Gorilla
(Gorilla gorilla)

There are three subspecies within the species Gorilla gorilla:

• Gorilla gorilla gorilla: Western Lowland Gorilla
• Gorilla gorilla beringei: Mountain Gorilla
• Gorilla gorilla graueri: Eastern Lowland Gorilla

MORPHOLOGY

The western lowland gorilla has a brownish-gray pelage color with reddish highlights. The eastern lowland gorilla has a black pelage color. The mountain gorilla is black like the eastern lowland gorilla, but the hair is longer. The adult male who controls the group develops silver airs on his back, thus the name silver-back. In the western lowland gorilla these silver hairs also go down to the rump and thighs. The male has a sagittal crest which is larger than that of the female. The teeth of the gorilla have high crests that are used to help break down the leaves that make up a large proportion of their diet (Fleagle, 1988). This species has relatively long forelimbs and short hindlimbs (Fleagle, 1988). The gorilla has a short trunk and a wide pelvis (Fleagle, 1988). Individual gorillas can be distinguished from the nose, each gorilla has a unique nose pattern, like the human fingerprint.

BODY MASS:
western lowland gorilla:

• male: 140 kilograms (Estes, 1991)
• female: 75 kilograms (Estes, 1991)

mountain gorilla:

• male: 160 kilograms (Estes, 1991)
• female: 85 kilograms (Estes, 1991)

eastern lowland gorilla:

• male: 175 kilograms (Fleagle, 1988)
• female: 80 kilograms (Fleagle, 1988)

RANGE WESTERN LOWLAND GORILLA:
The western lowland gorilla is found in the countries of Cameroon, Central African Republic, Congo, Equatorial Guinea, Gabon, and Nigeria. This species lives in the lowland rainforests, and more specifically in Northern Congo they live in swamp forests. The western lowland gorilla prefers open-canopy forests that allow light to reach the forest floor.

MOUNTAIN GORILLA:
The mountain gorilla is found in the countries of Rwanda, Uganda, and Zaire, on the slopes of the Virunga volcanoes (Estes, 1991). This species prefers to live in hagenia woodlands and are found up to 3500 meters in altitude. The mountain gorilla prefers open-canopy forests that allow light to reach the forest floor.

EASTERN LOWLAND GORILLA:
The eastern lowland gorilla is found in the country of Zaire. This species lives in lowland rainforests. The eastern lowland gorilla prefers open-canopy forests that allow light to reach the forest floor.

ECOLOGY

The gorilla is a folivorous species, but will also eat fruit, seeds, flowers, roots, herbs, insects, and clay. The clay is eaten for the minerals that the gorilla can not obtain from its regular food sources (Estes, 1991). The parts of the plants mostly eaten are the leaves, stems, and pith. The favorite plants of the mountain gorilla are the leaves of the hagenia and hypericum trees and wild celery. Western lowland gorillas in Gabon will eat insectes, the most favorite being the weaver ant. Some populations consume such a large amount of succulent herbs that they do not need to drink much water (Estes, 1991). The lowland gorillas tend to consume more fruit than the mountain subspecies (Fleagle, 1988). This is a diurnal and a semi-terrestrial species, although the mountain gorilla is much less arboreal mainly the young play in trees. Each night the gorilla will construct a nest of leaves on the ground which is generally a crude platform with a circular rim (Estes, 1991).

MEAN GROUP SIZE:

• western lowland gorilla: 5 (Estes, 1991)
• mountain gorilla: 6 (Estes, 1991)
• eastern lowland gorilla: 11 (Estes, 1991)

LOCOMOTION

The gorilla moves quadrupedally, and has a special kind of quadrupedal gait called knuckle-walking (Fleagle, 1988). Unlike other quadrupedal primates which support their weight on the palms of the hands, gorillas support their weight on the dorsal surface of the third and fourth digits of the curled hands (Fleagle, 1988). When they travel through trees they climb, but are unable to move by suspensory behavior (Fleagle, 1988).

SOCIAL BEHAVIOR

The gorilla has a unimale social system and a polygynous mating system. The basic group is composed of one mature silver-back male (the leader of the group), one subadult or black-back male (about 8 to 12 years old), three adult females, and two to three young (less than 8 years old) (Estes, 1991). Both males and females emigrate from their natal troop (Estes, 1991). When females leave they travel a long distance from their natal troop and either join a lone silver-back male or a group with only a few females (Estes, 1991). Females may emigrate to avoid inbreeding because when she reaches sexual maturity the silver-back male is most likely her father (Stewart and Harcourt, 1987). There is a hierarchy amongst the females within the group, and a female's rank is passed on to her offspring (Estes, 1991). The female's rank is based upon when she became a member of the group, so the first female a lone silver-back makes part of his group would be the highest ranking female (Estes, 1991). High rank allows that female and her offspring to be closer to the silver-back male, and this could decrease the risk of predation (Estes, 1991). Grooming occurs between the silver-back and the adult females and the juveniles and the silver-back and between juveniles (Stewart and Harcourt, 1987). Males will only stay in their natal group if they are likely to be able to mate with females (Estes, 1991). This occurs if the silver-back male is old and is ready to die soon, the son will inherit the group (Estes, 1991). Males generally become solitary until about the age of 15 when they are strong enough and have a home range to start a group of their own (Estes, 1991). Lone silver-back males will challenge a resident silver-back and try to get the females to come with him (Estes, 1991). The resident male does actively prevent the females from going with him, but rather performs elaborate displays to keep the lone silver-back from approaching (Estes, 1991). A lone silver-back will generally approach a group that has a female undergoing estrus (Estes, 1991). Infanticide has been known to occur when a lone silver-back challenges the resident silver-back, he would do this because a female will start estrus sooner if her infant has died and is no longer nursing (Estes, 1991).

Immature gorillas at around the age of two will start to spend more time around the silver-back male (Stewart and Harcourt, 1987). The immatures will sit in close proximity to the silver-back and will sometimes groom the silver-back, and will sometimes groom them (Stewart and Harcourt, 1987). The silver-back will protect the immature gorillas from outside and within group threats (Stewart and Harcourt, 1987). Immature gorillas also spend much time playing with each other and grooming each other, and they tend to groom direct siblings than other immature gorillas (Stewart and Harcourt, 1987).

VOCAL COMMUNICATION

roar: This call is given by silver-back and large black-back males (Estes, 1991). This call is low in pitch and is an outburst of sound through the open mouth (Estes, 1991). This call is given when the individual is under stress or threat, and the individual may give false charges and short lunges (Estes, 1991). The group hides behind the silver-back when hearing this (Estes, 1991).

wraagh: This call is also an outburst, but not deep as the roar, and is monosyllabic in nature (Estes, 1991). This call is mostly emitted by the silver-back male (Estes, 1991). This is emitted when the individual is experiencing sudden stress, and group members scatter when hearing this call (Estes, 1991).

hoot series: This call is low-pitched and consists of a series of "hoo-hoo-hoos" (Estes, 1991). This call is emitted by the silver-back male (Estes, 1991). This call is given in response to seeing members from another group, and it is used to maintain group spacing (Estes, 1991). This is given before chest- beating (Estes, 1991).

scream: This call is loud and is a shrill sound repeated many times (Estes, 1991). This call is emitted by all gorillas, and is given when the individual is upset or fighting with other gorillas (Estes, 1991).

question bark: This call consists of a short series of three notes, the first and third being lower in pitch than the second (Estes, 1991). Mostly this is given by the silver-back male, and he emits this when he discovers someone that was concealed or another individual that is making noise but can not be seen (Estes, 1991).

cry: This is given by infants, and is a response to extreme distress (Estes, 1991). This is a wail-like call that can build up to a shriek (Estes, 1991).

chuckle: This is a rasping sounding call, and is given by infants during play (Estes, 1991).

belch vocalizations: This call is given by all gorillas, generally when they are stationary, and generally communicates contentment (Estes, 1991). These noises consist of purring, humming, rumbling, crooning, moaning, and soft-grunting noises (Estes, 1991).

pig grunts: This call consists of a series of short guttural noises (Estes, 1991). This call is given by adult males and females, and communicates mild aggression (Estes, 1991). This call is emitted when the adult wants access to preferred foods or right of way (Estes, 1991).

whine: This call is given by infants and adult males when they are being left behind (Estes, 1991).

copulatory panting: This is a series of "o-o-o-o" sounds that are loud and low-pitched (Estes, 1991). This is given by the males during copulation (Estes, 1991).

OLFACTORY COMMUNICATION

fear smell: This is produced by the silver-back male, and comes from glands under his armpits (Estes, 1991). This signals excitement or an aggressive threat (Estes, 1991).

VISUAL COMMUNICATION

chest-beating: This behavior is done by all gorillas and the either one or two open-fist hands are clapped against the chest (Estes, 1991). Adult males produce a sound when doing this because of air sacs they have which are located on both sides of their throat (Estes, 1991). For the adult male this is a threat display (Estes, 1991).

strutting walk: This is a rigid walk with arms bowed and the hair bristled so that the individual looks bigger (Estes, 1991). The individual makes short steps and has the side facing the receiver and only looks at the receiver but for a few glances (Estes, 1991). This is a dominance display and is mainly performed by silver-back males especially when a lone silver-back is attempting to lure a female away from the group (Estes, 1991).

staring: This where the sender has its eyes fixed on the receiver, the eyebrows are lowered, the head is angled down, and the lips are parted and pursed (Estes, 1991). This communicates aggression or annoyance (Estes, 1991).

tense-mouth face: This is like staring but the gums and teeth are displayed having the lips curled back (Estes, 1991). This is a threat display for potential predators, mostly given by adult males, and is often accompanied by a mock charge and scream and roars (Estes, 1991).

pout face: This is where the lips are pursed, mouth is slightly parted or compressed, and the eyebrows are raised (Estes, 1991). This is most often given by infants when their mothers leave them or they do not receive what they want, it is a display of light distress (Estes, 1991).

open-mouth grimace: This is where the mouth is wide open, the corners of the mouth are drawn back, the eyebrows are raised, the head is tilted back a little, and the eyes move back-and-forth (Estes, 1991). This is a fear display (Estes, 1991).

play face: This is where the mouth is open but not showing the teeth or gums (Estes, 1991). This is seen during play (Estes, 1991).

TACTILE COMMUNICATION

social grooming: This is where an individual will remove dead skin or parasites from the hair of another. This occurs between females and the silver-back, between immature gorillas, but not between adult females (Estes, 1991). This behavior is used to maintain the social bonds between individuals (Estes, 1991).

REPRODUCTION

The gorilla gives birth to a single offspring every four years (Estes, 1991). The female is usually the one to solicit copulations from the male (Estes, 1991). Mating is usually dorso-ventral, but ventro-ventral (face-to- face) mating does occur (Estes, 1991). Copulatory panting is emitted by the males during mating (Estes, 1991). Immature males might also try to mount preadolescent and adolescent females, the adolescent females have small sexual swellings (Estes, 1991).

REFERENCES

Burton, F. 1995. The Multimedia Guide to the Non-human Primates. Prentice-Hall Canada Inc.

Estes, R. D. 1992. The Behavior Guide to African Mammals. University of Californis Press.

Fleagle, J. G. 1988. Primate Adaptation and Evolution. Academic Press.

Stewart, K.J. and Harcourt, A.H. 1987. Gorillas: Variation in Female Relationships. In Primate Societies. eds. B.B. Smuts, D.L. Cheney, R.M. Seyfart, R.W. Wrangham, and T.T. Struhsaker. University of Chicago Press.

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