Gorillas (genus Gorilla) are as genetically distant from chimpanzees (genus Pan) as they are from humans (genus Homo) and are currently classified as two species (Gorilla gorilla and Gorilla beringei, respectively) and five subspecies (Gorilla gorilla gorilla, Gorilla gorilla diehli, Gorilla beringei beringei, the Bwindi gorillas [no scientific name] and Gorilla beringei graueri). Gorillas are the largest and the most sexually dimorphic of all extant primate species. Adult male gorillas weigh on average 140 kg - 175 kg and have an average body length ranging from 170 cm - 175 cm. Adult females weigh on average 75 kg - 85 kg and have an average body length of 150 cm.
The color of the pelage varies across subspecies, from brownish-grey with reddish highlights in western gorillas to black in eastern gorillas. Each gorilla has a unique nose pattern, as with the human fingerprint. The Cross River gorillas (circa 250 individuals), the Bwindi gorillas (circa 300 individuals), and the Virunga gorillas (circa 360 individuals) are the rarest and most endangered subspecies. More numerous but less known are the eastern lowland gorillas, which make up 8–9% of the total gorilla population (circa 3,000 individuals) and the western lowland gorillas, which constitute the most common gorilla subspecies (circa 100,000 individuals).
Distribution and ecology
Gorillas occur in two widely separated regions in Central Africa—one in the west and one in the east—and they are found in a variety of forest habitats including primary lowland rainforest, secondary forest, swamp forest, marshy clearings (bays), and even montane forest.
Gorillas live primarily on the ground, spend 5–20% of the day in trees, and sleep mainly in ground nests. The average day range of western lowland gorillas (200–5500 m) is 2–3 times greater than that of eastern gorillas (500–1100 m). In addition, the annual home range size of western lowland gorillas (7–23 km) is at least double that reported for mountain gorillas. These home ranges frequently overlap with those of other groups. Traditionally, gorillas have been characterized as folivores. Recent research, however, has emphasized the importance of fruit consumption and dietary flexibility of gorillas at most study sites in lower altitudes.
Social structure and behaviour
The classic description of a Virunga mountain gorilla group is that of one silverback with several females and their offspring, all of which stay together while moving and resting. The social system is age-graded, “non-female-bonded”, with the strongest long-term relationships being formed between adult males and females; relationships between females are weak. Furthermore, the social system has been described by female transfer and male dispersal, although individuals of both sexes have been observed to stay in their natal groups.
Although comparable detailed data on social behavior and life history are still lacking for western gorillas, researchers suggest that lowland gorillas' social units are constrained by within-group feeding competition and are therefore less cohesive and interactions between social groups seem to occur more frequently.
For a more detailed overview and references see Pika, 2007.