Just as we were about to give up and go back, we tried one more turning, and suddenly the forest seemed to be thick with gorillas. A few feet above us a female was lounging in a tree idly stripping the bark off a twig with her teeth. She noticed us but was not interested. Two babies were cavorting recklessly four metres from the ground in a very slender tree, and a young male was chugging through the undergrowth nearby on the lookout for food. We stared at the two babies in astounded fascination at the wonderful wild abandon with which they were hurling themselves round each other and the terrible meagreness of the tree in which they had elected to do it. It was hard to believe the tree could support them, and indeed it couldn't. They suddenly came crashing down through it, having completely misunderstood the law of gravity, and slunk off sheepishly into the undergrowth.

We followed, encountering one gorilla after another until at last we came across another silverback lying on his side beneath a bush, with his long arm folded up over his head scratching his opposite ear while he watched a couple of leaves doing not very much. It was instantly clear what he was doing. He was mooching. It was quite obvious. Or rather, the temptation to find it quite obvious was absolutely overwhelming.

They look like humans, they move like humans, they hold things in their fingers like humans, the expressions which play across their faces and in their intensely human-looking eyes are expressions which we instinctively feel we recognise as human expressions. We look them in the face and we think, `We know what they're like,' but we don't. Or rather we actually block off any possible glimmering of understanding of what they may be like by making easy and tempting assumptions.

I crept closer to the silverback, slowly and quietly on my hands and knees, till I was about eighteen inches away from him. He glanced round at me unconcernedly, as if I was just someone who had walked into the room, and continued his contemplations. I guessed that the animal was probably about the same height as me - almost two metres - but I would think about twice as heavy. Mostly muscle, with soft grey-black skin hanging quite loosely on his front, covered in coarse black hair.

As I moved again, he shifted himself away from me, just about six inches, as if I had sat slightly too close to him on a sofa and he was grumpily making a bit more room. Then he lay on his front with his chin on his fist, idly scratching his cheek with his other hand. I sat as quiet and still as I could, despite discovering that I was being bitten to death by ants. He looked from one to another of us without any great concern, and then his attention dropped to his own hands as he idly scratched some flecks of dirt off one of his fingers with his thumb. I had the impression that we were of as much interest to him as a boring Sunday afternoon in front of the television. He yawned.

It's so bloody hard not to anthropomorphise. But these impressions keep on crowding in on you because they spark so much instant recognition, however illusory that recognition may be. It's the only way of conveying what it was like.

After a quiet interval had passed I carefully pulled the pink writing paper out of my bag and started to make the notes that I'm writing from at the moment. This seemed to interest him a little more. I suppose he had simply never seen pink writing paper before. His eyes followed as my hand squiggled across the paper and after a while he reached out and touched first the paper and then the top of my biro - not to take it away from me, or even to interrupt me, just to see what it was and what it felt like. I felt very moved by this, and had a foolish impulse to show him my camera as well.

He retreated a little and lay down again about four feet from me, with his fist once more propped under his chin. I loved the extraordinary thoughtfulness of his expression, and the way his lips were bunched together by the upward pressure of his fist. The most disconcerting intelligence seemed to be apparent from the sudden sidelong glances he would give me, prompted not by any particular move I had made but apparently by a thought that had struck him.

I began to feel how patronising it was of us to presume to judge their intelligence, as if ours was any kind of standard by which to measure. I tried to imagine instead how he saw us, but of course that's almost impossible to do, because the assumptions you end up making as you try to bridge the imaginative gap are, of course, your own, and the most misleading assumptions are the ones you don't even know you're making. I pictured him lying there easily in his own world, tolerating my presence in it, but, I think, possibly sending me signals to which I did not know how to respond. And then I pictured myself beside him, festooned with the apparatus of my intelligence - my Gore-Tex cagoule, my pen and paper, my autofocus matrix-metering Nikon F4, and my inability to comprehend any of the life we had left behind us in the forest. But somewhere in the genetic history that we each carry with us in every cell of our body was a deep connection with this creature, as inaccessible to us now as last year's dreams, but, like last year's dreams, always invisibly and unfathomably present.

It put me in mind of what I think must be a vague memory of a movie, in which a New Yorker, the son of East European immigrants, goes to find the village that his family originally came from. He is rich and successful and expects to be greeted with excitement, admiration and wonder.

Instead, he is not exactly rejected, not exactly dismissed, but is welcomed in ways which he is unable to understand. He is disturbed by their lack of reaction to his presence until he realises that their stillness in the face of him is not rejection, but merely a peace that he is welcome to join but not to disturb. The gifts he has brought with him from civilisation turn to dust in his hands as he realises that everything he has is merely the shadow cast by what he has lost.

I watched the gorilla's eyes again, wise and knowing eyes, and wondered about this business of trying to teach apes language. Our language. Why? There are many members of our own species who live in and with the forest and know it and understand it. We don't listen to them. What is there to suggest we would listen to anything an ape could tell us? Or that it would be able to tell us of its life in a language that hasn't been born of that life? I thought, maybe it is not that they have yet to gain a language, it is that we have lost one.

The silverback seemed at last to tire of our presence. He hauled himself to his feet and lumbered easily off into another part of his home.

On the way back to the but I discovered that I had a small tin of tuna in my camera bag, so we greedily devoured this on our return, along with a bottle of beer, and that, at two o'clock in the afternoon, marked the end of fun for the day, unless you count listening to a couple of German, sorry, Latvian students explaining how good their penknives are as fun.

At this Mark started to get quietly ratty, which meant that he grasped the beer bottle very tightly between his hands and stared at it a lot. Kurt asked us what we were planning to do next and we said we were flying up to Garamba National Park to see if we could find any northern white rhinos. Kurt nodded and said that himself he thought he would probably walk to Uganda tonight.

Mark's knuckles grew whiter round his beer bottle. Mark, like most zoologists, tends to prefer animals, to people anyway, but in this case I was with him all the way. It occurred to me that we had spent a day rapt with wonder watching the mountain gorilla, and being particularly moved at how human they seemed, and finding this to be one of their most engaging and fascinating features. To find afterwards that a couple of hours spent with actual humans was merely irritating and a bit confusing.