69.
Western Gorilla
(Gorilla gorilla)
CR
Overview
The gorilla is a heavily built primate and is the largest of the living apes.  Western and eastern gorillas are more genetically distant from one another than are chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). Western gorillas live in lowland, swamp, and montane forests from sea level to 1600 m. Western lowland gorillas have the largest home ranges and travel the farthest of all gorilla subspecies because of their reliance on fruit. Despite being the more numerous and widespread gorilla species, the severe threats of hunting, infectious disease and habitat loss make western gorillas at a greater risk of a population collapse in 30 years.
Urgent Conservation Actions
The effective implementation of protection laws and enforcement of protected areas are a priority.
Distribution
Angola, Cameroon, Central African Republic, Congo, Equatorial Guinea, Gabon, Nigeria
Associated Blog Posts
14th May 12
  The gorilla is one of the most impressive primates and the largest of the living apes: in some species males can weigh up to 140kg! With an EDG...  Read

Media from ARKive
ARKive video - Cross river gorilla - overview
ARKive image - Western lowland gorilla female with newborn
ARKive image - Western lowland gorilla infant asleep on mother's back
ARKive video - Western lowland gorilla infant with adult
ARKive video - Infant western lowland gorillas
ARKive image - Female western lowland gorilla walking, carrying infant on her back
ARKive image - Young western lowland gorilla, portrait
ARKive video - Western lowland gorilla infant in tree
ARKive video - Young western lowland gorillas playing
ARKive image - Western lowland gorilla female with infant
ARKive image - Infant riding on back of unrelated sub-adult western lowland gorilla, both orphans
ARKive video - Western lowland gorilla wading through water
ARKive video - Western lowland gorilla feeding
ARKive image - Young western lowland gorilla feeding
ARKive video - Western lowland gorilla - social behaviour
ARKive image - Juvenile western lowland gorilla knuckle walking
ARKive image - Young western lowland gorilla
ARKive video - Western lowland gorilla territorial defence
ARKive image - Young western lowland gorilla sitting on a fallen tree
ARKive video - Western lowland gorilla running through water
Evolutionary Distinctiveness
Order: Primates
Family: Hominidae
The family Hominidae comprises seven living species in four genera: Homo (humans), Gorilla (gorillas), Pan (chimpanzees) and Pongo (orangutans). Apes are thought to have diverged from Old World monkeys between 22 and 30 million years ago (mya). They were confined to Africa until around 15-17 mya, when they colonised Eurasia across a newly formed land bridge between the two continents. Here they underwent rapid dispersal and diversification, with the evolution of many new species including the ancestors of modern orangutans. This Eurasian ape radiation also includes the extinct genus Sivapithecus (formerly known as Ramapithecus), which was once thought to be one of the earliest human ancestors. Out of all the great apes, orangutans are the least closely related to humans, having split off from the early hominid lineage 10 to 12 million years ago. Gorillas were the next lineage to diverge, followed finally by the chimpanzee-bonobo lineage

For most of the twentieth century it was thought that there was a single species of gorilla with three subspecies; the western lowland, the eastern lowland and mountain gorillas. However, with DNA analysis it became clear that the significant differences were between eastern and western population rather than lowland and mountain gorillas. These forms were so distinct that they were reclassified as separate species. Today, the western gorilla (Gorilla gorilla) consists of two recognized subspecies: Gorilla gorilla gorilla (western lowland gorillas) and Gorilla gorilla diehli (Cross River gorillas). The taxonomic status of the gorilla populations in the Maiombe Forest (Cabinda, Democratic Republic of Congo and Republic of Congo) and in Ebo/Ndokbou (Cameroon) awaits clarification.
Description
Size: 
Standing height: 1.2 – 1.8 m
Weight: 68-181 kg
Western lowland gorillas tend to be a bit smaller than their mountain cousins. They also have shorter hair and longer arms. These great apes have black to greyish or reddish-brown hair that covers the body except for the face, soles of hands and feet, and upper chest. All males acquire a silver-gray colour across the back and upper thighs at sexual maturity. Males have an enlarged sagittal crest, which is a bone ridge on the top of the cranium. Males are larger than females.

Cross River gorillas are difficult to distinguish from other western gorillas, except that they differ significantly in their skull measurements and in particular in mean cheek tooth surface and the usual absence, or relatively poor development, of the sagittal crest in many males.
Ecology
Staple foods are pith, leaves and shoots. The fruit component of the diet is generally high but varies with seasonal availability. Insects also form an occasional component, when they can be caught. Like other subspecies of gorilla, the western lowland gorilla spends most of its time on the ground but will climb trees to feed on ripe fruits and to make a sleeping nest.

Gorillas live in groups averaging 10 and occasionally over 20 individuals, composed of at least one adult male, several adult females and their offspring. Each group’s home range may be as large as 20 km² and group ranges overlap extensively. Patterns of male and female emigration appear to be similar, but large groups (>20) containing more than one adult male are relatively uncommon in Gorilla g. gorilla.

Compared with the much larger demographic datasets from almost three decades of observation of mountain gorillas (Gorilla beringei beringei), it appears that western lowland gorillas reproduce more slowly due to longer inter-birth-intervals and higher infant mortality. Female gorillas give birth to one infant after a pregnancy of nearly nine months with inter-birth-intervals of 4 to 6 years. These newborns weigh around 2 kg and ride on their mothers' backs from the age of four months through the first two or three years of their lives.

Two studies of G. g. diehli demonstrated flexible grouping patterns with groups ranging in size from 2 to 20. These grouping patterns likely occur for several reasons, including restricted habitat, feeding competition related to fruit consumption, high hunting pressure, and limited opportunities for male migration. Each group’s home range may be as large as 20 km² and group ranges overlap extensively.

In captivity, gorillas have displayed significant intelligence and have even learned simple human sign language.
Habitat
Found primarily in lowland tropical forest (primary and secondary), particularly where there is dense ground-level herbaceous growth, and in swamp forests. Cross River gorillas are restricted mostly to hilly areas, and range from lowland to submontane forest although they occasionally use lowland areas between hills.
Distribution
The western gorilla Gorilla gorilla is found in Cameroon, Central African Republic (CAR), mainland Equatorial Guinea (Rio Muni), Gabon, Nigeria, Republic of Congo (RoC), Cabinda (Angola), and possibly in the Democratic Republic of Congo (DRC). The core population had until recently an almost continuous distribution in suitable habitat from southern CAR to the Congo River and west to the coast. Small outlying populations remain on the Nigeria-Cameroon border at the headwaters of the Cross River and in the Ebo/Ndokbou forest in Cameroon, just north of the lower Sanaga River. The species also persists in the Maiombe region of Democratic Republic of Congo contiguous with Cabinda. G. g. diehli occurs in a small area on the Nigeria-Cameroon border, extending a short distance on either side of the border in the forests on the upper drainage of the Cross River.
Population Estimate
Gorilla gorilla gorilla

The commonly cited figure of 95,000 is based on estimates that all suitable remaining habitat contains gorillas at densities similar to those in 1980 and is therefore subject to error. Furthermore, in 2007, WCS discovered a new population of 125,000 western lowland gorillas living in northern Republic of Congo.

Gorilla gorilla diehli

Intensive surveys over the last decade have found that approximately 250 to 300 G. g. diehli persist in a forested area of roughly 8,000 km². This estimate is of uncertain accuracy and is based primarily on nest counts and estimated range size.

The total western lowland gorilla population is estimated to include 100,000 to 200,000 individuals. No exact numbers are possible, as these elusive apes inhabit some of Africa’s densest and most remote rainforests.
Population Trend
Decreasing. This species has a predicted population reduction of more than 80% over three generations.
Status
Classified as Critically Endangered (CR A4cde) on the 2010 IUCN Red List of Threatened Species.
Threats
Gorilla gorilla gorilla
There are two primary drivers of rapid western lowland gorilla decline: commercial hunting and the Ebola virus. Since the early 1980s improvements in transportation infrastructure, devaluation of the regional currency, declining oil stocks, and timber depletion in other tropical regions have led to an explosion in mechanized logging. Logging vehicles are also used to transport bushmeat, and logging employees eat more bushmeat than local villagers. The gorillas’ very low reproductive rates mean that even low levels of hunting are enough to cause population decline. The threat posed by logging promises to continue and even intensify in the foreseeable future. Rates of timber production in the region are increasing, in the case of Gabon exponentially.

The second major driver of rapid gorilla decline is disease, specifically the Ebola virus. From 1992 to 2007, it is thought that around one third of the total population found in protected areas was killed by this lethal virus. Since the early 1990s, Ebola has caused a series of massive gorilla and chimpanzee die-offs in remote forest blocks at the heart of their range. For example, the 2003 outbreak of Ebola in the Republic of Congo killed 114 people and up to 800 western lowland gorillas. If the Ebola epizootic continues at the same rate and trajectory, it could reach most of the remaining protected areas with large populations of western lowland gorillas within the next 5 to 10 years.


Finally, there are suggestions that climate change may pose a serious future threat. Most of the western gorilla range receives rainfall only slightly higher than the amount necessary to maintain closed canopy forest. The last few decades have seen a decline in mean rainfall and a lengthening of dry seasons, which increase the risk of forest fires.

Gorilla gorilla diehli
The remaining population of G. g. diehli is small and fragmented, occurs mostly outside of protected areas (especially in Cameroon) and is surrounded by some of the most densely populated human settlements in Africa. This subspecies is at risk from its small size and associated increases in inbreeding and loss of genetic diversity. The lack of strictly protected areas throughout much of the range of G. g. diehli makes the future of sizeable portions of gorilla habitat uncertain. Conversion of forest for agriculture and grazing is occurring rapidly in many parts of the gorillas’ range and the largest current protected area in which Cross River gorillas occur (the Okwangwo Division of Nigeria’s Cross River National Park) contains enclaves of human settlements whose farmlands have spread beyond their legal boundaries and threaten to divide the park into two. Though legal prohibitions against the killing of gorillas exist in Nigeria and Cameroon, enforcement of wildlife laws is often lax, and most protected areas suffer from poorly-controlled poaching.
Conservation Underway
National and international laws controlling hunting or capture of gorillas exist in all habitat countries, but all lack the funds and infrastructure for enforcement. G. gorilla is listed under Appendix I of CITES and in Class A of the African Convention. The Cross River gorilla, Gorilla gorilla diehli, is listed on Appendix I of the Convention on Migratory Species (CMS).

Conservation areas exist in most gorilla range states and several National Parks have been created recently specifically to protect great apes and other large mammals. Strongholds identified as exceptional priority areas for ape conservation and not yet affected by Ebola are: the Dja Conservation complex and Boumba-Bek/Nki complex in Cameroon, the Loango/Moukalaba-Doudou/Gamba complex in Gabon, the Lac Télé Likouala complex in RoC, and the Sangha Trinational complex of the Republic of Congo, Central African Republic and Cameroon. National Parks offer no protection from Ebola and only one of these remaining gorilla strongholds (Loango/Moukalaba-Doudou/Gamba) is remote from recent epidemics.

Over 1998 to 2002, conservation efforts undertaken by the local people in collaboration with the Cross River Gorilla Research Project (Cameroon) and the Ministry of Forestry and Wildlife (MINFOF) project PROFA have markedly reduced gorilla hunting in these areas.
Projects

Ensuring the long-term survival of the western lowland gorilla and its forest habitat across the region, through a combination of approaches including research into status and threats and improving wildlife management in timber concessions.

Ensuring the long-term survival of the western lowland gorilla and its forest habitat across the region, through a combination of approaches including research into status and threats, supporting and improving wildlife management and exploring alternatives to bushmeat for communities.

Conservation Proposed
Urgent conservation needs include the effective implementation of protection laws. Conservation areas must be adequately protected and managed, and some additional areas should be gazetted. Surveys of outlier populations are needed, as are conservation education programmes. Extensive resources are required to identify appropriate conservation actions in the face of the spread of Ebola. Recommendations for reducing the negative impacts of selective logging on large mammals, including gorillas, have been formulated, but considerable efforts are needed to establish partnerships with the logging industry such that protective measures are enforced, and concessions bordering National Parks are a priority. Finally, the poor understanding of the current size of the population of western gorillas must be addressed. New surveys using consistent methods as well as regular monitoring of populations in protected areas are urgently needed throughout the western gorilla’s range. This will enable the conservation community to design and implement optimal conservation strategies in the face of this potent cocktail of threats.

A series of three workshops has identified priority actions for the conservation of G. g. diehli, and these have been formulated into an IUCN Action Plan. Of these recommendations the most notable are; increasing conservation education and awareness, fostering improved community participation in conservation issues, increasing trans-boundary conservation activities, such as joint patrols to control timber and bushmeat between the two countries, and further research.
Associated EDGE Community members

Madelaine is actively involved in Great ape conservation and has been involved in the production of many wildlife documentaries

West and North Africa Programme Manager

Central, East and South Africa Programme Manager

Links
Zoo population

There are western lowland gorillas resident at ZSL London Zoo

References
Redmond, I. 2008. Primates of the World: The Amazing Diversity of Our Closest Living Relatives. Marshall Editions, UK.

Walsh, P.D., Tutin, C.E.G., Oates, J.F., Baillie, J.E.M., Maisels, F., Stokes, E.J., Gatti, S., Bergl, R.A., Sunderland-Groves, J. & Dunn. A. 2008. Gorilla gorilla. In: IUCN 2010. IUCN Red List of Threatened Species. Version 2010.4. <www.iucnredlist.org>. Downloaded on 15 November 2010.

if you can provide new information to update this species account or to correct any errors, please email us at info@edgeofexistence.org


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