|Scientific Name:||Gorilla gorilla (Savage, 1847)|
|Infra-specific Taxa Assessed:|
Troglodytes gorilla Savage, 1847
|Taxonomic Source(s):||Mittermeier, R.A., Rylands, A.B. and Wilson D.E. 2013. Handbook of the Mammals of the World: Volume 3 Primates. Lynx Edicions, Barcelona.|
The Western Gorilla (Gorilla gorilla) has two recognized subspecies: the Western Lowland Gorilla (Gorilla gorilla gorilla) and the Cross River Gorilla (Gorilla gorilla diehli: Sarmiento and Oates 2000, Groves 2001). Genetic data suggest that the two subspecies of Western Gorilla diverged approximately 18,000 years ago (Thalmann et al. 2011) and that the Cross River Gorilla population can be clearly differentiated from Western Lowland Gorillas (Prado-Martinez et al. 2013). The taxonomic status of the Gorilla populations in Ebo (Cameroon) awaits clarification; however, measurements from a single Ebo Gorilla skull indicate this may be a relict population of a previously more widespread population living north of the Sanaga River (Groves 2005).
|Red List Category & Criteria:||Critically Endangered A4bcde ver 3.1|
|Assessor(s):||Maisels, F., Bergl, R.A. & Williamson, E.A.|
|Reviewer(s):||Mittermeier, R.A. & Rylands, A.B.|
|Contributor(s):||Baillie, J., Butynski, T.M., Dunn, A., Gatti, S., Oates, J.F., Stokes, E., Strindberg, S., Sunderland-Groves, J., Tutin, C. & Walsh, P.D.|
Gorilla gorilla has a large geographic range, covering over 700,000 km². The size of the population is currently being evaluated, but thought to be in the order of a few hundred thousand (Strindberg et al. in prep). Only a very small number of Western Gorillas are the G. g. diehli subspecies, therefore this rationale focuses on the G. g. gorilla subspecies. The country of Gabon lost over half its Gorilla population between 1983 and 2000 (Walsh et al. 2003). More recent population declines have been estimated using a predictive model that incorporated survey data collected between 2003 and 2013 across the entire range of Western Lowland Gorillas. The results reveal an 18.75% decline between 2005 and 2013, corresponding to an annual loss of ~2.56% (Strindberg et al. in prep). These population decreases were driven by poaching and disease (Ebolavirus) outbreaks.
Despite their abundance and wide geographic range, Western Gorillas qualify as Critically Endangered under criterion A: a population reduction of more than 80% over three generations (one generation is ~22 years). This listing is based on ongoing population losses due to illegal hunting, disease and habitat loss: poaching is intensifying with the expansion of access routes into forests and Zaire Ebolavirus remains a highly significant threat. At a conservative rate of reduction (2.56% per year rather than 4%, calculated from Walsh et al. 2003), the reduction in the Western Gorilla population is predicted to exceed 80% over three generations (i.e., 66 years, 2005–2071). Illegal hunting has not ceased despite intense anti-poaching efforts, and the threat of Ebolavirus has not been removed. In addition, the scale of habitat conversion to industrial agriculture will increase, and the effects of climate change will become more evident. Gorilla gorilla thus qualifies as Critically Endangered (A4bcde).
|Previously published Red List assessments:|
|Range Description:||Gorilla gorilla is found in Angola (Cabinda enclave), Cameroon, Central African Republic (CAR), mainland Equatorial Guinea (Rio Muni), Gabon, Nigeria and Republic of Congo. Until recently, the core population had an almost continuous distribution from southern CAR to the Congo River and west to the coast. Rivers are the primary geographic barrier for this taxon, followed by habitat fragmentation: the two subspecies are separated by a major river (the Sanaga), and Western Lowland Gorillas are divided into subpopulations by other major rivers in the region (Anthony et al. 2007, Fünfstück et al. 2014, Fünfstück and Vigilant 2015).|
The northwestern limit of the western lowland subspecies distribution is the Sanaga River in Cameroon; the northern limit is the forest-savanna boundary to a maximum of roughly 6°N; the eastern limit is the Ubangi River; the Congo River south of its confluence with the Ubangi then becomes the southeastern and southern limits all the way to the coast. Small outlying populations of the Cross River subspecies remain on the Nigeria-Cameroon border at the headwaters of the Cross River and in the proposed Ebo National Park in Cameroon. Most Western Gorillas are found below 500 m asl, but those living on mountains occasionally reach elevations of 1,900 m asl.
Native:Angola (Cabinda); Cameroon; Central African Republic; Congo; Equatorial Guinea (Equatorial Guinea (mainland)); Gabon; Nigeria
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||Estimates of Gorilla population size are usually made using a standard index of abundance: night nest abundance and distribution, sometimes combined with predictive modelling.|
G. g. diehli
Intensive surveys carried out in 1990–2005 suggested that approximately 250–300 Cross River Gorillas persist in a forested area of roughly 12,000 km² (Bergl 2006, Oates et al. 2007). Monitoring of the Gorillas at multiple sites and genetic analysis at select locations (Arandjelovic et al. 2015) has confirmed the small size of the subspecies population, while also documenting a larger geographic range than previously recorded (Bergl et al. 2012, Dunn et al. 2014). Cross River Gorillas occur in at least geographically distinct 11 localities (ibid.) Genetic evidence suggests that these subpopulations have had reproductive contact in the recent past (Bergl and Vigilant 2007); however, field surveys paired with remote sensing analysis reveal that forest corridors connecting subpopulations may no longer be functional as routes for dispersal (Imong et al. 2014).
G. g. gorilla
Western Lowland Gorillas are found in almost all protected areas and many of the logging concessions in their range; they can persist at high densities in well-managed logging concessions (Morgan et al. 2013). Extensive surveys carried out since the mid-2000s suggested a total population in the areas surveyed in the order of 150,000–250,000 (Williamson et al. 2013, Sop et al. 2015). Their numbers declined by 18.75% between 2005 and 2013, corresponding to an annual loss of approximately 2.56% (Strindberg et al. in prep). The current Western Lowland Gorilla population size is being evaluated, but is in the order of a few hundred thousand (ibid.)
|Current Population Trend:||Decreasing|
|Habitat and Ecology:||Western Gorillas are diurnal and semi-terrestrial. They build nests to sleep in every night, usually on the ground but sometimes in trees. They are social and live in stable, cohesive groups composed of one “silverback” adult male, several adult females and their offspring. Gorillas are not territorial and group ranges overlap extensively.|
G. g. diehli
Cross River Gorillas inhabit forests largely restricted to rough terrain in remote areas, although they occasionally use lowland areas between hills (Oates et al. 2003, Bergl and Vigilant 2007). This distribution appears to be directly related to greater levels of human activity (especially poaching) in lowland areas, rather than selection of areas that provide preferred foods (Imong et al. 2013, Sawyer and Brashares 2013). Group sizes range from 2 to 20 individuals and home ranges may be as large as 30 km² (McFarland 2007, Sunderland-Groves et al. 2009). The highly-seasonal nature of the Cross River Gorillas’ habitat (more markedly seasonal than that of other Gorilla populations) likely contributes to grouping and ranging patterns. Their diet reflects this seasonality, with terrestrial herbs and bark eaten as staples throughout the year, and fruit consumed during periods of seasonal abundance (McFarland 2007, De Vere et al. 2011, Etiendem and Tagg 2013, Sawyer and Brashares 2013).
G. g. gorilla
Western Lowland Gorillas occur in both swamp and terra firma lowland forests throughout Western Equatorial Africa. They are especially common where ground vegetation is dominated by monocotyledonous plants such as Haumania liebrechtsiana and Megaphrynium macrostachyum (Rainey et al. 2009). Staple and fallback foods are pith of Aframomum spp. and leaves and shoots of the Marantaceae family, whereas fruit consumption varies greatly between seasons (Rogers et al. 2004). Some populations spend hours feeding on aquatic herbs in baïs and swamps. Social ants and termites are the only animal matter deliberately ingested. Group size averages 10, but is occasionally over 20 individuals, and annual home ranges are usually 10–25 km² (Williamson and Butynski 2013).
Life History (as summarised in Williamson et al. 2013): Male Western Gorillas take 18 years to reach full maturity, and females take ca10 years. Length of the reproductive cycle is unknown. There is no birth season. Mean length of gestation is 256 days in captive Gorillas. Infant mortality up to three years of age is 22–65%. Infants suckle for 4–5 years, causing lactational amenorrhea in the mother. Interbirth intervals are 4–6 years. Western Gorillas appear to reproduce more slowly than Eastern Gorillas (G. beringei). Maximum life span is unknown, but likely to be ca 40 years. Generation time is estimated to be 22 years (see Appendix I in the supplementary material).
|Generation Length (years):||22|
|Movement patterns:||Not a Migrant|
|Use and Trade:||
Gorillas are completely protected by national and international laws in all countries of their range, and it is, therefore, illegal to kill, capture or trade in live Gorillas or their body parts.
The two subspecies of Western Gorilla face similar threats, but to varying degrees in different regions.
National and international laws controlling hunting or capture of Gorillas exist in all habitat countries, but enforcement of protective legislation is inconsistent or lacking throughout much of the species’ range. Gorilla gorilla is listed under Appendix I of CITES and in Class A of the African Convention.
Only ~22% of Western Lowland Gorillas live inside protected areas, which cover ~14% of their geographic range (Strindberg et al. in prep). A further 20% are found in FSC-certified logging concessions, adding up to 8% of the range (Strindberg et al. in prep). Although forest guards work in many protected areas and in the well-managed logging concessions, 58% of Western Lowland Gorillas and 78% of their range are unprotected and highly vulnerable to poachers.
The Cross River Gorilla population is small and fragmented and their habitat is surrounded by some of the most densely populated human settlements in Africa. Approximately 30% of Cross River Gorillas occur outside of protected areas and many important habitat corridors between subpopulations exist as ungazetted land. The recent creation of new protected areas in Cameroon combined with existing protected areas in Nigeria, provides the legal framework for habitat protection and the control of hunting. However, effective management of these reserves remains a challenge. Illegal activities, including poaching, occur in all the region’s protected areas. Resources, equipment and training, in addition to proper oversight are needed for these areas to operate effectively.
Targeted conservation action plans have been produced for both Cross River Gorillas (Oates et al. 2007, Dunn et al. 2014) and Western Lowland Gorillas (Tutin et al. 2005, IUCN 2014). A series of recommendations have been outlined, which can be broadly encapsulated as:
|Errata reason:||This is an errata version of the 2016 assessment to correct some minor typos (e.g., to correct "Ebola virus" to "Ebolavirus") and grammatical errors in the Justification, Habitat & Ecology, Threats, and Conservation Actions sections.|
|Citation:||Maisels, F., Bergl, R.A. & Williamson, E.A. 2016. Gorilla gorilla (errata version published in 2016). The IUCN Red List of Threatened Species 2016: e.T9404A102330408.Downloaded on 27 May 2018.|