Gorilla gorilla (Savage, 1847)

Russell A. Mittermeier, Anthony B. Rylands & Don E. Wilson, 2013, Hominidae, Handbook of the Mammals of the World – Volume 3 Primates, Barcelona: Lynx Edicions, pp. 792-854 : 848-849

3.

Western Gorilla

Gorilla gorilla

French: Gorille de I'Ouest / German: Westlicher Gorilla / Spanish: Gorila occidental

Other common names: Lowland Gorilla; Cross River Gorilla (diehli), Western Lowland Gorilla (gorilla)

Taxonomy. Troglodytes gorilla Savage, 1847

Gabon: Mpongwe.

In 2001, a small population ofgorillas was discovered in the Ebo Forest of western Cameroon, geographically intermediate between the subspecies gorilla and diehli Genetic analyses are neededto determine if the Ebo gorillas are a distinct taxonomic group. Two subspecies are recognized.

Subspecies and Distribution.

G.g.gorillaSavage,1847—SCameroon SWCentralAfricanRepublic, EquatorialGuinea, Gabon, RepublicoftheCongo, andNAngola (Cabinda).

G. g. diehli Matschie, 1904 — SE Nigeria and W Cameroon.

Descriptive notes. Head-body 103-107 cm (males); weight 145-191 kg (males) and 57-73 kg (females); standing height 138-180 cm (males) and 109-152 cm (females) The Western Gorilla displays extreme sexual dimorphism; males are about twice the size offemales. Ears are small and hidden in the hair. Teeth arerelatively small except for large incisors. With the exception ofthe face, ears, hands, and feet (which are naked and black), bodyis covered in coarse brown hair; brows are sparsely haired | Fully mature adult male Western Gorillas develop a series of secondary sexual characteristics: a pronouncedsagittal crest (a bonystructure on top of the cranium, to which massive jaw muscles areattached); large canine teeth; broad chest that becomes bare with age; broad shoulders; and hair on the saddlethat is replaced with short grayishwhite hairs, hence they are popularly referredto as “silverbacks.” Some male “Western Lowland Gorillas” (G. g. gorilla ) have a rich chestnut-color crown and sagittal crest, which contrasts with the rest of the body. “Cross River Gorillas” (G. g. diehlr) differ from Western Lowland Gorillas both in the shapeofthe skull and their genetics.

Habitat. Western Lowland Gorillas inhabit a variety of lowland forest types, including openand closed-canopy moist mature, seasonally inundated, and disturbed and secondary (regenerating) forest, usually below 500 m elevation. In northern Republic of the Congo, they occur at high density in vast swamps. Gorillas in the Ebo forest range up to 1200 m. Cross River Gorillas are confined to forested hill country in a mountainous landscapeat elevations of 200-1900 m, in patches of lowland, submontane, and montane forest. Their populationis highly fragmented and scattered across c.8000 km?, tenuously connected byforest corridors, much of which is under no for-mal protection. Cross River Gorillas persist in steep terrain that is difficult for hunters to reach.

Food and Feeding. Western Gorillas feed predominantly on ripe succulent fruit and terrestrial herbaceous vegetation, mainly pith of Aframomum (Zingiberaceae) and shoots of Marantaceae . Whenfruit is scarce, large quantities ofterrestrial herbaceous vegetation are consumed, supplemented by protein-rich mature leaves and bark of Milicia (Moraceae) trees. Some populations of Western Lowland Gorilla specialize on aquatic herbs. Cross River Gorillas endure a longer than average dry season, during which their consumption offruit drops to only 1-24%offecal weight. The only animal matter ingested deliberately is weaver ants (Qecophylla longinoda, Formicidae ) or termites ( Cubitermes , Termitidae ), which Western Lowland Gorillas eat every day when available.

Breeding. Male Western Gorillas take 18 years to reach full maturity, and females take c.10 years. Females usually transfer to another group when they reach maturity. Young males either becomesolitary or remain in their natal group iftolerated by the domi-nant male. The dominant male mates with all adult females in his group. In captivity, length of the menstrual cycle is ¢.32 days and mean length of gestation is 257 days. There is no birth season. Infant mortality up to three years ofage is 22-65%. Forthe first few years oflife, infants are highly dependent on their mothers’ care. Females carry their young ventrally in the early months; infants begin to travel dorsally within a few weeks. Infants suckle for 4-5 years, causing lactational amenorrhea in the mother. Offspring sharetheir mothers’ night nests until they are weanedorthe next sibling is born. Young Western Gorillas are weaned at 4-5 years ofage, from which point, they no longertravel on their mothers’ back. Interbirth intervals are 4-6 years. Western Gorillas appearto reproduce more slowly than Eastern Gorillas (G. beringer). Maximum life span is unknown but likely c¢.40 years.

Activity patterns. The Western Gorilla is diurnal and semi-terrestrial. They are more arboreal than generally presumed, and even adult males often climb to heights of 40 m in the canopy to feed. They are active for c.12 hours/day and spend most of that time foraging and resting. They build nests to sleep in every night, usually on the ground but sometimes in trees.

Movements, Home range and Social organization. Daily movement of the Western Gorilla averages 2 km (1-1-5-2 km) as they travel widely in search of ripe fruit. Annual home range sizes are 10-6-15-4 km?*, and home ranges of neighboring groups overlap extensively. Western Gorillas are not territorial. On the rare occasions that two groups meet, impressive posturing by adult malesis likely, but mutual avoidance is the usual response. Western Gorillas live in cohesive, relatively stable groups composed of an adult male (silverback), multiple females, their offspring, and immature relatives. Median group size is ten weaned individuals, and the maximum observed is 22 individuals. Multimale groups are rare.

Status and Conservation. CITES Appendix I. Classified as Critically Endangered on The IUCN Red List, along with both subspecies. Listed under Class A in the African Convention on the Conservation of Nature and Natural Resources. Gorillas are protected by national and international laws in all of their native countries, but law enforcement is generally weak. Western Lowland Gorillas total ¢.140,000-160,000 individuals, perhaps two-thirds of which are in the northern Republic of the Congo. Important populations also remain in Gabon and south-eastern Cameroon. The population in Equatorial Guinea was recently assessed at a few thousand individuals. It is unlikely that Western Gorillas are still resident in the Mayombe region of DR Congo, but transient individuals may cross into DR Congo from Cabinda (Angola), where a few hundred Western Lowland Gorillas are thought to persist. The number of Cross River gorillas totals only 200-300 individuals, fewer than the “Mountain Gorillas” (G. beringei beringei) in the Virunga Volcanoes region, but distributed over an area 17-6 times larger. Cross River Gorillas are found at 10-11 localities in rugged highlands, where they have so far found refuge from hunters. Major threats to the survival of the Western Gorilla are intense poaching for bushmeat, habitat loss, and disease (notably Ebola haemorrhagic fever). Poaching of great apes is usually linked to the commercial bushmeat trade, which supplies bushmeat to cities and towns. There is strong cultural attachment to bushmeat in Central Africa, and this trade has become one of the principal threats to wildlife throughout Central Africa. As wildlife populations have been diminished, hunters move progressively further into remote areas, creating what is now known as the “empty forest” syndrome. Studies of poaching and this illegal trade have estimated that nine tons of bushmeat are extracted from an area of 50,000 km? in the Congo Basin every day, which is clearly unsustainable. Mechanized logging in the Western Gorilla’s range is usually selective, with relatively low levels of offtake and disturbance, so gorillas can survive if hunting is controlled. More detrimentalis infrastructure, such as logging roads that penetrate previously inaccessible forests. Slash-and-burn clearing of forest for small-scale agriculture is a threat in heavily populated areas. Numbers of Western Lowland Gorillas in Gabon were halved by poaching and a deadly strain of Ebola hemorrhagic fever between 1980 and 2000. The Ebola epidemic then moved into the Republic of the Congo and killed another 5000 gorillas. Ebola is still considered to be a high risk to gorillas and Chimpanzees ( Pan troglodytes ) in this region. The major international conservation organizations have programs throughout Central Africa and are working toward the long-term survival of Western Gorillas.

Bibliography. Bourry et al. (2006), Cousins (1990), Dowsett-Lemaire & Dowsett (2001), Groves (1986, 1992, 2001), Oates (2011), Oates et al. (2007), Rainey et al. (2010), Robbins et al. (2004), Ruffler & Muray (2012), Sarmiento et al. (1996), Stokes et al. (2010), Tutin et al. (2005), Williamson & Butynski (2013b).

Figures

Plate 56 Species Accounts

Subspecies and Distribution. G.g.gorillaSavage,1847—SCameroon SWCentralAfricanRepublic, EquatorialGuinea, Gabon, RepublicoftheCongo, andNAngola (Cabinda). G. g. diehli Matschie, 1904 — SE Nigeria and W Cameroon.