitable. One adult became the visible alpha male. Others monitored his presence and behavior. They would allow him access to all resources and become subservient around him. These results correlated closely with other studies showing gibbons in smaller areas develop heightened aggression and some dominant behavior (Esser et al., 1979).
Hall's
Island, Bermuda has served as a gibbon research community for decades.
Its relatively large size and natural boundaries assimilate it to the wild
better than any other captivity sites. Hall's Island is pear-shaped with
a surface area of approximately 1.5 acres. The food naturally growing
on the island makes up 50% of the gibbons' daily diet consisting of leaves,
seeds, berries, buds and flowers. Several modifications were made to make
it even more like a natural habitat. An arboreal pathway was constructed
using 1,000ft of braided nylon rope attached to cedar and bamboo skeletons.
Cleared footpaths, storage sheds, and observation platforms were added
but completely ignored by the animal inhabitants. Monkey chow is supplied
three times a week at feeding stations. These additions help to simulate
a forced environment where random gibbons could live in close monitored
proximity. As previously mentioned, Esser et al. observed possible dominance
relationships. Their study observed 6 random white-handed gibbons (Esser
et al., 1979). Baldwin and Teleki observed distinct behavior patterns among
these gibbons and provide excellent descriptions of 59 behavioral patterns
including facial expressions, gestures, postures, and movements, maintenance,
stationary and locomotor actions (Baldwin et al.,
1976). Both studies have
allowed observations difficult to have discovered in the wild.
Mated Gibbons live in a large home range and defend a territory within it. Average home ranges can be anywhere between 5-500 hectares with an average of 20-40 hectares. Within this home range, between 70-95% is defended (aggression) (Fuentes, 2000). The heavily defended territory consists of the sleeping tree and some food resources. There is usually only one sleeping tree found in each territory. Since each sleeping tree is a limited resource, having more than one in a territory would cause unnecessary and extra energy used in defense. For this reason, most territories are set up around one sleeping tree along the periphery. Each home range can vary in size depending on resources and population density. Crowded smaller areas can consist of larger groups per territory as each sub-adult has less chance to find its own territory. These larger families provide more defense from intruding groups trying to take over a territory (cooperation) (Brockelman et al., 1998).
As a result of their large size for arboreal animals, gibbons have very few natural predators. Adaptations for strictly arboreal life probably resulted to minimize predation risks by land animals. Tigers have preyed on unsuspecting gibbons in very low branches or occasional visits to the forest floor (Ellefson, 1976). The main predator to the gibbon are humans and deforestation. Immature gibbons can fall prey to some species of pythons that feed on primates as well as large birds of prey . However, predation rates among gibbons are very low (Tenaza, 1975).
With a large variation of food sources and minimal predation risks, the only real need for defense is protection of a territory. Gibbons compete with small animals for food. This allows groups to be small as they do not need large groups for protection, resource defense, or hunting.
Aggressive Displays and Encounters
Courtesy of Thomas Geissmann; The
Gibbon Research Lab
Aggressive Behavior
Usually initial intergroup aggression remains harmless. This can be "mock rushing," chasing, vocalizations, or other signal exchanges (Palombit, 1993). More serious behavior can consist of lunging, grappling, or biting. Lunging usually begins with eye contact, a raised brow, grimaces or squeals, and may follow chasing. This is usually escalated behavior for defense or displacement of another individual. Grappling includes rolling around holding on to one another with both arms and legs while trying to gain positioning. This is a very escalated form and usually ends in injury or one running off. Biting occurs after chasing and lunging and mostly during grappling. The goal is to either bite and hold, hence grinding or cutting, or to nip and slash. This usually incurs injury (Baldwin et al., 1976). Gibbons have "saber-like" canines that can be up to 2 cm long (Palombit, 1993).
The most common type of aggression displayed is same-sex aggression. Males constantly guard their territory and mate from other males while the female fends off any other females. This is one reason why family numbers are so small. Usually sub-adults are forced from the group upon reaching maturity. As sub-adults reach maturity, their same-sex parent begins to push them further and further away from the group during feeding. Eventually the mature individual leaves before aggression turns into danger. This stems from jealousy by the parent. With this aggression, both the male and female defend their territory depending on the intruder. This display is also partially responsible for their monogamous relationships (Chivers et al., 1980).
Aggressive Encounter
Palombit describes one observation
where no preemptive ritualized aggression occurred and one male was in effect
killed by intergroup fighting. These observations were of White-Handed Gibbons
(Hylobates lar) on Hall's Island.
After duetting with his female mate, another male
approached the female. Male A intervened. The female fled while the unknown male
turned and attacked Male
A. Physical contact was observed and a tuft of hair on Male A's left shoulder
was observed missing. Male A showed decreased duetting time daily and limited
social interaction with his mate and group. Flies were seen circling the wound,
the male spent more time grooming his wound, and within a month disappeared.
While mortal wounds are unusual within gibbon intergroup aggression, this incident
appears to provide evidence that they exist more than originally thought. This
wound probably resulted from a bite from the sharp canines. This attack and its
unprovoked beginning goes against normal gibbon territorial defense. However,
this may lead
towards
theories of
higher male responsibility in resource protection or demonstrate higher same-sex
mating competition throughout monogamous males (Palombit,
1993).
Top of Page
Return to homepage