San Francisco State University
Department of Geography

Geography 316:  Biogeography 

The Biogeography of GORILLA  (Gorilla gorilla)
  by John Sighamony, student in Geography 316, Fall 1999

Kingdom:Animalia Phylum:Chordata Class:Mammalia Order:Primates Family:Pongidae Genus:Gorilla Species: Gorilla gorilla

Description of Species:
     The lowlands gorilla has two subspecies:  the Western{G. gorilla gorilla} and the Eastern{G. gorilla graueri}.  The larger eastern version lives in the moist Zaire rain forests near the Rwanda-Uganda border (MacDonald 1985).  The smaller western version lives in the tropical jungles of the Congo Basin (MacDonald 1985).  The western lowland male has distinctive almost white saddle on its back while the eastern version has short, black fur.  For the most part, they live in societies, or families, of about five to ten gorillas.  Also related is the very rare mountain gorilla{G. gorilla beringei} that inhabits the mountain forests(up to 11,000 ft.) of Eastern Zaire, bordering Rwanda (MacDonald 1985).
    The differences between the two races of gorilla are quite subtle.  The western gorilla, as stated before, is the smallest version between the two.  The western gorilla has small jaws and teeth set in a broad face (Dixson 1981).  The eastern lowland gorilla has short, black body hair and a narrow face, and is the largest version (Dixson 1981).  Gorillas stand about 5.8 feet tall.  They have short broad hands and feet that are no longer adept to swinging beneath branches.  The longer arm mean that the mammal has a sloping backline (Dixson 1981).  The gorilla’s big toe is opposable, much like a thumb.  The sole of the gorilla’s feet lie flat on the ground.  The males weigh around 300 kilos, while the females weigh half that (about 150 kilos) (Dixson 1981).
     The gorillas’ habitat is restricted to central Africa.  Studies in the field have suggested that the gorilla shows both intrapopulational and seasonal variation in diet as well as daily distance traveled (Schaller 1963).  The western lowlands gorilla (G. gorilla gorilla) eats a variety of fruits, somewhat similar to what the chimpanzee eats in the forests.  The strategies that the gorilla employs is dependent on its food resources.  This isn’t surprising because the gorilla isn’t a migrating species (Schaller 1963).  When fruits are available in large quantities, lowland gorillas travel long distances to search for food, and when fruits are scarce, gorillas travel less and consume primarily low-quality terrestrial foods (Uchida 1996).  This strategy probably enables the gorillas to live in forests.

Habitat
    Gorillas typically live in dense forests.  They generally are adept climbers, but are more frequently found roaming the ground (Dixson 1981).  Lowland gorillas are more frequently found in the forest belts with a damp climate.  From what we know, they stay clear of large openings in the forests.  They really are very much a private animal.  Gorillas spend most of their time on the ground, although at night the females usually build a platform from bent twigs.  The adult males stay on the ground, building their nests beneath the trees where the herd sleeps (Dixson 1981).  Lowland gorillas aren’t very good swimmers, so they don’t usually come near water.  The current population for the Western Lowland Gorillas and the Eastern Lowland Gorillas is about 40,000 and 10,000, respectively.  The mountain gorilla numbers about 250-350.
    For both the lowlands gorillas, an easy way to determine the habitat in the forests is to look for key signs.  Gorillas bulldoze, like elephants:  they lumber ponderously through the undergrowth often in single file, leaving a trail of broken stems(Nowark 1988).  Pieces of foliage are usually ripped by a swiping hand.  They even leave behind nests arranged in a circular fashion on the ground which usually last for up to a year.  A way to determine where gorillas have just been would be their smell, which smells much like human sweat.  Another sign are the large amounts of dung it leaves behind.
 

Natural History:
     All gorillas eat vegetation.  They rarely kill for fodder.  In fact, it has almost never been seen.  Most of their food consists of shoots, young leaves and other foliage, bark, roots, as well as various kinds of fruits and seeds.  Large quantities of this non-nutritious food with its high cellulose content are naturally required so that the animals spend a great deal of their time feeding (Dixson 1981).  Due to their vegetarian diet, they do not require much drinking water.
    Most gorillas live in a group led by a single male, known as a silverback (Dixson 1981).  The groups usually number between five and twenty, although they sometimes number into forty (Dixson 1981).  Groups tend to be smaller and denser in lowland forest, or where food sources are more scarce (Dixson 1981).  Having stated this, the group is akin to a harem, where adult females have different ranks among themselves.  There is always one dominant silverback (Dixson, 1981).  he is central, while females dominate juveniles and juveniles pull rank according to age (Dixson 1981).  The silverback takes care of the group, always facing up to deadly intruders, and tolerates the infants that scramble over him (Dixson 1981).
     In contrast to many other primate species, the gorilla’s sexual behavior is of minor significance (Berger 1985).  Most of the sexual behavior-such as genital display and the mounting of another gorilla-has rarely been observed.  What is known is that the gorillas have a tendency to be more vocal when they mate.  The males usually become sexually active at eight or nine years of age.  When living in the wild, female gorillas give birth at intervals of about four years (Berger 1985).  Gestation is about 245 to 255 days.  Since females have a generally large abdomen, pregnancy is not discovered until birth (Berger 1985).  When born, young gorillas are very playful, which, in the future, helps develop social ties.  The mother teaches the child and introduces it to the rest of the society.  It is  also the mother who develops the child and produces a child that behaves normally toward its family members.  In most gorilla societies, there is a solitary male that runs the troop, but the group of females that he has in the troop play a major role in the raising of the young gorilla.
    For the next six months of the infant's life, the mother has the responsibility of never letting it out of its sight.  The young gorilla spends two to three years suckling (Dixson 1981).  This is also the time that it interacts more with the other group members (Dixson 1981).  By the time that the infant is four years old, the male gorilla may separate from the mother, while the female gorilla stays in the group for a few more years (Dixson 1981).  Eventually she will also leave to find a mate.  Female gorillas may raise two or three young throughout her entire life (Dixson 1981).

 
 
  Evolution:
    It would seem to be very easy to find various information on the evolution of the gorilla, but this is a very rare animal to study.  Many of the things that we know about gorillas has come up in the past thirty years of field research and observation.  We now the gorilla developed as a single species, but developed into three different species over the course of time.  What do we know about the evolution of gorillas?  That question has not been fully answered.  It may take longer to figure out fully where the species has evolved from.  To show gorilla evolution, it is appropriate to look at primate evolution.
     About fifteen million years ago, apes and many other of its forms evolved.  What the large apes gained in brute strength, they lost in agility (Kimbel & Martin 1993).  They lost the ability to move through the branches.  Early apes swung along by their arms from tree to tree, many evolving short legs and arms to suit this mode of travel (Kimbel & Martin 1993).  The tail was also eventually lost, which was needed mostly for balance (Kimbel & Martin 1993).  The apes then moved through wide distribution when Africa drifted into Asia (Kimbel & Martin 1993).  Asian apes diverged from the African apes 12.5 million years ago, which then gave rise to the orangutans and gibbons.  Between four and eight million years ago, at which point man, chimps, and gorillas began to evolve along three independent paths (Kimbel & Martin 1993).
     Primate ancestry is very difficult to research, since it involves the evolution of man.  The fossil record begins with teeth from Montana, dated about seventy million years ago (Uchida 1996).  Most likely, they were small and rodent-like.  They probably ate insects, but then developed a taste for the  abundant fruit to be found in moist tropical rain forests (Uchida 1996).  We are not sure what the early relative of the gorilla was, but it can be traced back to an early ape known as Proconsul africanus.  Roughly the size of a mandrill, it leaped or ran about the forests of East Africa, feeding on fruit (Kimbel & Martin 1993).  The chart on the last page details the development of primates.  From what science knows, all primates started developing during the Paleocene era (58-63 million years ago).  Gorillas developed during the Pleistocene era (2 million years ago).

     There is also evidence to suggest that there is a disjunct range between all the types of gorillas:  Western lowlands (G. g. gorilla), Eastern lowlands (G. g. graueri), and the Mountain (G. g. beringei).  Minor differences between the lowland and mountain forms suggest that the divergence between the two occurred in the past.  The present distribution of the two subspecies also suggests that at one point the population was continuous throughout a rain forest belt from western Africa to central Africa, north of the Congo River (Schaller 1963).  There seems to be two primary factors that limit the growth of gorillas as a species.  For one, gorillas frequent only humid forested areas where succulent herbs and vines are readily available for forage (Schaller 1963).  Secondly, gorillas are apparently unable to swim and hesitate to wade in water (Schaller, 1963).  Rivers are barriers to gorillas.  In the past gorillas may have been a single population, but due to climatic change, their population has become changed.  Maybe at one time, central and western Africa were an uninterrupted stretch of rain forest.  Due to climatic change where there was a much drier period caused the rain forest to be in pockets
    Volcanic activity also suggests the population may have splintered into different directions.  In the Pleistocene period there seems to have been considerable tectonic activity which caused the split.  When the environment changed, the gorillas adapted to whatever their new habitat’s conditions supplied, such as food.
     This change in habitat is likely to reflect different dietary adaptations acquired during the isolation (Uchida 1996).  Western lowland gorillas have relatively wide incisors, relative to both molar size and body weight, while mountain gorillas have large, high-crowned molars with expanded crushing basins (Uchida, 1996).  From the development of teeth, we can note that the lowland gorillas have a varied diet of leaves and fruits, while the mountain version is adapted to eating mainly leaves and shoots of the higher elevation.  The habitat disjunction is related to the landscape change due to climate, which has allowed the three subspecies to evolve into different types of gorilla.
     Most of the evidence has not contributed to an agreed theory.  Many different scientists have a different view of how the gorillas evolved.  Some say it is due to climate change and some say it is due to the availability of food.  It may even be a combination of both.  For sure it has to do with the its diet and habitat.  This is a question that has soon to be resolved and answered.

 

Distribution

Map of Distribution:

According to data from various studies, the lowland gorilla far outnumbers the eastern and mountain races and is more widespread.  The western race of gorilla lives in the Central African Republic, equatorial Guinea, Gabon, and western Congo to the Congo River(Nowark 1988).  It has also been known to be found in small areas of northwestern Cameroon.  Most of the gorilla’s range includes a large section of the Congo Basin.  This is the area where four-fifths of Africa’s tropical rainforest can be found(Parker et al. 1990).  The jungles where the western version of the gorilla resides is very moist and where most of the rain falls.  The rains then drain into the Congo River which is subsequently carried into the Atlantic Ocean.  It shares its home in the forest with the tsetse fly, which carries sleeping sickness.  As a result, there has not really been a significant sojourn in to the gorilla’s domain.  This subspecies is also one of the least studied in the field because most of the attention has gone to the mountain gorilla.  The western lowland gorilla stays at an altitudes no higher than 6,000 feet(1,800 m.), so its homeland is never too cold(Nowark 1988).

The eastern lowlands gorilla is ten times rarer than the western lowlands gorilla.  The range of this ape is considerably smaller than its western cousin.  It lives on patches of lowland rain forest and in the western foothills of the Rift Valley, all within eastern Zaire (Nowark 1988).  On some mountain slopes, it lives on altitudes of around 6,560-8,200 feet (2,000-2,500 m.).  The easy way to determine the habitats of the gorillas is to note that the lowlands version stays in a hot and wet climate, whereas the mountain version stays in a cool, temperate climate.
    

Other interesting issues:
    For many centuries, Africans have been hunting and killing the lowlands gorilla.  For some centuries, gorilla meat was their only source of meat.  For as long as the gorilla has been discovered, there has been considerable fear and loathing of the ape.  Just the mention of its appearance would scare many people.  This has lead to many attacks on the gorilla population.  Since firearms are becoming increasingly widespread in rural Africa, the fear with which many locals regard gorillas, often leads to impulsive shootings (MacDonald 1985).  Most of the hunting that is done is out of sheer fright (MacDonald 1985).
     In the past the gorilla probably benefited from human activity.  As villagers shifted their farms and allowed former pastures to return to secondary forest, which provides the plentiful ground cover (Dixson, 1981).  Today most of the hunting is in the form of deforestation and logging (Dixson 1981).  Logging means big money, especially in the form of mahogany and other hardwoods (Dixson 1981).  The logging companies move deep into the forest, hiring locals to game hunt the gorilla.  They then trap gorillas and the preserved flesh is then sent back to the big towns for consumption (MacDonald 1985).  The main reason for the increased logging and hunting is the population growth of equatorial Africa.  While the lowland gorilla is still relatively stable, the mountain gorillas of Rwanda and Zaire are surrounded by high population density of humans (MacDonald 1985).
     Most of the threat lies within the majority of hunting and poaching.  During the early 20th century, the gorilla was shot for game (Berger 1985).  This is due through the use of fetishes.  These are cured animal parts collected and sold locally for their supposed magical powers (Berger 1985).  Many pounds of gorilla meat are used for consumption and is still prized by many local African cultures.  Logging and deforestation seem to be the biggest problems in the survival of the gorilla (Berger 1985).
     Gorillas can survive, only if humans lend a helping hand.  Most of the protection lies within governments taking action.  Although there are laws and decrees, enforcement is very difficult and almost ineffective (MacDonald 1985).  One major factor in the protection of the gorilla has been the national park system.  Tourism in areas of Rwanda have raised money for the protection of the endangered species (MacDonald 1985).  It is hoped that now, governments and local people can work together to provide a safe future for both gorillas and humankind.
 
 
 

Bibliography  

Berger, Gotthart.  1985.  Monkeys And Apes. New York, Arco Publishing.

Dixson, A.F.  1981.  THE NATURAL HISTORY OF THE GORILLA (PP. 180-196); foreward by RD Martin. New York.  Columbia University Press.

Kimbel, William & Lawrence Martin.  1993.  PRIMATE EVOLUTION CONCEPTS.  PP. 393-99.  New York, NY.  Plenum Press.

MacDonald, Dr. David. 1985.  “Gorilla” pp. 432-439.  Complete Mammal Encyclopedia.  New York. Equinox Publishing.

Nowark, Ronald M.  1988.  WALKER’S MAMMALS OF THE WORLD.  VOL. 1.  Baltimore, MD.  Johns Hopkins University Press.

Parker, Sybil.  1990.  GRZIMEK’S ENCYCLOPEDIA OF MAMMALS. VOL. 2.  New York, NY.  McGraw Hill Publishing co.

Schaller, George B.  1963.  THE GORILLA: THE BEHAVIOR AND ECOLOGY. pp. 27-36.  Chicago, Ill.  University of Chicago Press.

Uchida, Akiko.  1996.  “What We Don’t Know About Great Ape Variation.”  TRENDS IN ECOLOGY & EVOLUTION.  pp. 163-67.  White Plains, NY.

PICTURE BIBLIOGRAPHY
COVER: MacDonald, Dr. David.  1985.  "Gorilla" pp. 436.  Complete Mammal Encyclopedia.  New York.  Equinox Publishing.

PHOTOS: www.snap-shot.com/picpages/gor16.htm    (UNDER GORILLA PICTURES ONLINE)
                  www.snap-shot.com/picpages/gor13.htm    ALL PICTURES FROM ORIGINAL WEBSITE: www.snap-shot.com/pages/zoos/gorilla/gorr2.htm. Snapshot Wallpaper. Copyright 1999.

DISTRIBUTION MAP: Kimbel, William & Lawrence Martin.  1993.  PRIMATE EVOLUTION CONCEPTS.  pp. 396.  New York.  Plenum Press.
 
 
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