The Gorilla Species Survival Plan® manages the genetic and demographic health and oversees the care of gorillas in AZA zoos.








The Gorilla Health Project is an SSP-endorsed research initiative for gorillas in zoos.




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Western gorilla
Gorilla gorilla
There are two subspecies: the western lowland gorilla (G. g. gorilla) and the Cross River gorilla
(G. g. diehli).
Natural History
Cross River gorillas are found at the westernmost edge of the western gorilla range, in a small area along the Nigeria-Cameroon border. Western lowland gorillas range from Cameroon eastward to the Republic of Congo and southward to Angola. Western lowland gorillas are found in a variety of habitats, including primary and regenerating forests as well as lowland areas. Their distribution appears to be closely linked to the presence of terrestrial herbs, which are an important component of their diet.
Western gorilla diet varies considerably by season. During the wet season, western gorillas eat a large amount of fruit; during the dry season, when fruit is less available, their diet consists primarily of fibrous vegetation and herbs. In comparison to eastern gorillas, western gorillas have larger day and home ranges, probably as a result of the higher fruit content of their diet.

Reproduction has not been thoroughly studied in the wild. It has been estimated that females give birth to an offspring every five years, but birth intervals can be greatly affected by seasons and food availability. They do appear to have longer interbirth intervals than mountain gorillas, probably as a result of their less consistent diet.

Like the other apes, western gorillas make both day and night nests. However, western gorilla nests are often on the ground or in the very low branches of trees.

Social Behavior
Western gorilla groups range from 2 to 32 individuals. Group size averages 10 individuals, generally including one adult male (silverback), three to four females, and multiple immature offspring. Most gorillas emigrate from their natal groups after reaching maturity. In all gorilla subspecies, silverback-female relationships are key to gorilla sociality. Because females do not meet until adulthood, they generally have very weak social bonds, as demonstrated by their extremely low rates of grooming and other affiliative behaviors. As a result, when the silverback dies, the females disperse to new groups rather than staying together.

Communication between gorillas consists of grunts, barks, screams, hoots, and facial expressions.

Threats and Conservation
The western gorilla is considered Critically Endangered by the International Union for the Conservation of Nature (IUCN). The Cross River gorilla is the most critically endangered great ape subspecies, with an estimated 300 individuals remaining. Numbers for western lowland gorillas are estimated at 120,000 individuals; however this estimation has a large margin of uncertainty. Threats to western gorillas include: forest clearings, fragmentation, and degradation for agriculture, road building, and logging; hunting by private collectors and local people. Western gorilla populations have also been significantly affected by Ebola, which is estimated to have a 95% mortality rate in gorillas.
Eastern gorilla
Gorilla beringei
The two subspecies are the eastern lowland or Grauer's gorilla (G. b. graueri) and the mountain gorilla (G. b. beringei).
Natural History
Mountain gorillas reside in two known populations within the Democratic Republic of the Congo (DRC), Rwanda, and Uganda. The Virunga population lives in three national parks: the Virunga National Park (DRC), Volcanoes National Park (Rwanda), and Mgahinga Gorilla National Park (Uganda); the Bwindi population lives in Bwindi National Park in Uganda. Grauer's gorillas only live in eastern DRC in the Kahuzi-Biega National Park and bordering Kasese region, Maiko National Park, Itombwe Forest, and North Kivu. The subspecies are geographically separated.

In the Virungas, the mountain gorillas reside at the ecological extreme for the species: montane forest ranging from 2000 m to 4100 m. Bwindi and Grauer's gorillas generally live in lower-altitude habitats (1,110 m to 2,600 m for Bwindi gorillas; 600 m to 2,900 m for Grauer's gorillas). The high altitude (up to 4,100 meters) of the Virunga mountain gorilla habitat limits their diet to primarily herbaceous vegetation, as little fruit is available. Bwindi mountain gorillas and Grauer's gorillas both range at lower altitudes and incorporate considerable fruit into their diet during the seasons it is available. As a result, ranging behavior varies between the subspecies. Virunga mountain gorillas have home ranges ranging between 5.5-11 km2; these values for Bwindi and Grauer's gorillas are 20-40 km2 and 13-17 km2.

Females reach maturity at about eight years and the birth interval is about four years. Between the first sign of menstruation and conception, there is about a two-year sterility period. Mating outside of a group is rare.

Nests are built by adults and weaned young each night for sleeping. Unlike most other apes, mountain gorillas primarily nest on the ground.

Social Behavior
Group size ranges from 2-65 with an average group size of 10. Unlike western gorillas, eastern gorillas are known to form multi-male breeding groups, particularly in the mountain gorilla subspecies. In some groups up to 10 adult males have been observed to live together. This can provide for more group stability than found in western gorilla groups because there is a subordinate male to take over if the dominant male dies. Natal dispersal is observed in both males and females, although roughly 50% of Virunga male gorillas do not disperse. As with western gorillas, grooming and other social support is uncommon between females and female-female relationships are weak.

Communication is similar to western gorillas and consists of grunts, barks, screams, hoots, and facial expressions.

Threats and Conservation
Eastern gorillas are considered endangered by the International Union for the Conservation of Nature (IUCN). The Grauer's subspecies is listed as endangered and the mountain gorilla is listed as critically endangered. There are currently 700 mountain gorillas remaining and this subspecies of ape is the only one known to be increasing. Because of the long civil war in DRC, conservationists to do not have a reliable estimate on the number of Grauer's gorillas. The projected range is from 5,000 to 26,000 individuals. Threats to eastern gorilla survival include: hunting, war and political unrest, habitat loss and modification, and disease transmission from humans.

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