This web page was designed by Andre Carelock as an assignment for Animal Behavior, a Biology course at Davidson College.
Mountain Gorilla
(Gorilla beringei beringei)
Habitat and Utilization:
The mountain gorilla is found in southwest Uganda, northwest Rwanda, and Eastern Zaire. The Virunga Volcano chain at an altitude of 2,100-3,650 is home to about half the remaining mountain gorillas. With the Hygenia tree being the most dominant, the rain forest in which the mountain gorilla dwells is filled with a wide range of forest and vegetation types. Gorillas are most frequently found in the Hygenia forest where food availability is most high, but seasonally in the bamboo forest (the second most resourceful area) and sub-alpine vegetation zone. The Hagenia and bamboo forest are temperate zones with an average temperature of 60 degrees Fahrenheit and a minimum of 40 degrees Fahrenheit The average rainfall and humidity are 72 in. and 67-76% (Maple and Hoff). The rain forest also houses many of the plant species (Tshinononono, Musakule, Mbobwe, etc.) used for nest building, displaying aggression, and feeding. Mountain gorillas are terrestrial folivores whose food selection is not as vast as other gorillas. They feed on perennially available herbaceous items such as herbs, vines, wild celery, flowers, bulbs, occasionally bamboo shuts, and etc. Food is densely distributed in super patches and there are no locally superabundant food sources. There is spatial variability in food abundance (Watts).
Animal Description:
Both sexes exhibit divergent bodily features of size, height, and markings. Dimorphism in mountain gorillas is exhibited through different ontogenic processes, bimaturism and sexual selection. Newborns small, covered in black fur, and weigh about 2.3 kg (5lbs.). Juveniles are aged three to six years old. Before reaching six, both sexes are usually similar in size, black coloring, and growth rate. At age six, the female matures with a weight of 113-136kg (250-300) and a height of 4ft. Males continue to grow beyond the age a six and mature around 10 years old. Full-grown males reach a maximum height of 1.5-1.8m (5-6ft) and weight about 204-227kg (450-500lbs). Mature males are designated by a patch of gray/silver hair on their backs and by unusually long canines.
Courtesy of Chris Kreger
Social System:
Gorillas have several social group types ranging from lone silverback, one-male, multi-male, and all male groups (Robbins). Sixty percent of all gorilla social structures are uni-male. The silverback (dominant male) is the leader of the group. The family structure of gorilla is usually composed of one dominant silverback male, one to six adult females, and several juveniles/infants. Group size may range from five to thirty individuals. If the group is multi-male, the other male will usually be a son of the dominant male raised in the group since birth. The dominant male will not allow outside males to immigrate into the group. Females do not have an overt dominance hierarchy. Once reaching maturity, females will usually emigrate to prevent incest, however, males sometimes stay in the natal group.
Courtesy of Tim Knight
Social Spacing:
Mountain gorillas have an exclusive home range spanning from 5 sq. km to 35 sq. km. They travel between 600 and 1100 meters a day. Food is a spacing mechanism that influences migration (Casimar and Butenandt). Because food is distributed in variable abundance, quality, and without any superabundant sources, gorillas must travel from area to area based upon food availability. Such conditions are hard to defend. Annually, migration occurs based upon food availability/abundance between hygenia and bamboo forest (Casimir and Butenandt). Migration varies from regular to rapid (Elliott). Traveling is also influenced by predation from humans and leopards. By regularly migrating to new locations within the home range the dominate male is able to evade predators. Lastly, constant travel enables the group to obtain full knowledge of total home range conditions, increase overall food availability, avoid overlapping ranges with others, and decrease digestive problems posed by a restricted diet.
Courtesy of Tim Knight
Social Relationships:
Mountain gorillas exhibit a female defense polygyny. A polygyny exists because a male has a greater chance of increasing individual fitness by breeding with a large number of females. One advantage of this is the accumulation of subordinate males who aid in defense. Polygyny exists because if fewer males perform viable tasks (reproductive success, defense, finding food, etc.) more efficiently than many, then selection will favor polygynous mating (Eisenberg). Females depend on the dominant male for protection, reproductive success, and finding food resources. Dominant males aggressively prevent other males either within or outside the group from mating with adult females. Dominant males only allow subordinate males to mate with already pregnant and/or cycling females (Sicotte and Watts). Moreover, the silverback can monopolize several females in a polygyny because females are the primary care givers of offspring, females put forth energy to maintain the male-female relationship only with the dominant male, and she mostly 'chooses' to mate with the dominant male.
The bonds existing between the dominant male and the adult females are critical to the cohesiveness of their social structure. Both male and females play role in maintaining the social structure of the group. Grooming, huddling, and other non-sexual behaviors strengthen bonds (Eisenberg). Females invest energy into maintaining a high quality male-female relationship (Sicotte). To strengthen bonds, females spend more times around the silverback than any other male. A female will increase proximity to the silverback after child birth (Stewart and Hancourt). If the relationship is not strong, females tend to emigrate to other groups or a lone male in the face of intense male reproductive competition. Female-female relationships tend to be agonistic. Females form unclear agonistic dominance hierarchies whereby they maintain distance from one another unless they are kin. Relationships among female kin in female-resident species are typically more affinitive than those among non-kin (Stewart and Hancourt). The existing competition (which is connected to age and dominance rank) between females controls the social structure of the group as it encourages females to emigrate if it outweighs the benefit of being in the group (Stewart). In multi-male groups the dominant male is usually the father of the other residing male(s) in the group. The non-dominant male(s) or blackbacks help maintain social structure by aiding the silverback in defense. Male reproductive competition plays a major role in the formation/maintenance of the social structure of the group. In inter-group encounters males exhibit such competition in the form of contact aggression and agonistic displays in the attempt to maintain the presence of females their the group (Sicotte).
Mountain gorillas have a parental family structure. Mothers play the primary role in infant care. Females are responsible for the infant's socialization, nurturing and protection. Males provide the infant with both food and protection from infanticide. Both the male and female groom the infant (Eisenberg).
Courtesy of Tim Knight
Social Cooperation:
In uni-male groups the the males exhibit selfish behavior as he defends his offspring to ensure his reproductive success. Relationships among female kin in female resident species are typically more affinitive than those among non-kin. Related female gorillas form cooperative alliances to aid in competition and agonistic interactions. The mating structure of the group may inhibit/constrain the formation of kin groups. Thus, the frequency of kin selection/related individuals being present in a group is low. To prevent inbreeding, once reaching maturity, the daughter female usually emigrates from the natal group. Silver backs control the amount of aggression/fighting among females and more specifically he prevents the formation of kin-based alliances (Stewart and Hancourt). Females display such behavior most often in competition for food. Because the presence of both female relatedness and cooperative behavior is rare, it would be difficult to accurately define the females' behavior as selfish, altruistic, or reciprocal altruism. This issue is left open to further analysis and/or debate.
Courtesy of: www.FotoClipart.com
Social System:
The social structure of the mountain gorilla has been influenced by density dependent selection. The social system is dependent upon the availability of resources offered by the gorilla's habitat (Eisenberg). As state earlier, food is distributed in variable abundance, quality, and without any superabundant sources. The size of the gorilla group is directly proportional to the supportability of the habitat. The uni-male (i.e. smaller group size) social structure is adaptive because selection selects against large group sizes that require many individuals to perform viable tasks as efficiently as small groups and more importantly are not maintained as easily by the habitat. The uni-male group is considered to be the most adaptive social structure (Eisenberg).
Courtesy of: www.FotoClipart.com
Last eight photos courtesy of: www.anamilpicturesachives.com
References:
Casimir, Michael J., Butenandt, Eckart (1973) "Migration and Core Area Shifting in Relation to some Ecological Factors in a Mountain Gorilla Group in the Mt. Kahuzi Region." Z. Tier psychologie 18 (6): 514-22.
Eisenberg, J. F., N. A. M., R. Rudran (1972). "The Relation between Ecology and Social Structure in Primates." Science 176(4037): 863-874.
Elliott, R. C., (1976). "Observations on Small Group of Mountain Gorillas." Folia Primatologica 25: 12-24.
Leigh, S.R. (1995) "Ontogeny and the Evolution of Adult Body Size Dimorphism in Apes." American Journal of Primatology 36(1): 37-60.
Robbins, M. M. (1999). "Male Mating Patterns in Wild Multi-male Mountain Gorilla Groups." Animal Behavior 57 (5): 1013-1020.
Robbins, M. M. (1995). "A demographic analysis of male life history and social structure of mountain gorillas." 132:21-47.
Sicotte, P. (1994) "Effect of Male Competition on Male-Female Relationships in Biamael Groups of Mountain Gorillas." Ethology 97: 47-64.
Sicotte, P. (1993). "Inter-Group Encounters and Female Transfer in Mountain Gorillas: Influence of Group Composition on Male Behavior." American Journal of Primatology 30 (1): 21-36.
Stewart, K.J., and Harcourt, Alexander H. (1987). Gorillas: Variation in Female Relationships. Primate Societies. D. C. Barbara B. Smuts, Robert M. Seyfarth, Richard W. Wrangham, and Thomas T. Struhsaker, The University of Chicago Press: 155-164.
Watts, D. P. (1985). "Relations Between Group Size and Composition and Feeding Competition in Mountain Gorilla Groups." Animal Behavior 33: 72-85.
Watts, D. P. (1994). "Agonistic Relationships Between Female Mountain Gorillas." Behavioral Ecology Sociobiology 34: 347-358.
Watts, D. P. (1990). "Mountain Gorilla Life Histories, Reproductive Competition, and Sociosexual Behavior and some implications for Captive Husbandry." Zoo Biology 9:185-200.
http://www.selu.com/bio/gorilla/info/nh.html (2/19/2001).
http:www.cotf.edu/ete/modules/mgorilla/mgbiology.html (2/19/2001).
For Questions or comments e-mail: vecase@davidson.edu
© Copyright 2000 Department of Biology,
Davidson College, Davidson, NC 28036
