Mammals

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.

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.

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.

Decreasing. This species has a predicted population reduction of more than 80% over three generations.

Classified as Critically Endangered (CR A4cde) on the 2010 IUCN Red List of Threatened Species.

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.

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.

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.

National Geographic

WWF 

 WCS

 Primate Info Net

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.