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Gorilla beringeieastern gorilla

By Tracy Lindsley and Anna Bess Sorin

Geographic Range

Mountain, or eastern gorillas, Gorilla beringei, are found in the Virunga volcanoes that separate the Democratic Republic of Congo from Rwanda and Uganda.

Habitat

Mountain gorillas inhabit the montane cloud forest of the Virunga range. Occasionally they go into the afro-alpine meadows (4,000 m) where temperatures are subfreezing at night and there is little suitable food to forage on.

  • Range elevation
    4000 (high) m
    13123.36 (high) ft

Physical Description

Gorillas are the largest primate, with average lengths of 150 cm for females and 185 cm for males. They are highly sexually dimorphic, with females weighing 70 to 114 kg and males averaging 160 kg. They have robust bodies, long muscular arms, short legs, massive heads, and males have large, sharp canine teeth. Mountain gorilla coats are silky and long, ranging in color from blue-black to brownish-grey. Mature males develop a large patch of silver or grey hair on their backs, giving them the name silverbacks. Males also have apocrine glands in their armpits that emit a strong odor when the animal is under stress.

Mountain gorillas differ from other gorillas in having longer hair, larger jaws and teeth, smaller nose, and shorter arms.

  • Sexual Dimorphism
  • male larger
  • Range mass
    70 to 200 kg
    154.19 to 440.53 lb
  • Range length
    150 to 185 cm
    59.06 to 72.83 in

Reproduction

Mountain gorillas are polygynous; the dominant male in each group has exclusive access to all the females in the group.

Reproductive rates are slow and a female may leave only 2 to 6 offspring over a 40 year life-span. Males that have a harems of 3 to 4 females increase their reproductive output by fathering 10 to 20 offspring over 50 years. These animals don't mature sexually until well into their teens.

Mating behavior is initiated by the female, with a series of slow and hesitant approaches to the male. A female is receptive only during estrus, and she will cease to ovulate for several years after giving birth. The length of the estrous cycle of a female mountain gorilla is 28 days, and there is no visible external menstrual flow.

A single, dependent young is born after a eight and a half month gestation period. Weaning often doesn't occur until three years of age, and juveniles may remain with mothers for years after that. Females are sexually mature by 10 years of age, but males are unlikely to start breeding before 15 years. Reproductive output for females is about one surviving offspring every 8 years (survival implying reaching breeding age).

  • Breeding interval
    The interval between reproductive events depends upon infant survival. Females are capable of producing an infant every 4 to 5 years.
  • Breeding season
    These animals breed throughout the year.
  • Average number of offspring
    1
  • Average gestation period
    8.5 months
  • Range weaning age
    36 to 48 months
  • Average age at sexual or reproductive maturity (female)
    10 years
  • Average age at sexual or reproductive maturity (male)
    15 years

Females provide most of the parental care in this species. Females nurse and carry their young for about 4 years. They also play with the young, teach them, and groom them.

The role of males in parental care is less direct, although no less important. Males protect the females and the young within their social group from potentially infanticidal rival males who may take control of the group.

  • Parental Investment
  • altricial
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • male
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • male
      • female
  • pre-independence
    • protecting
      • male
      • female
  • extended period of juvenile learning

Lifespan/Longevity

Gorillas can reach ages of 40 to 50 years.

  • Typical lifespan
    Status: wild
    50 (high) years

Behavior

The main social unit is a male with a harem of females and their offspring. These groups are nonterritorial, but severe intergroup conflicts can occur when groups encounter each other and especially if a lone male contacts the group. The dominant male of a group is massive compared to the other members and they all defer to him.

Females transfer from their natal group to a new group before breeding. This generally occurs at around 8 years of age. Often they join a lone male and start a new group, rather than join an established group and be a lower ranking female. Males often leave the natal group at around 11 years of age. Males, however, can't join an establish group, and they spend much time in solitary existence until they can gain females and begin a group of their own at age 15 or older.

Grooming often occurs between females and males, or among females.

Gorillas spend about 30% of the day feeding, 30% traveling, and 40% resting. They make nests to sleep and rest in that can be in trees, on steep slopes or even on the ground.

Communication and Perception

All primates have complex patterns of communication. Gorillas are known to use vocalizations to communicate with one another. Tactile communication, in the form of grooming, play, and sexual contact, also occurs. Males emit a strong odor when stressed, which appears to function as a type of chemical communication. In addition to these, gorillas use body postures and facial expressions, as well as other visual signals, to communicate with one another.

Food Habits

Mountain gorillas occasionally eat invertebrates, but they are primarily folivorous. They eat the roots, leaves, stems, and pith of herbs, vines, shrubs, and bamboo. Their diet is supplemented by small amounts of bark, wood, roots, flowers, fruit, fungi, epithelium stripped from roots, galls, invertebrates, and gorilla dung.

  • Animal Foods
  • insects
  • Plant Foods
  • leaves
  • roots and tubers
  • wood, bark, or stems
  • fruit
  • Other Foods
  • fungus
  • dung

Predation

These animals are very large, and live in regions where not many potential predators exist. It is not likely that they fall prey to any particular species with any regularity.

Ecosystem Roles

These animals may be important in structuring plant communities, as they feed heavily on vegetation.

Economic Importance for Humans: Positive

Gorillas may be visited by ecotourists, enhancing local economies.

  • Positive Impacts
  • body parts are source of valuable material
  • ecotourism
  • research and education

Economic Importance for Humans: Negative

There is continuing political pressure to convert the remaining gorilla reserves into areas for farming or commercial use. Due to the high population density, many people feel that the land would be better put to commercial use.

Conservation Status

Mountain gorillas are highly endangered. This is due both to habitat destruction and severe poaching pressures. Gorilla species are subjected to heavy pressure from poaching for body parts and for young animals collected for zoos and private collections, generating illegal income. The civil war occurring in the region they inhabit has only added to their plight, increasing mortality through accidents and the breakdown of patrol units against poachers.

Other Comments

Gorillas are usually very gentle creatures. Their only effective predators are humans. They are often thought to be "slow" or "dumb" because of their sluggishness, but in fact they are intelligent and capable of learning sign language.

There is an ongoing debate as to the relationship of gorillas, chimps and humans. Gorillas are clearly one of our closest relatives if not the closest, sharing 98% of our genome.

Contributors

Nancy Shefferly (editor), Animal Diversity Web.

Tracy Lindsley (author), University of Michigan-Ann Arbor, Anna Bess Sorin (author), Biology Dept., University of Memphis.

Glossary

Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

World Map

acoustic

uses sound to communicate

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

diurnal
  1. active during the day, 2. lasting for one day.
dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates

ecotourism

humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

fertilization

union of egg and spermatozoan

folivore

an animal that mainly eats leaves.

herbivore

An animal that eats mainly plants or parts of plants.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

mountains

This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.

native range

the area in which the animal is naturally found, the region in which it is endemic.

polygynous

having more than one female as a mate at one time

rainforest

rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

tactile

uses touch to communicate

terrestrial

Living on the ground.

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

year-round breeding

breeding takes place throughout the year

References

Estes, R.D. 1991. The Behavior Guide to African Mammals. University of California Press: Los Angeles.

Mittermeier, Russell A. and Dorothy L. Cheney. 1987. Conservation of Primates and Their Habitats. Primate Societies. 189- 202.

Watts, David P. 1994. Agnostic relationships between female mountain gorillas. Behavioral Ecological Sociobiology, vol.34. 347-358.

Watts, David P. 1990. Ecology of gorillas and its relation to female transfer in mountain gorillas. International Journal of Primatology, vol.11. 21-43.

Stewart, Kelly J. and Alexander H. Harcourt. 1986. Gorillas: Variation in Female Relationships. Primate Societies. 155-164.

Blaffer Hrdy, Sarah and Patricia L. Whitten. 1986. Patterning of Sexual Activity. Primate Societies. 370- 384.

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