Gorilla Natural History

by Tim Knight, University of Washington

Historical Background

"None of the three great apes is considered ancestral to modern man, Homo sapiens, but they remain the only other type of extant primate with which human beings share such close physical characteristics. From them we may learn much concerning the behavior of our earliest primate prototypes, because behavior, unlike bones, teeth, or tools, does not fossilize.

Several million years ago the chimpanzee and gorilla lines had already separated from one another, and the orangutan line even earlier than that. Throughout the eighteenth century there remained a considerable amount of confusion in distinguishing between orangutans, chimpanzees, and gorillas. The orangutan was the first to be recognized as a distinct genus -- only because of its remote habitat in Asia. It was not until 1847, on the basis of a single skull from Gabon, that the gorilla was confirmed as a separate genus from the chimpanzee." (Fossey, 1983)

Subspecies of Gorillas

Just as there are separate subspecies among orangutan and chimpanzee, there are separate suspecies of gorilla, also with morphological variations related probably primarily to habitat.

The gorillas, previously considered a single species, were recently divided into two species and five subspecies. The eastern gorilla (Gorilla beringei) includes the mountain gorilla (Gorilla beringei beringei) of the Virunga Volcanoes area of Rwanda, Uganda and the Democratic Republic of Congo, the yet unnamed, but distinct, population of Uganda's Bwindi (Impenetrable) Forest, and the eastern lowland gorilla (Gorilla beringei graueri). Western Africa is home to at least two additional taxa, the western lowland gorilla (Gorilla gorilla gorilla) and the Cross River gorilla (Gorilla gorilla diehli). The mountain, Bwindi and Cross River gorilla populations all number only in the hundreds and are considered critically endangered. (Once and Future Primate Order, Conservation International, 5 April 2000)

In western Africa there remain approximately 94,000 western lowland gorilla (Gorilla gorilla gorilla ) in the wild, but recent surveys indicate a decline of up to 56 percent across their range, due to poaching and disease. In areas hard hit by the Ebola virus, over 90 percent of great apes have been killed. It is this subspecies most frequently seen in captivity and mounted in museum collections.

Some 1000 miles to the east within the Virunga Volcanoes of Zaire, Uganda, and Rwanda live the last surviving mountain gorillas (Gorilla gorilla beringei ) see photo by Harcourt. The Virunga population were the subjects of Dian Fossey's field study. According to a recent census of mountain gorillas in the Virunga montane forests, led by the WWF-funded International Gorilla Conservation Program, about 380 mountain gorillas remain in the wild. This is a 17 percent increase in the population based on the count of 324 mountain gorillas more than a decade ago. The Bwindi gorilla population — previously classified as mountain gorillas by some scientists — numbers approximately 325 animals in the Bwindi Impenetrable National Park in Uganda. No mountain gorillas live in captivity.

Another recognized subspecies is known as Grauer's gorillas or the eastern lowland gorilla (Gorilla beringei graueri). An estimated population of 16,000 eastern lowland gorillas remain in the lowland and Albertine Rift montane forests of Democratic Republic of Congo, where they have been long-time victims of poaching in a historically war-torn area (WWF 2005). These figures are far higher than previously reported, both as a result of more sophisticated calculations on the basis of estimates of density, and of area of available habitat, and because of exciting new findings of gorillas in huge areas of west Africa where they were not previously suspected to be (Harcourt 1996). While the high numbers are good news, most live outside protected areas, and calculations of rate of forest disappearance in Africa indicate that numbers will crash in the next 150 years (Harcourt 1996).

A recent report from Conservation International states:

"The world population of the Endangered eastern lowland gorilla (Gorilla beringei graueri), found almost exclusively in eastern Democratic Republic of Congo (DRC), has plummeted by more than 70 percent in the past decade. Scientists estimate that fewer than 5,000 individuals remain, down sharply from 17,000 in 1994." (Conservation International, March 2004)

A subspecies of eastern gorilla, the eastern lowland gorilla now inhabits just 13 percent of its historic range. The eastern lowland gorilla is located approximately 600 miles from the nearest population of western lowland gorillas (WWF 2005). In 2005, there were only two female eastern lowland gorillas living in captivity at the Antwerp Zoo (ISIS Abstracts, February 2005).

The Cross River gorilla, which differs from the western lowland gorilla in skull and tooth dimensions, is restricted to the forested hills on the Nigerian-Cameroon border (WWF 2005). "The Cross River gorilla is a good example of why we must be very careful not to neglect possible diversity. In the nick of time we have realized these gorillas are distinct, just before it is finally too late to save them from oblivion," said Dr. John F. Oates, primatologist with Hunter College - CUNY, and co-author with Esteban Sarmiento of the American Museum of Natural History, of the re-description of the Cross River gorilla." (Primates on the Brink, Conservation International 10 January 2000). The Cross River gorilla population is estimated to be 200 animals, making it one of the rarest apes in the world.

There is one Cross River gorilla living in the Limbe Wildlife Centre (LWC) near Douala, in the West of Cameroon. The LWC takes in rescued animals, often orphans of the pet and bushmeat trades.

[Population estimates were updated in 2005, based on new information from Conservation International and the World Wildlife Fund ]

Distribution

The distribution of gorillas in Africa is discontinuous. The western lowland gorilla (Gorilla gorilla gorilla ) is found in south-east Nigeria, Cameroon, Equatorial Guinea, Gabon, Congo (Brazzaville), and the Central African Republic. A wide gap exists between this large area of distribution and the two other subspecies, each with extremely limited ranges; the mountain gorilla (Gorilla beringei beringei ) from the Virunga Volcanoes and the Bwindi Forest at altitudes of 2,100-3,650m (7-12,000 ft) on the Zaire-Rwanda border; and the eastern lowland gorilla (Gorilla beringei graueri ) from Zaire, to the west of Lake Tanganyika and Lake Edward, from the lower altitudes of 760-2255m (2500-7400 ft). See the distribution map (based on Harcourt 1996).

Terrestrial vs. Arboreal

Gorillas are mainly ground-living, but where trees are strong enough and fruiting, whole groups can be seen up them, even the mature males. The height above the ground that animals build the nests in which they sleep at night is inversely proportional to their weight, with the silverbacks usually being on or close to the ground. Gorilla and chimpanzee nests look alike, but can usually be told apart because chimpanzees nest so much higher up trees than do gorillas.

Diet

Gorillas are primarily vegetarians, and large quantities of food are needed to sustain their massive bulk. They will eat large quantities of fruit when it is available, sometimes as much as the far more usually frugivorous (fruit-eating) chimpanzee. The main difference between the species is that when fruits become scarce, the far larger gorilla readilly switches to a fairly coarse diet of ground vegetation. In the Virungas, where fruit is very scarce, food is almost wholly derived from ground plants. Thus the mountain gorilla's diet consists basically of leaves, bark, pith, coarse stems, roots, vines, bamboo, wild cherry, thistles and nettles, and occasionaly insects, snails and slugs. See Dian Fossey's list of food vegetation types of mountain gorillas. The western lowland gorilla is more frugivorous.

Physical Description

The coat is black with a small white tuft on the rump in infancy. Mature males of all races develop a saddle of grey hairs across the back, hence the term "silverback" for an adult male. In adulthood, the western lowland gorilla is greyish or brownish; the lighter saddle extends to the thighs and is not sharply defined from the rest of the coat; in Gorilla beringei graueri, the fur is fairly short, but in Gorilla beringei beringei, it is long and silkly.

The male gorilla skull bears massive sagittal and nuchal crests and heavy brow ridges. The mandible is stout and chinless, with a thick bony simian shelf uniting the two halves. The dentition is typical of the great apes, with only a few minor differences. Not being a fruit-eater, the gorilla has less spatulate incisors than either the orang-utan or the chimpanzee; like the chimpanzee, the molar enamel is not wrinkled.

The thumb is rather short and stubby, but manipulative ability is greater than in chimpanzees or orang-utans. Paradoxically, however, there is no evidence of tool-using or tool-making like in the chimpanzees observed by Jane Goodall or in orangu-tans that I have observed.

Locomotion

The gait is quadrupedal, with the weight of the forequarters carried on the backs of the knuckles. Bipedalism is not often employed, except during the chest-beating display where the gorilla may run bipedally for up to six metres (20ft). With long arms and short legs, the gorilla's intermembral index is high - 117. Arm-swinging never takes place except in infancy.

From birth infants may travel ventrally until about four months old, when they begin riding dorsally (Fossey, 1983). Infants initially cling upside-down to the long chest hairs of the mother - with occassional support from the mother's hand - while traveling. As the infant begins to physically mature the mother will lift the infant onto her back where the infant will ride with hands and feet clinging to the mother's hair (see photo). I have also observed an infant clinging to her mother's arms or legs while the mother walks quadrupedal.

Height and Weight

 Western Lowland
Gorilla  (Gorilla gorilla gorilla) height: males 1666mm (5ft 6in); arm
span: 2336mm (7ft 8in); wt: 140kg (307lb)

Mountain Gorilla  (Gorilla beringei beringei) height: males 1725mm (5ft
8in); arm span: 2286mm (7ft 6in); wt: 155kg (343lb)

Eastern Lowland Gorilla  (Gorilla beringei graueri) height: males 1750mm
(5ft 9in); arm span: 2590mm (8ft 6in); wt: 165kg (360lb)

Mature females are approximately half the weight of males. Infant gorillas weigh about 2.3kg (5 lb) at birth.

Social Behavior

The primary sources of information on the social behavior of gorillas comes from studies of the mountain gorilla by Schaller (1963), and by Dian Fossey (1983) and others at the Karisoke Research Centre in the Virunga Volcanoes. Groups contain 5-30 individuals with usually a single silverback male who acts as the leader, one or two black-backed subadult males, several adult females, and up to ten juveniles and infants. Groups are very cohesive and generally peaceable. There is little overt dominance among females, but access to the breeding females is the prerogative of the silverback male.

As a young male matures and becomes a silverback at about 11-13 years of age, he may not be tolerated by the dominant males, and may be forced to emigrate from the troop. He lives a solitary life as a lone male, probably traveling some distance, until an opportunity occurs to start a group of his own by luring maturing females from other groups. Adult females usually transfer from their natal groups to another group, but without any period of solitary existence. Because females leave the group in which they were born, an unacceptable degree of inbreeding is prevented.

Tool Use in Captive Gorillas

GOMA, Congo (AP) -- An infant gorilla in a Congo sanctuary is smashing palm nuts between two rocks to extract oil, surprising and intriguing scientists who say they have much to learn about what gorillas can do -- and about what that says about evolution.

It had been thought that the premeditated use of stones and sticks to accomplish a task like cracking nuts was restricted to humans and the smaller, more agile chimpanzees.

Then, in late September, keepers at a Dian Fossey Gorilla Fund International sanctuary in this eastern Congo city saw 2 1/2-year-old female gorilla Itebero smashing palm nuts between rocks in the "hammer and anvil" technique, considered among the most complex tool use behaviors.

"This is a surprising finding, given what we know about tool use in gorillas," Dian Fossey Gorilla Fund primatologist Patrick Mehlman said earlier this month at his Goma office.

Mehlman said that the finding indicates that complex tool use may not be a trait developed only by humans and chimpanzees, and could have its origins earlier in the evolutionary chain, among ancestors common to both humans and our closest relatives the great apes.

(source: Nut-cracking gorilla surprises scientists - CNN.com, October 18, 2005)

Tool Use in Wild Gorillas

Descriptions of novel tool use by great apes in response to different circumstances aids us in understanding the factors favoring the evolution of tool use in humans. Although there are reports of tool use by captive gorillas, including object throwing and use of tools in feeding, there has been to our knowledge no reported case of tool use in by wild gorillas, despite decades of field research. In 2005, researchers reported observing gorillas in the Republic of Congo's rain forests using simple tools, according to a team led by Thomas Breuer of the Wildlife Conservation Society at the Bronx Zoo. A recent research paper documents what scientists believe to be the first two observations of tool use in wild western gorillas (Gorilla gorilla).

Genetics

The gorilla has 48 chromosomes, like the chimpanzee, bonobo, and orangutan, with a very similar banding pattern to the 46 chromosomes of humans (Knight 1995).

Reproduction

There is no fixed birth season. The menstrual cycle lasts 31-32 days, and oestrus is marked by slight swelling of the genitalia in young females. Menstrual bleeding is minimal. Gestation lasts for about 258 days. Infants are weaned at about two years of age, and sleep in their mother's nest until about three. Females first give birth at about ten years of age in the wild, and breed every 3.5 to 4.5 years. A male acquires his silver back at 11-13 years of age.

Longevity

The oldest animal that we know of in the wild died at 35 years of age. Gorillas in the wild normally would live to age 30 or 35. A male gorilla named "Bul Bul" lived to 44 years of age at the Ueno Zoological Gardens in Tokyo. The Philadelphia Zoo's Massa set the longevity record of 54 years at the time of his death.

In May 2008, a female Western lowland gorilla named Jenny - who is recognized as the oldest gorilla in captivity - turned 55 at Dallas Zoo. Jenny's caretakers at the Dallas Zoo say she's having a few joint issues and her eyesight isn't what it used to be but she still looks good for an old ape. The International Species Information System, which maintains records on animals at 700 institutions around the world, said Jenny is the oldest gorilla in its database. (Source: AP) Read more.

Gorillas in Captivity

According to the ISIS Abstracts (June 2008), there are a total of 759 gorillas in zoos that report to the International Species Information System. Of these captive gorillas there are 331 males, 416 females and 10 unknown gender (infants). The overwhelming majority of captive gorillas are the western lowland gorilla subspecies (757); while 2 are classified as eastern lowland gorillas. There are apparently no mountain gorillas in captivity. The ISIS Abstracts lists a total of 19 captive gorilla births in the last 6 months.

A list of zoos that maintain captive gorillas is available in the ISIS Abstracts.

References

Breuer T, Ndoundou-Hockemba M, Fishlock V (2005) First Observation of Tool Use in Wild Gorillas. PLoS Biol 3(11): e380
(Read the article)

Cardiff University (2007, December 11). Ice Ages And Rivers May Have Affected Gorilla Diversification. ScienceDaily. Retrieved May 3, 2008, from
http://www.sciencedaily.com­ /releases/2007/12/071210212200.htm

Conservation International - Press Release
Eastern Lowland Gorilla Population Plummets 70 Percent Since 1994, 30 March 2004
(Read the article)

Conservation International - Press Release
Once and Future Primate Order, Closer look shows several species had not been identified,
5 April 2000

Conservation International - Press Release
Primates on the Brink, New List Spotlights World's Top 25 Most Endangered,
10 January 2000

Fossey, D. 1983. Gorillas in the Mist, Houghton Mifflin Company, Boston

Harcourt, A.H. 1996. Is the gorilla a threatened species? How should we judge? Biological Conservation, 75, 165-176.

Knight, T.F. 1995. Primate Cytogenetics Network, Web site. Seattle, Washington. http://homepage.mac.com/wildlifeweb/cyto/

Napier, J.R.; Napier, P.H. 1985. The natural history of the primates, The MIT Press, Cambridge, Massachusetts.

Sarmiento, Esteban E., Oates, John F.
The Cross River Gorillas: A Distinct Subspecies, Gorilla gorilla diehli Matschie 1904.
American Museum Novitates 2000 3304: 1-55 (abstract)

Wildlife Conservation Society (2008, February 13). Unique Mating Photos Of Wild Gorillas Face To Face. ScienceDaily. Retrieved May 3, 2008, from
http://www.sciencedaily.com­ /releases/2008/02/080212134818.htm

Wildlife Conservation Society (2008, April 22). World's Rarest Gorilla Finds Sanctuary. ScienceDaily. Retrieved May 3, 2008, from
http://www.sciencedaily.com­ /releases/2008/04/080419020546.htm

Wolfheim, J.H. 1983. Primates of the World: Distribution, Abundance, and Conservation, University of Washington Press, Seattle and London.

World Wildlife Fund 2005. WWF Gorillas, Web site. Washington, DC.
http://www.worldwildlife.org/gorillas/index.cfm

Online Information

Eastern Gorilla - BBC Wildfacts

Facts and pictures on the eastern lowland gorilla (G. beringei graueri)

First Observation of Tool Use in Wild Gorillas.
Descriptions of novel tool use by great apes in response to different circumstances aids us in understanding the factors favoring the evolution of tool use in humans. This paper documents what we believe to be the first two observations of tool use in wild western gorillas (Gorilla gorilla).

Gorilla Links

Explore a variety of online information on gorillas.

Gorilla Questions and Answers

How closely are gorillas related to us? How intelligent are gorillas? What is a Silverback? These questions and many more are addressed on this website.

Gorilla References

A list of gorilla references on a variety of topics.

International Gorilla Conservation Programme

The goal of the International Gorilla Conservation Programme (IGCP) is to ensure the conservation of mountain gorillas and their regional afromontane forest habitat in Rwanda, Uganda and the Democratic Republic of Congo (DRC).

WWF Gorillas

Covers gorilla species, natural history, threats, science, field notes, and how to contribute to programs that protect gorillas in the wild.

Photo Credits

Western lowland gorilla photos by Tim Knight; Bwindi gorilla photo contributed by Debbie Bloom; Distribution map contributed by Alexander Harcourt. See the Gorilla Gallery for more photos of western lowland, bwindi, and mountain gorillas.

Taxonomic Note: The taxonomy used on this page was revised in June 2000 and will not correspond to older taxonomies that have been published in books and scientific journals. New information about the anatomy, vocalizations, behavior and chromosome numbers of primates, as well as sequencing of their nuclear and mitchondrial DNA, influenced the analysis, which called into question previous categorizations.

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