The modern apes are all essentially vegetarian: gorillas are predominantly leaf-eaters; only the chimpanzees include a significant proportion of meat in their diet, sometimes hunting cooperatively for colobus monkeys or other medium-sized mammals and they will even share food. Their hunting behaviour, as well as their use of tools such as stones for cracking nuts and twigs for extracting termites, has attracted attention from field researchers for its relevance to the behaviour of early humans.

There is only one species of gorilla, divided into three subspecies: Gorilla gorilla gorilla (Western Lowland Gorilla); Gorilla gorilla graueri (Eastern Lowland Gorilla) and (Mountain gorilla). Until recently, the Eastern Lowland Gorilla was considered to be a form of Mountain Gorilla. Mountain gorillas live in two isolated populations, one in the Virunga volcanoes which sprawl across the borders of Rwanda, Zaire and Uganda in Central Africa, and the other in the Bwindi National ("Impenetrable") forest in Uganda. There is some speculation that this Bwindi population may be yet another subspecies.

Mountain gorillas are physically distinct from Lowland Gorillas; they are larger, have longer, thicker fur and a slightly different nose shape among other skeletal differences. They are the largest living primates, an adult male weighing up to 180 kilograms (400 pounds), with an arm span of about two metres (seven feet). Adult females weigh about half as much as males. When he reaches maturity, a male develops silvery grey hairs on his back and is called a "silverback" - in a group of gorillas, a silverback is usually the sole dominant member and living with him are several females, infants, juveniles and young adults.

Gorillas are diurnal, sleeping each night in a fresh nest built from leaves and branches. They are nomadic within their range and so, usually, end up in a new location each night. Normally they become active around dawn but if it is a cold overcast morning they may lie in for a while; although close to the equator, it can be very cold where the mountain gorillas live in the Virunga volcanoes at around 3,000 metres or more. On occasion they even venture into the sub-alpine zone in search of different foodplants such as the pith of the giant Senecio and where, at an altitude of over 3,500 metres there can be frost in the mornings.

The mountain gorillas' day is a routine of alternating periods of feeding and resting. They are almost exclusively vegetarian: bamboo, nettles and Gallium being some of their favourite foods. They will occasionally eat grubs, which they find in rotten wood, and even safari ants, scooping them up in huge handfuls to stuff into the mouth until the bites of the ants become overpowering and drive the gorillas away. It is the silverback leader who decides when the activities of the day begin and finish; when he moves, everyone moves; when he stops to rest, everyone stops. He is the emotional centre, the magnet of the group. His power not only derives from his size but the fact that he is the protector and everybody follows him. In a typical rest period, the silverback dozes surrounded by the rest of the group while the juveniles and infants play. Rest periods present good opportunities for social bonding, not only in play behaviour but also in grooming one another. The younger members of the group spend a good proportion of their time climbing, and swinging from branches, but adult gorillas are too heavy, only occasionally hauling their pear-shaped bodies up into a tree to reach an irresistible item of food spotted from the ground. On the ground, gorillas usually walk on all fours, supporting most of their weight on the feet and walking on the large front knuckles.

Being very social, communication is important manifesting itself in a variety of grunts, howls, hoots and barks. There are nearly twenty different vocalisations, each one with its own particular meaning. Gorillas also communicate by beating on their chests, or on the ground. For the silverback male, chestbeating is a show of power, designed to intimidate, but even the infants beat their chests as a kind of displacement activity during play, perhaps in mimic of their elders.

Mountain gorillas may live for thirty-five to forty years, reaching sexual maturity between the ages of eight and eleven. Full maturity for a male is a long haul for, although he begins to develop the "silver back" at the age of twelve or thirteen, he usually leaves his parental group at that time to wander alone, or in the company of other males, for a few years before managing to attract females from other groups to join him, thus forming his own family. It is a logical evolutionary process that sorts out the strong from the weak. When a female leaves her group to join another male, the new silverback will most likely kill an accompanying infant, a seemingly cruel and callous act to us but one which brings the female into oestrus for him to mate and thus ensure his own blood line. In the gorillas' social structure where the breeding in any one group is almost exclusively by a single silverback male, periodic movement of females between groups is essential to ensure genetic variety and to prevent inbreeding, a peril in small populations