Gorillas are the largest living primate. These terrestrial apes can attain a standing height of nearly 6 feet and a body weight of 430 pounds. Currently, Gorilla is categorized as two species each divided into two subspecies. Western gorillas are represented by the western lowland gorillas, Gorilla gorilla gorilla, and the Cross River gorillas, Gorilla gorilla diehli. Eastern gorillas are classified as either eastern lowland gorillas, Gorilla berengei graueri, or mountain gorillas, Gorilla berengei berengei. The majority of gorillas remaining in the wild today are the western lowland and this is the species most commonly seen in zoos throughout the world. According to Vigilant, “Gorillas contain twice the genetic diversity found in humans. The divergence to the lineage creating Gorilla was between 6.3 – 8.5 mya. The Homo-Pan LCA is separated from this divergence by only 1.5 – 2.1 million years.”
The sexual dimorphism exhibited in Gorilla is large. At over 400 pounds, males are 50% bigger than the females, have pronounced sagittal crests, and massive canines. The most telling feature of sexual dimorphism is the “silverback.” During their mid teens males reach full sexual maturity and begin to develop their signature silver hair. In his prime it will extend from atop the shoulders through the base of the lower back with fringes running through the arms and legs. Female gorillas reach sexual maturity at approximately 7 – 8. Sexually receptive females are courted through a series of mock challenges issued by the silverback. These actions typically consist of biting, hooting, barking, chest thumping, and jump kicking. Single infants are born after a gestation period of 8.5 months. The newborn will cling to the chest or stomach of its mother for the first 4-5 months of life and will eventually transition to the shoulders or back. Life expectancy both in the wild and the zoological setting are approximately 40 – 45 years.
Gorillas typically live in one of two categories of social groups. Each of the two systems is composed of 3 – 20 individuals in a strict dominance hierarchy with a mature silverback male at the top. One male polygynous groups occur when one mature male lives with as many females as he can monopolize. It is the responsibility of the silverback to protect the females and his offspring, their food surplus, and territory from outside intruders. Polygynandrous systems contain multiple males and females. The dominant silverback is aided by the presence of other possible males, most likely his brothers and/or sons, in the group when it comes to the protection of assets.
Each species of Gorilla is recognized as endangered. According to The World Conservation Union’s Red List of Threatened Species and the Max Planck Institute for Evolutionary Biology, “The combined effects of poaching and Ebola have caused western gorilla populations to drop by at least 60% in the last twenty years. Eastern gorillas probably number in the mid thousands while western gorillas number in the tens of thousands. Habitat loss is the greatest threat to eastern gorillas but a relatively minor threat to western gorillas. Eastern gorillas are also threatened by illegal hunting but to a lesser extent than western gorillas. Only western gorillas and common chimpanzees are currently threatened by Ebola.”
Gorilla gorilla gorilla:
Western Lowland Gorilla
Western lowland gorillas live in lowland tropical forests in Cameroon, the Central African Republic, the Republic of Congo, Equatorial Guinea, Gabon, and Nigeria. This is a polygynous society consisting of one silverback male, multiple females, juveniles, and/or infants. There day consists of grooming, juvenile play, and eating fruits, leaves, termites, and stems. This silverback will sire all of the infants in the group and his reign is absolute. He is under constant pressure to defend his troop at all costs against any challenger. If and when he is deposed it will become the newcomers chance to monopolize and defend both the females and territorial resources. Each of the females has two options: stay and breed or go. A female who is already invested with a child will not become receptive to the new silverback until her offspring are killed. If the new silverback does not complete this task, the mother will lose respect for him and leave for another group.
Research in the area of ecological and social factors and their influence on the ranging activities indicate that all subspecies of gorilla differ in respect to their ranging patterns. According to Doran-Sheehy, et al., “Male – male competition has a strong effect on ranging behaviors. Males range for a much greater distance on days after encountering another male. Western gorillas encounter rival groups much more frequently than the other subspecies of gorilla and have the greatest daily range length of approximately 6600 feet. High male – male competition is also reflected by the smaller group size of this subspecies (1 fully mature silverback male), elevated gorilla group density, and increased home range overlap.”
Life history and social structure patterns indicate that Eastern gorillas live in both multi-male and single male troops. Multi-male groups are rare in Western gorillas. Data provided by Robbins, et al. indicate, “Female Western gorillas prefer single male groups which seems consistent with increased feeding competition. Whereas the polygynous group formation may improve the reproductive success of the male. Western gorillas exhibit the highest rate of infanticide amongst the genus, which may explain why females of the Eastern species prefer multi-male groups.”
Gorilla berengei graueri:
Eastern Mountain Gorilla
Mountain gorillas, the rarest of the subspecies, live in mountain forests (up to 11,000 feet) at the borders of Rwanda, Uganda, and the Democratic Republic of the Congo. Mountain gorillas roam a home range between 1000 – 2000 acres in search of leaves, roots, flowers, fruits, bamboo, shoots and grubs. Polygynandrous societies are common within this subspecies. Approximately 40% of all mountain gorilla groups contain more than one silverback (Vigilant). According to Strier, “The behaviors of female mountain gorillas are highly influenced by the males of the group. This is due to an indifference towards other females and an easily accessible food source.”
Gorilla berengei graueri:
Eastern Lowland Gorilla
The eastern lowland gorilla is the largest of the four gorilla subspecies. Eastern lowland gorillas, also called Grauer’s gorillas, live in tropical forests from low elevations up to 8,000 feet in the Democratic Republic of the Congo (formerly Zaire) and along the border with Uganda and Rwanda. Eastern lowland gorillas feed primarily on leaves, shoots, and stems. Galium vines, wild celery and three or four other species make up a high proportion of the diet. Small amounts of wood, roots, flowers, fruits, and grubs also are eaten. Males of this species either emigrate to form their own group or take over an existing group.
Gorilla gorilla diehli:
Cross River Gorilla
Status: Critically Endangered, the rarest of the Gorilla subspecies with less than 300 individuals. The Cross River State is a coastal state in southeastern Nigeria, bordering Cameroon to the east and it is named for the Cross River (Oyono), which passes it.