The feeling I had looking at my first silverback gorilla in the wild was vertiginous. It was as if there was something I was meant to do, some reaction that was expected of me, and I didn't know what it was or how to do it. My modern mind was simply saying, `Run away!' but all I could do was stand, trembling, and stare. The right moment for something seemed to slip away and fall into an unbridgeable gulf between us and the gorilla, and left us simply gawping helplessly on our side. The gorilla, meanwhile, seemed to notice that we had been busy photographing its dung and merely stalked off into the undergrowth.

We set off to follow it, but it was in its own element and we were not. We were not even able to tell whereabouts in its own element it was and after a while we gave up and started to explore the area more generally again.

The gorilla we had seen was a large male silverback. `Silverback' simply means that its back was silver, or grey-haired. Only the backs of males turn silver, and it happens after the male has reached maturity. Tradition has it that only the chief male of a group will develop a silver back, and that it will happen within days, or even hours, of it taking over as leader, but this apparently is nonsense. Popular and beguiling nonsense, but nonsense. And while we are on the subject of nonsense I should mention something that we discovered a few days later when talking to Conrad Aveling, a field researcher in Goma who has for years been responsible for gorilla conservation work in the area.

We told Conrad how alarmed we had been by Murara and Serundori's accounts of simply going out and mowing down the local poachers, and he sat back in his chair, kicked up his heels and roared with laughter.

`It's incredible what these guys will tell the tourists! I bet they told you they were ex-commandos as well, did they?

We admitted, rather sheepishly, that they had. Conrad clasped his hand to his brow and shook his head.

'The only thing about them that's ex-commando,' he said, 'is their uniforms. They buy them off the commandos. The commandos sell them to buy food because they hardly ever get paid. It's all complete nonsense. I heard another great story the other day. A tourist had asked a guide - and this was at Rawindi where there are no gorillas - the tourist asked, "What happens when a gorilla meets a lion?" And instead of answering, "Well, that's a silly question, because lions and gorillas live in completely different areas and would never ever meet," the guide obviously feels obliged to think of some sort of colourful answer. So he says, "What happens is that the gorilla beats the hell out of the lion, then wraps his body in leaves and twigs and then stamps on him." I only heard about it myself because the tourist came to me afterwards and said how fascinated he had been to hear about it. It bothers me when they make up these colourful answers. I wish I could make them understand that if they don't know the answer, or they think the right answer isn't very interesting, it's better to say so rather than just invent absolute nonsense.'

One thing that was beyond dispute, however, was that when our guides were not inventing stuff or acting out Rambo fantasies, they really knew the forest, and they really knew the gorillas. They had (and Conrad Aveling confirmed all this enthusiastically) themselves 'habituated' two of the gorilla groups for human contact. `Habituating' is a very long, complicated and delicate business, but, briefly, it is the process of contacting a group in the wild, and visiting them every day, if you can find them, over a period of months or even years and training them to accept the presence of human beings, so that they can then be studied and also visited by tourists.

The length of time it takes to habituate gorillas depends on the dominant silverback. He's the one whose confidence you have to win. In the case of the family group we were visiting it took fully three years. Conrad Aveling spent the first eight months of his time on the project crawling around in the undergrowth with them, but never actually saw them once, though he was often no more than twenty or thirty feet away.

`One of the problems of habituating in this sort of habitat,' he explained, 'is that it's so thick you can't see each other and what happens is you end up having these sudden confrontations at about three or four metres or less, and you still can't see each other. Everybody's jumping out of their skin. The gorilla's jumping out of his skin, I'm jumping out of mine. It's extremely exciting. You get a real adrenalin rush. A problem with the Bukavu group was that the silverback wouldn't charge. I actually wanted him to, because then, having charged, he would expose himself and then realise that I didn't mean him any harm. But he wouldn't do that, he just kept circling. Usually they do charge, and if they do you have this face-to-face, and you have a moment to understand that neither of you means the other any harm and the gorilla backs off.'

`But you go into a submissive posture, do you? asked Mark. `You don't confront him??

'No, I usually don't go into a submissive posture. I'm usually too frightened to move.'

Once a silverback accepts humans, the rest of the group will quickly fall into line, and, interestingly enough, any other groups in the area will usually become habituated much more quickly. There is hardly ever any trouble provided everybody treats everybody else with respect. The gorillas are perfectly capable of making it clear when they don't wish to be disturbed. There was one occasion on which a gorilla group had had a particularly stressful morning as a result of an encounter with another gorilla group, and the last thing they wanted was to be bothered with humans in the afternoon, so when a tracker brought some tourists and overstayed his welcome the silverback took hold of the tracker's hand and gently bit his watch off.

Now the business of tourism is obviously a vexed one. I had myself wanted to visit the gorillas for years, but had been deterred by the worry that tourism must be disturbing to the gorillas' habitat and way of life. There is also the risk of exposing the gorillas to diseases to which they have no immunity. It is well known that the famous and extraordinary pioneer of gorilla conservation, Dian Fossey, was for most of her life passionately opposed to tourism and wished to keep the world away from the gorillas. However, she did, reluctantly, change her mind towards the end of her life, and the prevalent view now is that tourism, if it's carefully controlled and monitored, is the one thing that can guarantee the gorillas' future survival. The sad but unavoidable fact is that it comes down to simple economics. Without tourists it's only a question of which will happen first - either the gorillas' forest habitat will be entirely destroyed for crop farming and firewood, or the gorillas will be hunted to extinction by poachers. Put at its crudest, the gorillas are now worth more to the locals (and the government) alive than dead.

The restrictions, which are tightly enforced, are these. Each gorilla family can only be visited once a day, usually for about an hour, by a party of a maximum of six people, each of whom are paying US$100 for the privilege. And maybe they won't even get to see the gorillas.

We were lucky; we did. Though after our first brief encounter with the silverback it looked, for a while, as if we would not find any more. We moved slowly and carefully through the undergrowth, while Murara and Serundori made regular coughing and grunting noises. The purpose of these was to let the gorillas know we were coming and reassure them that we meant no harm. The noises are imitations of a noise that gorillas themselves make. Apparently it doesn't actually matter much about trying to imitate them, though. It's hardly going to fool anyone. It just reassures the gorillas that you always make the same noise. You could sing the national anthem as far as they are concerned.