THE ULTIMATE patriarch, he eases himself up from buttocks which have flattened their own forest glade and looks at me thoughtfully. On all fours he is the size of a small car. His hands are as big as bolsters, with fingers like black puddings and long, oddly elegant nails. He prods the infant playing between his feet, gives an instructive grunt, and together they join the family. The youngster's mother is nearby, but if she is lost to disease or poachers her child will be raised and protected - defended to the death, if necessary - by his colossal father: King Kong, the man-beast, greatest of the great apes, the beleaguered primate whose identity has evolved over the past century from homicidal monster to gentle giant.

There are tears in my eyes. I realise I am almost hyperventilating and steady my breathing. Nothing has prepared me - no work of prose or film-maker or veteran naturalist - for this encounter with mountain gorillas in the cloud forests of the Virunga Mountains. My first sight of the great ape, sitting statue-still on the forest floor, brings a crazy thought. Is he real? Or has some misguided functionary of the gorilla tourist industry placed a fibreglass model from the workshops of Special Effects on the slopes of Mount Gahinga? The silverback turns his head, with its Amy Winehouse beehive of glossy hair, and disbelief becomes awe. I am close enough to see a red glint in his eyes and feel the authority of his absolute power, which arrives with a silver spray of hair and fearsome canines when he reaches sexual maturity about the age of 12. He is strong enough to uproot trees. He has, literally, the strength of 10 men. But his name, Kwitonda, is the Kinyarwanda word for patience, and like all his kind he is really rather shy. At 19, Kwitonda is the youngest of the dominant silverbacks of the seven families who, by a gradual process of habituation, have been made accessible to visitors to Rwanda's Volcanoes National Park.

But not too accessible. Gorilla tourists must be active, if not reasonably fit, with the stamina for strenuous climbs in thin mountain air up the dense, tangled, often rain-soaked slopes of the Virungas. They must also be in good health, free of symptoms of infections like flu, measles or even colds; gorillas share our susceptibility to some diseases, but not our resistance. I meet the criteria, more or less, and join the day's restricted complement of international trekkers: seven widely dispersed groups of eight, each with an escort of guides, trackers, porters and Rwandan soldiers hefting AK-47s, "just in case we need to warn off buffalo or elephant".

The well-informed know better. Each small party of puny primates is assigned to one large family of muscular ones, and the annual revenue generated by our permits ($500 US a head, 56 tourists a day, 365 days a year) helps us save them from ourselves: from human encroachment, loss of habitat, poaching, the abduction of infant gorillas for unscrupulous collectors (going rate $80,000 US) or from merely becoming collateral damage in the crossfire of an unstable frontier region of sub-Saharan Africa. The military, with an army unit stationed in the national park, are there to protect both species of primate.

Straddling the borders of Rwanda, Uganda and the Democratic Republic of Congo (DRC), the Virunga massif is a chain of nine freestanding volcanoes, six dormant and three active, the highest 14,557ft. Their forests constitute one of only two habitats of the world's tiny population of Gorilla beringei beringei, the subspecies named for a German explorer, Robert von Beringe, who first described "a herd of big, black monkeys" when climbing Mount Sabyinyo in 1903. "We succeeded in killing two of these animals," he reported blithely, " a big, human-like male monkey of one and a half metres in height and a weight of more than 200 pounds."

Thus, just over 100 years ago, the mountain gorilla was dragged on to the European stage. The smaller lowland gorilla was already well known to adventurers in the Congo basin, where the western race was formally described in 1847. Its current population of 80,000 or so is more secure, although threatened by bush meat hunters and the Ebola virus, while the eastern race of lowland gorillas has suffered from the running conflicts and civil unrest in the eastern Congo. Their numbers are down to about 17,000.

But the mountain gorilla is among the planet's rarest creatures. In the Virungas and Uganda's Bwindi Impenetrable Forest, neighbourhoods which have witnessed much human savagery in recent years, the population is somewhere between 750 and 800. "The last census was taken in 2003, when the estimate for the two ranges was 700," says Francis Bayimgana, who has been a wildlife guide in Volcanoes National Park for eight years. "But there have been many births since then. Last year alone we had 20 new babies."

On the Rwandan and Ugandan sides of the Congolese border this is a conservation success story. Across the border, security for both apes and people is more problematic: 120 rangers in the DRC's Virunga National Park have been killed by poachers or insurgents in the past 10 years, and heroic efforts to save its gorilla population continue. There are some anxieties about gorilla tourism throughout the whole region, of which more later, but conservationists agree that if it weren't for tourists there would be no mountain gorillas. They would be gone, not just from the wild but from the world. Unlike the lowland subspecies, none has been successfully bred or survived long in captivity.

Around the middle of the last century the gorilla's ferocious image came under review. The raving, red-eyed beast of popular imagination and a wardrobe of Hollywood monkey suits was exposed as a tolerant, easy-going family man and committed vegetarian whose inner demons are released only when his troop is threatened. Even then, before launching an attack, he will allow challengers (usually other silverbacks looking to hijack his harem) to retreat before his displays of snarling, chest-thumping, branch-waving and general biting of the forest carpet. The mouthy primate is more jaw-jaw than war-war, although, as I discover, he does have his irritable moments.

On our pre-trek briefing Francis Bayimgana rehearses the protocols of gorilla tourism: no flash photography, no sneezing, no closer than seven metres (about 22ft), no longer than an hour in their company and, above all, no touching - which turns out to be easier said than done. He also reassures us. "The adults can look very intimidating, but we know their personalities, we know their behaviour and if they get upset for any reason they will give us warnings." So that's all right then, I think; and complete the permit registration form by naming, as requested, my next of kin.

Despite the fact that serious research into mountain gorilla behaviour began in the 1950s, it was again Hollywood which had the widest impact on revising the ape's reputation. I'm staying at Virunga Lodge, an hour's drive from the park headquarters, and its hilltop site must be among the most spectacular in Africa. Climbing to its highest point I reel before an immense, round-the-compass view that soars above Lake Bulera, the largest lake in the region, and takes in five volcanoes - Mutahara, Gahinga, Sabinyo, Karisimbi and Visoke.

Dian Fossey, the American primatologist, turned the mountain gorilla into a conservation icon with her best-selling book Gorillas In The Mist and its film adaptation.

Fossey arrived in the Virungas in 1967 and spent 18 years studying the gorillas. She did ground-breaking research, learning to identify individuals by their nose-markings and contributing enormously to scientific understanding of the subspecies and its place in a diminishing ecosystem. But she became obsessively attached to "her" gorillas, and her attitude to conservation was combative. When she organised anti-poaching patrols she alienated many of the locals, some of whose livelihood depended on a dismal trade in infant gorillas and sad souvenirs, including gorilla-hand ashtrays.

Her aggressive tactics almost certainly led to her unsolved murder in 1985. The gorilla population in Volcanoes national park reached an all-time low in the early 1970s, when only an estimated 250 remained, and the global influence of Gorillas In The Mist helped secure their future. If Fossey were alive today she would be astonished by their recovery, which has been achieved not just by her successors at the Karisoke Research Centre and other conservationists, but by a very different approach to their protection. As another famous ape woman, Jane Goodall, has said, "We can't possibly have any wildlife conservation unless we solve the people problems, too."

The Rwandan genocide is the elephant in the room for visitors to the beautiful little country, less than half the size of Scotland. The heart-wrenching Gisozi Genocide Memorial in Kigali testifies to Rwandese determination to confront their recent history, and the locals refer matter-of-factly to events before and after "the genocide". But do not ask who is Hutu and who is Tutsi; the question is crass. Under the sage and humane government of President Paul Kagame "the land of a thousand hills" has made huge progress since the appalling events of 1994, and gorilla tourism has played a major part in building an economy that gives the two ethnic groups a shared future.

It is now the leading industry, overtaking tea and coffee in the past year. Nine million people are squeezed between Rwanda's ranges of green hills, each of which, apart from the forest reserves, is cultivated to the summit in terraced fields. It was the pressure on land that mobilised conservationists in 1979, when plans were made to clear a large area of Volcanoes National Park for cattle grazing. Loss of forest remains the greatest risk to mountain gorillas, and to pre-empt this Amy Vedder and Bill Weber, researchers from the Karisoke Research Centre, designed and initiated the first gorilla tourism programme.

Forty years later, gorilla trekking brings over 20,000 visitors a year to the country and earns $3m US a year from permits alone. A further $20m is earned from tourist spending in Rwanda, Uganda and the Congo, and in Rwanda 5% of the revenue is devoted to community projects. The gorillas' rural neighbours on the edge of the Virunga forests are the most direct beneficiaries. Eric Nkurunziza, the young man who carries my rucksack up the slopes of Mount Gahinga for a modest $10, grows potatoes on his smallholding. But his income will grow faster than his crops if he graduates from porter to tracker to ranger to wildlife guide, the route taken by many employees of the national parks.

Live gorillas, not dead ones, now represent a career path for men like Nkurunziza. So much so that the birth of an ape to a family in Volcanoes is now celebrated with the same kind of naming ceremony as the Rwandese hold for their own new babies. In Rwanda, if not Congo, Fossey's gorilla wars have largely been won, and the professional poaching industry has been brought under control by improved surveillance and income-generating initiatives.

At Volcanoes the forest is completely surrounded by a high drystone dyke 70 miles in circumference. Its stones were dug and sold to the wildlife authority by villagers, who were also paid to build the wall; it represents a negotiated compromise that today allows smallholders to cultivate to the very edge of the forest but contains its foraging population of buffalo and elephant. The gorillas, of course, make light work of the wall, and sometimes climb over to sample eucalyptus trees grown for firewood; but they don't have an appetite for the local cash crops, mainly potatoes.

Thus every tourist expedition begins with "climbing the wall". Visits are made only in the morning, during the apes' "social time". In the absence of danger the gorilla day is an enviable one. They get up with the sun for some serious breakfasting - bamboo shoots and wild celery are favourite dishes - then spend most of the morning relaxing with the family. Mothers suckle their young, adults groom each other, infants and juveniles release their energy with inventive games, turning their seniors into bouncy castles and the forest vines into swings and climbing ropes.

We are gone by their siesta time, which lasts at least two hours. Then there's supper (a silverback needs about 30 kilos of vegetation a day to keep up his strength), followed by the evening ritual of nest-building. When dusk falls they retreat to the trees and their home-made beds of branches and leaves. Below them sleeps their guardian silverback.

The Kwitonda family are refugees from Congo. Unless they are spooked, gorillas rarely move more than a mile within their territory, which lets the rangers give them round-the-clock protection and facilitates the work of the trackers, who have located the group long before we arrive. After two hours of climbing (some treks take up to six) we are led, sweating and stumbling behind their energetic machetes, into the bosom of the 13-strong family. Gorillas galore, snacking, lolling, grooming, nursing, play-fighting. Bayimgana is keeping an eye on one of them, Karevuro, a nine-year-old blackback or adolescent male. This strapping teenager has developed a habit of pushing over visitors.

Balanced on muddy slopes, hemmed in by thickets, we find it's often impossible to maintain the regulation distance of seven metres if a gorilla chooses to approach. When Karevuro collides with three of our group, sending them backwards into the undergrowth, Bayimgana tut-tuts disapprovingly. "He is getting too old for this behaviour. It unsettles the group. In another three years or so he will be a silverback, and must learn to behave responsibly."

To my shame I am not too bothered about Karevuro's emotional development but I am envious of the Canadians. How glorious to be manhandled, ever so slightly, by a gorilla! Twenty minutes later I have been similarly collapsed on to a mattress of undergrowth by the wayward teenager. All I remember is a sensation of strength as he barges past me with a swift, sideways shove. The pressure from his huge hand on my breastbone is painless but irresistible, and the whole episode is over before I have time to feel alarm.

There is a serious side to such close encounters, which not only unsettle the group but increase the risk of disease transmission. Dr Liz Williamson, who was director of the Karisoke Research Centre until 2001, has made an analysis of the costs and benefits of gorilla tourism. She concluded that so far it "has not been deleterious to the gorillas' overall health, behaviour and ecology, and any negative impacts seem to have been outweighed by the improved monitoring and protection". But she also warned that the small population in the Virungas could be devastated by an infectious disease, and urged international visitors to respect the regulations that have been put in place.

Most people do; but the gorillas don't know the rules, and Karevuro's mischief-making shows how habituation can turn into over-familiarity, making both people and gorillas more vulnerable to each other. Non-habituated gorillas are shy and elusive, but last summer 10 apes from a habituated family were easily and senselessly shot in Congo's Virunga NP; possibly to frighten the rangers and make a statement about forest rights in a turf war over charcoal-burning, which is illegal. And although no gorilla attack on tourists has ever been reported, there are signs that occasionally the apes do tire of all the attention. Two weeks before my visit a silverback picked up one over-intrusive tourist by his shirt, carried him away from the group and dropped him into a shallow ravine. The man was only bruised, but the incident had the makings of the kind of tragedy that could undermine all the benefits of gorilla tourism.

I have two permits, and am out again next morning to meet the Amahoro family on the slopes of Mount Visoke. Here the silverback, Ubumwe, greets us with a heart-stopping display of acrobatic power. He climbs into a slender tree and jumps and swings on its branches until he and the tree crash to the ground. Then he starts stripping and eating the foliage; this foraging exercise, which begins as soon as we arrive, is surely also designed to impress.

Nearby is a nursing mother, Mbele. Her infant finishes his meal and starts twirling on a creeper, commanding our attention with a stare that says: "Look at me!" He is nine months old, cuddly as a soft toy, with a punk's coxcomb of hair. Mbele and I exchange looks; there is such intelligence in her tender brown gaze that I am tempted to say something soppy, mother to mother. We have in common 98% of our genes and an ancestral tree. Does she recognise in me something of herself?

She heads towards us, and the woman at my side gasps. But, as Mbele gently brushes past us to supervise her baby's play, the guide says: "She is kind, Mbele. Not like that bad boy Karevuro."

I never did find out the meaning of Karevuro's name. It was probably Hoodie. Perhaps all he needed was a hug.