He had to say it three times before I finally believed that I wasn't dreaming it - he said he just wanted to count the eggs in the nest of a saddlebilled stork at the top of the tree we were fast approaching.

He banked sharply over the top of the tree, and then appeared to put the hand brake on while he leant out of the window and counted the eggs. The cockpit was thick with the sound of 'Abide With Me' as the plane seemed slowly to start tumbling sideways out of the sky. He seemed to miscount twice before he was happy with the final tally, whereupon he hauled his head back through the window, turned to ask if we were doing all right, then turned back, refastened the window, and at last scooped the plane back up into the air moments before death.

From the air, the savannah looks like ostrich skin stretched across the land. We passed a small group of elephants nodding and bowing their way across the plains. Charles shouted over his shoulder at us that they have a project in Garamba National Park for training elephants, and have achieved the first major success in this field since Hannibal. African elephants are intelligent but notoriously difficult to train, and in the old Tarzan movies they used to use Indian elephants and stick bigger ears on them. The ultimate aim of this project is to use elephants on anti-poaching patrols, and also on tourist safaris. Once again, tourist revenue is seen as the one certain way of ensuring the future survival of the threatened wildlife of the area.

We wheeled around in ever increasing circles, looking out for anything resembling a rhinoceros. From up here they would clearly be much easier to distinguish from termite hills, if only for the sheer speed with which they move.

Suddenly there was one.

And there, as we passed a screen of trees, was another.

There, in fact, were another two: a mother and daughter, quite close to us moving rapidly across the plain like trotting boulders. Even seen from a couple of hundred feet in the air the sense of massive weight on the move is extraordinarily impressive. As we crossed the steady path the mother and daughter were keeping and wheeled round back over them, descending as we did so, it felt as if we were participating in a problem of three-body physics, swinging round in the gravitational pull of the rhinos.

We took another pass over them, lower and slower, directly following their path, coining as close to them as we could, and this time the sense was of taking part in military manoeuvres in which we were giving air cover to some monstrous cavalry hurtling across the plain.

Shouting above the noise in the cockpit we asked Charles if it didn't worry the rhinos having us flying so close to them.

`Not half as much as it worries you,' he said. `No, it doesn't bother them at all really. A rhino isn't scared of anything very much and is only really interested in what things smell like. We fly down low over them pretty regularly to get a good look at them, identify them, see what they're up to, check that they're healthy and so on. We know them all pretty well, and we'd know if they were upset about anything.'

I was struck again by something that was becoming a truism on these travels, that seeing animals such as these in a zoo was absolutely no preparation for seeing them in the wild - great beasts moving through seemingly limitless space, utterly the masters of their own world.

Or almost the masters. The next rhino we found, a mile or so further on, was engaged in a stand-off with a hyena. The hyena was circling warily round the rhino while the rhino peered at it myopically over its lowered horns. A rhino's eyesight is not particularly acute, and if it wants to get a good look at something it will tend to look at it first with one eye and then with the other - its eyes are on either side of its skull and it can't see straight ahead. Charles pointed out as we flew over that this rhino had had problems with hyenas before: half of its tail was missing.

By now I was beginning to feel seriously airsick and we started to head back The purpose of the trip was just to find out where the rhinos were, and out of a total wild population of twenty-two rhinos, we had seen altogether eight. Tomorrow we would set out overland to see if we could get close to one on ground level.

One of the things that people who don't know anything about white rhinoceroses find most interesting about them is their colour.

It isn't white.

Not even remotely. It's a rather handsome dark grey. Not even a sort of pale grey that might arguably pass as an off-white, just plain dark grey. People therefore assume that zoologists are either perverse or colour-blind, but it's not that, it's that they're illiterate. `White' is a mistranslation of the Afrikaans word 'weit' meaning `wide', and it refers to the animal's mouth, which is wider than that of the black rhino. By one of those lucky chances the white rhino is in fact a very slightly lighter shade of dark grey than the black rhino. If the white rhino had actually been darker than the black rhino people would just get cross, which would be a pity since there are many better things to get cross about regarding the white rhino than its colour, such as what happens to its horns.

There is a widespread myth about what people want rhino horns for - in fact two myths. The first myth is that ground rhino horn is an aphrodisiac. This, I think it's safe to say, is just what it appears to be - superstition. It has little to do with any known medical fact, and probably a lot to do with the fact that a rhino's horn is a big sticky-up hard thing.

The second myth is that anyone actually believes the first myth.

It was probably the invention of a journalist, or at best a misunderstanding. It's easy to see where the idea came from when you consider the variety of things that the Chinese, for example, believe to be aphrodisiacs, which include the brain of a monkey, the tongue of a sparrow, the human placenta, the penis of a white horse, rabbit hair from old brushes, and the dried sexual parts of a male tiger soaked in a bottle of European brandy for six months. A big sticky-up hard thing like a rhinoceros horn would seem to be a natural for such a list, though it's perhaps harder to understand, in this context, why grinding the thing down would be such an attractive idea. The fact is that there is no actual evidence to suggest that the Chinese do believe rhino horn to be an aphrodisiac. The only people who do believe it are people who've read somewhere that other people believe it, and are ready and willing to believe anything they hear that they like the sound of.

There is no known trade in rhino horn for the purposes of aphrodisia. (This, like most things, is no longer strictly true. It is now known that there are a couple of people in Northern India who use it, but they only do it to annoy.)

Much horn is used in traditional medicine in the Far East , but a major part of the trade in rhino horn is caused by something much more absurd, and it's this: fashion. Dagger handles made of rhinoceros horn are an extremely fashionable item of male jewellery in the Yemen . That's it: costume jewellery.

Let's see the effect of this fashion.

Northern white rhinos were unknown to the western world until their discovery in 1903. At the time, there were enormous numbers of them in five different countries: Chad , the Central African Republic , Sudan , Uganda and Zaire . But their discovery spelt disaster, because unfortunately for the northern white rhino it has two horns - which makes it doubly attractive to poachers. The front one, the longest, averages two feet in length; the world record-holder had an incredible horn six feet long and, sadly, was worth some US$5,000.