The chilling screams might have been human, if it hadn't been for the hooting too. "Hurry," muttered ranger Ronald Tindyebwa, lifting his rifle. "Something is wrong. I heard them like this once before, when a chimp fell out of a high tree and died."
With nervous excitement we ran towards the ruckus, slipping down a greasy hill to the edge of a swamp, just 50 metres from the chimps. They were huddled in two trees, shrieking at something terrifying on the floor. We couldn't see what the danger was because of a bush the size of a small house.
"Julia! You cannot rush into a swamp!" hissed Ronald, berating his colleague, the British primatologist Julia Lloyd, as she leapt forward. Instead, the guards led the way, guns raised, expecting something - an elephant, a buffalo, a lion - to burst from the foliage.
Then Ronald started flapping his gun about in excitement as we saw the head of the enormous rock python emerge. The beast slithered slowly past, swamp water glistening on its back in the midday sun, turning briefly to acknowledge us. I counted six metres of snake. We had inadvertently wandered into the middle of a chimp hunt - the sort of Attenborough documentary-style moment for which people cross the globe. It seemed we might have to watch the python feast on one of the apes.
The alpha male of the group, Mobutu, had unimpressively done a runner, and we later found his deputy, Flop, clinging to the top of a sapling, petrified. In their absence, a few of the troop's foolhardy teenage miscreants decided to prove their macho credentials. A tense stand-off developed: the python sizing them up while one of the biggest adolescent males, Bwana, hurled thick branches at it.
He and a pal, Matu, leapt down and pounded the floor. They ran after the snake, screeched, sniffed the ground and touched its tail to intimidate it. Their teamwork succeeded: the hungry python slid into the forest. They chased it across a small river, and just for good measure disturbed a cluster of huge fruit bats hanging in a tree, adding to the chaos.
Then the sky opened and dropped a day's rain on our heads. I looked miserably at my notebook, now a cake of mushy paper, then found some shelter to eat my lunch of boiled eggs and chicken wings, which had spent the morning warming in the jungle heat. When the deluge stopped, the chimps moved on.
Pursuing chimpanzees through the tall, tangled scrub that carpets western Uganda's rainforest demands a machete, heavy boots, thick trousers, a very silly hat to keep the thorns out of your head, patience, and a sense of humour. Such dramatic encounters are exceptional, but even on a slow day, tracking chimpanzees is thrilling.
We had set off across the forest floor at 5.30am in the pitch black. Half an hour later, we'd reached the treetop nests that the 85-strong Kanyantale chimpanzee community had built the previous night. We spent the next 14 hours watching them feed, play, laugh, hug, kiss and fornicate. The boys fought, slapped hollow buttress roots and chased each other, jostling for future primacy in the hierarchy.
We found them by following a trail of knuckle prints, dung and half-digested fruit - not forgetting the cacophony - and observed them from a distance, avoiding loud noise, sudden movement, standing, shaking branches and worst of all, staring. However tempting to gaze into the eyes of a creature that shares 98.4 per cent of our DNA, chimps consider it the height of bad manners.
I learned how to identify each by its scars, face, missing digits and behavioural traits. The chimps were complex, tender and charismatic characters, they deserve their comparison with humans. But they can be also extremely aggressive and utterly different from the cuddly infant apes we see on television - as the battle with the python proved. Wild chimpanzees are big, suspicious, clever and potentially dangerous. An adult male is four times stronger than a grown man.
Habituation means getting the apes to accept humans as a neutral presence. It takes years of breaking down wild chimpanzees' natural fear, otherwise they vanish at the sight of man. When tracking began in Uganda in 1991, the likelihood of tourists seeing even "black blobs in trees" was as low as 20 per cent. Successful habituation has now pushed that above 90 per cent.
However, scientists worry that the chimps, like gorillas, are being over-habituated. Some of the adolescents who grew up with human contact don't share their elders' fear of man. Guides allow photo-hungry tourists to go too close. As chimps lose that trepidation, they become aware of their strength in relation to us, and can threaten guides, tourists and locals: charging, knocking, hitting and biting. There are isolated reports of habituated groups chasing people from the forest, and of rogue individuals killing village children.
We were thrown menacing looks by the troop's third-in-command, Kaguta, known to rangers as The Terrorist, and his teenage apprentice. When three other chimps tired of our presence towards the end of the day they bombarded us with figs.
Ape tourism is vital for both the Ugandan people (among the poorest in the world) and the apes. The country's population is predicted to grow from 28 million to 128 million by 2050, and encroachment by humans on forest habitat and transmission of contagious diseases are the two biggest threats to chimps. They face extinction unless they are an economic asset to locals: tourists' money at Kibale, for example, funded the construction of a nearby primary school. But at what cost should that tourism come?
It was a privilege to see these close cousins in their natural habitat. As the most intelligent ape, perhaps man can figure out a way to keep them there.
THE COMPACT GUIDE
HOW TO GET THERE:
Discovery Initiatives (01285 643333; discoveryinitiatives.co.uk) offers 12-day Chimpanzee Insight Tours, departing 1 June and 14 September from £3,595 per person, based on two sharing. The package includes return flights, accommodation and meals, one day tracking gorillas, four with chimpanzees, game drives and a boat trip.
Uganda Wildlife Authority (00 256 41 355 000, uwa.org.ug).
A one-hour taxi ride from the airport at Entebbe. The Grand Imperial (00 256 41 311048; imperialhotels.co.ug) offers rooms from US$212 (£118) and has pleasant bars and a spa. Hotel Africana (00 256 41 348080; hotelafricana.com) has a decent pool with rooms from $139 (£77). Buy ape-tracking permits at the Uganda Wildlife Authority (00 256 41 346287; uwa.or.ug). Visit the chimpanzee sanctuary on Ngamba Island. To book a trip contact Wild Frontiers
(00 256 41 321479; wild frontiers.co.ug). And make some time to meet the locals.
2. Fort Portal
Meet your driver after breakfast. Make sure you've packed maps and binoculars for the seven-hour journey to Kibale Forest, near western Uganda's highlands. The luxury choice of accommodation there is Ndali Lodge (00 256 772 221309; ndalilodge.com) near Fort Portal, perched on the edge of a volcano crater. Or there's the budget camp of Kanyanchu Bandas (00 256 41 355000; uwa.or.ug), which has pit latrines. It is alive with noisy
primates and visited by elephants and leopards.
3. Kibale Forest
Teeming with birds, snakes, monkeys (vervet, blue and red-tailed), baboons, red, black-and-white colobus, and 700 chimpanzees, Kibale is also home to lions, leopards, buffalos, warthogs and elephants. With the all-day Chimpanzee Habituation Experience, you are on the forest floor by 5.30am to watch the chimps wake and search for their breakfast. Then you follow them - observing the full gamut of their behaviour - until they build new nests for themselves at around 7.30pm.
4. Research project in Kibale Forest
British primatologist Julia Lloyd works with ranger Ronald Tindyebwa and head guide and former poacher Silver Kyamukama to research the impact of human habituation on the chimps. They collect urine to measure levels of the stress hormone cortisol, either by taking droplets from leaves, or by standing beneath a urinating chimp with an open plastic sample bag to catch it in full flow. Another reason to wear a wide-brimmed hat.
5. Queen Elizabeth National Park
The big game in Uganda is not so plentiful as on the Serengeti or Masai Mara, but this national park offers buffalo, elephant, antelope, hyena, mongoose, giant forest hog, the full range of Ugandan primates and more than 500 species of bird. Not forgetting the park's famous and rare speciality - tree-climbing lions (below) - or the schoolchildren who nonchalantly skip through the tall grass 200 metres further along the road.
6. Jacana Lodge
Jacana Lodge (00 256 41 258273) is a cluster of luxury timber eco-cottages on the shore of Lake Nyamusingiri, with spectacular views from the restaurant. There's a swimming pool for morning dips but no power after 10pm. Monkeys play overhead. Check out the boisterous baboon who sits in the driving seat of a yellow digger, admiring his reflection in the wing mirror. And take a look in the cave, home to rock pythons and huge numbers of Egyptian fruit bats.
7. Kazinga Channel
The Kazinga Channel, between Lakes George and Edward, is a stop-off for lunch, or a second day in the park in itself. A two-hour boat trip along the channel costs $20 (£10) through Mweya Lodge (00 256 41 222646; mweyalodge.com) brings you face to face with hippos. Herds of elephants drink from the water's edge. The waterfowl impress ornithologists. Nearby, the deep Chambura River Gorge offers habituated chimpanzees and prides of lions.
8. The Rwenzoris
From Jacana Lodge, it's a four-hour drive along terrible roads, through thousands of acres of vertically-inclined coffee and banana plantations (main picture), to reach the rainforests of the Rwenzoris - the Mountains of the Moon - which are the home of the mountain gorilla, as well as of 350 bird species, 324 tree species, and 119 other mammals. It is worth making a short stop mid-journey to lie on the muddy track and photograph up-close some of the millions of canary and white butterflies that carpet the route.
Accommodation is in the comfortable Bwindi Eco Lodge, bookable through Volcanoes Safaris (0870-870 8480; volcanoessafaris.com/camps_bwindi.html), at the foot of the daunting mountain that must be scaled to reach one of three habituated gorilla groups, which do occasionally turn up in base camp. Local trackers leave before dawn to locate the apes. Meanwhile, rangers tell tourists the safety procedures (if charged by a gorilla, sit down, don't run) and check equipment: strong boots, waterproofs, water and a walking stick.
10. The Impenetrable Forest
Armed soldiers accompany tourists up the mountain to The Impenetrable Forest - stretcher-bearing villagers carry the infirm and lazy. The trek takes up to eight hours, through muddy, scratchy, sweaty jungle valleys. Then, suddenly, flattened bushes, some droppings, a pungent body odour. A branch is torn away, and you are metres from a silverback. The apes take no notice of the supposed minimum distance of eight metres. After an hour it's time to return, dazed, to base camp.