There are two subspecies: the western lowland gorilla (G. g. gorilla)
and the Cross River gorilla
(G. g. diehli).
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.
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.
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.
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.
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.
between gorillas consists of grunts, barks, screams, hoots, and
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.
two subspecies are the eastern lowland or Grauer's gorilla (G.
b. graueri) and the mountain gorilla (G. b. beringei).
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.
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
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.
are built by adults and weaned young each night for sleeping. Unlike
most other apes, mountain gorillas primarily nest on the ground.
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
is similar to western gorillas and consists of grunts, barks, screams,
hoots, and facial expressions.
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.