Gorilla beringei, Matschie, 1903

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 : 849-850

4.

Eastern Gorilla

Gorilla beringei

French: Gorille de I'Est / German: Ostlicher Gorilla / Spanish: Gorila oriental

Other common names: Eastern Lowland/Grauer’s Gorilla (graueri), Mountain Gorilla (beringei)

Taxonomy. Gorilla beringei Matschie, 1903 ,

Rwanda, Mount Sabyinyo.

Gorillas of the Bwindi Impenetrable National Park in south-western Uganda are currently listed as the subspecies beringei. The mtDNA of this population does not differ from beringe: in the Virungas, but they look more like graueri. Further research is needed to clarify their taxonomy. Two subspecies are recognized.

Subspecies and Distribution.

G. b. beringe: Matschie, 1903 — VirungaVolcanoesRegioninEDRCongo, SWUganda, andNWRwanda, alsoinBwindiImpenetrableNationalParkinSWUganda.

G. b. graueri Matschie, 1914 — E DR Congo, inhabiting three major regions (Kahuzi-Biéga-Kasese, [tombwe Massif, and Maiko-Tayna).

Descriptive notes. Head-body 101-120 cm (males); weight 120-209 kg (males) and c.60-98 kg (females); standing height 159-196 cm (males) and 130-150 cm (females). The Eastern Gorilla displays extreme sexual dimorphism; males can be twice the size of females. It is somewhat larger than the Western Gorilla ( Gorilla gorilla ) and is therefore the largest of all primates. Eastern Gorillas, the “Mountain Gorilla ” (G. b. beringei) in particular, have shorter arms and are stockier than Western Gorillas. Head is large, and ears are small and hidden in the hair. Teeth are relatively small except for large incisors. Mountain Gorillas have large jaws and a wider facial skeleton than other Gorilla taxa. With the exception of face, ears, hands, and feet (which are naked and black), body is covered in coarse black hair. Hair of the Mountain Gorilla is thick and long, especially on the arms of adult males. Fully mature adult males develop a pronounced sagittal crest and have large canine teeth; broad chest and shoulders; a bare chest with age; and hair on the back that is replaced with short grayish-white hairs, spreading to the thighs later in life; hence, they are popularly referred to as “silverbacks.”

Habitat. “Grauer’s Gorillas” (G. b. graueri) occur at 600-2900 m above sea level in dense mature and secondary lowland, submontane, and montane forest and, where present, in bamboo forest ( Cyperus , Cyperaceae ), swamp, and peat bog. Mountain Gorillas are restricted to elevations above 1850 m in the Virunga Volcanoes (1400 m in Bwindi Impenetrable National Park) by human occupation of lower elevations. They range up to 3800 m. During the rainy season, they frequent bamboo forest. They also visit alpine and subalpine grassland on the volcanic peaks. “Eastern Lowland Gorilla ”is a misnomer because there is considerable overlap in the elevational ranges of Grauer’s and Mountain gorillas.

Food and Feeding. Diet of the Eastern Gorilla varies greatly with elevation and its effect on food availability. Grauer’s Gorillas have a more diverse and seasonal diet at lower elevations. Grauer’s Gorillas in lowland forest and Bwindi gorillas are more frugivorous: fruit forms ¢.25% of their total diet. Other dietary items include leaves,pith, bark, vines, herbs, and ants. Mountain Gorillas in the Virunga Volcanoes Region are largely herbivorous and feed on stems, pith, leaves, bark, and occasionally ants. Their favored food items are wild celery, thistles, nettles, bedstraw, wood, and roots. Both Eastern Gorilla subspecies feed almost exclusively on young bamboo shoots when they are in season.

Breeding. Male Eastern Gorillas become “blackbacks™ at 8-12 years of age and are capable of reproducing. They are considered silverbacks at twelve years of age and reach their full adult size at 15 years. Female menarche occurs at 6-7 years of age, followed by a period of adolescent sterility. Female Eastern Gorillas emigrate upon reaching sexual maturity to join another group or a solitary male. Average age of first parturition is 9-9 years. Females have a menstrual cycle of ¢.28 days and are proceptive for 1-4 days around the time of ovulation. In unimale groups, one male mates with all adult females. In multimale groups, females often mate with more than one male, but the dominant male is the partner in most copulations. Subordinate males sometimes copulate surreptitiously, but they are harassed by a dominant male if detected. There is no birth season. Gestation is ¢.255 days. For the first few years oflife, infants are highly dependent on their mothers’ care and sleep in her nest at night. Females carry their young ventrally in the early months; infants start to travel dorsally within a few weeks of birth. Females experience lactational amenorrhea while suckling infants. Young are weaned at 3-4 years of age, from which point, they no longer travel on their mothers’ back. Females give birth every 3—4 years and generally produce 3—4 surviving offspring during their reproductive life span. Maximum life span is unknown, but it is certainly over 40 years.

Activity patterns. The Eastern Gorilla is diurnal and semi-terrestrial. After waking, they feed intensively and then alternate rest, traveling, and feeding until bedding down for the night. Mountain Gorillas spend 55% of the day foraging, 34% resting, 7% travelling, and 4% in other activities. Eastern Gorillas build nests in trees, but the majority of their nests are on the ground.

Movements, Home range and Social organization. The distance traveled daily by Eastern Gorillas varies with elevation. Fruit availability decreases with increasing elevation, and with less fruit in the diet, they range shorter distances. Grauer’s and “Bwindi” gorillas occupy 16-28 km® annually. Groups of Mountain Gorilla in the Virunga Volcanoes use areas as small as 6 km” but as large as 34 km”. Eastern Gorillas are not territorial, and there is extensive overlap between the home ranges of different groups. Grauer’s gorillas move ¢.1500 m/day, whereas Mountain Gorillas usually travel less than 600 m/day. Eastern Gorilla groups are polygynous or polygynandrous, with one or more adult silverback males, several females, their offspring, and immature relatives forming the core of relatively stable groups. Median group size is ten weaned individuals; maximum observed group size is 65 individuals. About 40% of groups contain more than one adult male, some as many as nine. Infanticide of unweaned offspring sometimes occurs when an outsider male challenges a resident silverback. If the dominant male of a multimale group dies, another resident male is likely to take over leadership, assuring some continuity and preventing group disintegration and infanticide.

Status and Conservation. CITES Appendix I. Classified as Endangered on The IUCN Red List, with the Mountain Gorilla classified as Critically Endangered and Grauer’s Gorilla as Endangered. Listed under Class A in the African Convention on the Conservation of Nature and Natural Resources. Although Eastern Gorillas are protected by national and international laws, both subspecies are threatened by illegal activities. Grauer’s Gorilla habitat is estimated to cover an area of 100,000 km? east of the Lualaba River and the Burundi-Rwanda-Uganda border. Surveys in the 1990s estimated a total population of 8660-25,500 Grauer’s Gorillas in discrete areas of their expansive habitat. Their current status is poorly known because few surveys have been possible since the mid-1990s due to insecurity in eastern DR Congo. Local declines have been documented and the total population remaining is perhaps 2000-10,000 individuals. If more precise data were available, Grauer’s Gorilla would without doubt be reclassified as Critically Endangered. Mountain Gorillas are restricted to two refugia that have been cut off from forest at lower elevation by advancing agriculture. One population occupies 215 km? in the 330km* Bwindi Impenetrable National Park in south-western Uganda, and its numbers are stable. The other population occupies ¢.375 km? in the 455km* Virunga Volcanoes region and its numbers are increasing—nonetheless a total of only ¢.780 individuals remain in this small and vulnerable region. The main threats to Eastern Gorillas are poaching, habitat loss, and civil war. Capture of live orphans is a secondary threat (after the mother has been killed and eaten), except where infant Mountain Gorillas are the principle target, fulfilling the demands ofa fictitious, illegal international market. DR Congo has a legal framework for managing national parks and wildlife, but has difficulty applying its laws, and political will is limited. Underlying this is the difficult sociopolitical context: breakdown in law and order during years of conflict, combined with poverty and economic insecurity, which exacerbate difficulties of enforcing the law within the Grauer’s Gorilla’s range. Armed conflict and collapse of law and order in DR Congo have caused a significant rise in the illegal circulation of military weapons and ammunition. Formertraditional hunters now have access to guns, notably the AK47, and commercial trade in bushmeat has increased with the spread of firearms, often supplied by government soldiers and rebel militia. Illegal miners are decimating the main stronghold ofthe Grauer’s Gorilla . Several international conservation organizations have been working in difficult circumstances for decades to support national park authorities and try to secure the Eastern Gorillas’ survival.

Bibliography. Bradley et al. (2005), Cousins (1990), Czekala & Sicotte (2000), Gray et al. (2010), Groves (1986, 1992, 2001), Guschanski et al. (2009), Hall et al. (1998), IUCN & ICCN (2012), Nixon et al. (2012), Schaller (1963), Sleeman et al. (2000), Watts (1988), Williamson & Butynski (2013a), Williamson & Fawcett (2008).

Figures

Plate 56 Species Accounts

Subspecies and Distribution. G. b. beringe: Matschie, 1903 — VirungaVolcanoesRegioninEDRCongo, SWUganda, andNWRwanda, alsoinBwindiImpenetrableNationalParkinSWUganda. G. b. graueri Matschie, 1914 — E DR Congo, inhabiting three major regions (Kahuzi-Biéga-Kasese, [tombwe Massif, and Maiko-Tayna).