Fico puffed on a cigarette as the line of gringos moved gingerly across the ankle-deep pool. Someone shouted. A large green snake lay half-submerged on a raft of water hyacinth. Fico grinned, triumphant. "See. The snake is not going to come to you. You have to go to him."
He had a point. If you want to see wildlife in South America you have to get stuck right in. Forget the jungle - which is too dark and dense to see a thing - and head for the swamps that ring the equatorial heart of the continent. Do not expect African-style safaris with balloon flights and champagne breakfasts. Nature here has rough edges.
In Bolivia these humid wetlands are called pampa and they swallow the back flow from the rivers that storm down the eastern Andes and into the Amazon. Their gateway is the small gold-trading river port of Rurrenabaque which lies at a geographical junction of mountain, jungle and pampa.
Leaving the town and the last green line of Andes, we had travelled by motor canoe up muddy rivers past sparse herds of cattle. Once we saw an old Dakota plane pancaked in a paddock, its propellers bent back. "Narcotrafficos" (drug-dealers), whispered the guide and the word was passed.
As the cattle herds thinned, so did the clumps of dry land. The riverbanks were wooded and dry, but beyond them, away from the river, lay fields of flooded pools choking with sedge grasses and water lilies. Among the drifting clouds of bugs stood wading birds of every type. Herons waited, heads cocked, for a frog to plop by. Less patient were flocks of parakeets that hustled among the moriche palms looking for fruit. Wading in the swamp and swatting mosquitoes, I felt like just another link in the food chain.
That night we made camp under plastic sheets in trees by the river. Sunshine the next morning led to refreshing thoughts of swimming. So we set off for the deepest part of the river. No one, however, ventured far from the canoe. "Don't be scared of the alligators," Fico had said. "These caiman only eat the fish."
What sort of fish? "Piranhas."
After our short swim we discovered that piranhas are mostly harmless. Some species are even vegetarian, but there is no movie mileage in seeing a cauliflower stripped to the stalk in three seconds, so the carnivorous ones get more press.
Flesh-eating piranhas mostly eat each other, or more likely the vegetarian types. A human is only in danger if swimming in isolated pools where carnivorous types have got trapped and not quite finished eating each other. Or if you are wearing red swimming trunks, explained Fico. And he showed us how to catch piranhas by hand. You need quick reactions and a bag of smelly meat. We dropped in chunks of bait until the water boiled with orange and silver piranhas. Then some leapt clear of the surface, gnashing their gin-trap jaws. With a flat hand, Alberto, our other guide, deftly batted one into the canoe to frenzy about in the bilge water, sowing terror among those wearing sandals. Then, before the craft capsized, he flipped it back into the river.
Our river antics went on under the benign gaze of a family of cabybara on the bankside. These rodents are the size of large pigs and are the New World equivalent of the hippopotamus. They spend most of the day up to their necks in water chewing aquatic grass. Colonial settlers found them easy to hunt so capybara numbers dwindled until the church declared them a "fish" (because of their webbed hind feet) and only food for Fridays.
That evening we kept a vigil for fishing bats. They came swooping down to a still pool to snag fish on their harpoon-like claws, their catches glistening silver in the moonlight as they flitted back to the dark trees. Equally deft, by day, were the fishing kites balancing on the breeze with their forked tails.
Next day we pushed on up the river. I had hoped to see a giant river otter, but had to make do instead with a tree iguana and a large terrapin hiding in the shallows. The terrapin had two tiny eyes and smelt of my dentist's aquarium. Its crinkle-cut shell was perfect to keep itself hidden in the rotten leaves - which it would have done except for its silly habit of panicking and trying to get away.
The tree iguana, on the other hand, kept confidently still but the vivid yellow-green colour was hard to miss. It was the sort of lizard that used to appear enlarged in 1950s sci-fi B movies knocking police cars around. Now it was in scale, 3ft long, and sprawled on a branch over the river. It wouldn't be so smug, I thought, if it had just read about the aruana, or water-monkey fish, which "can leap 6ft into the air to snatch reptiles from overhanging vegetation".
Another dead loss at camouflage was the red stick insect I found clinging to a green leaf. "Why is it bright red if it was trying to look like a stick?" Because it was poisonous, explained Fico. If I rubbed it in my eyes I would go blind. I put it back on the bush and, washing my hands in the river, resolved never to wipe any insect on my face.
Spiders, of course, dominated our campfire conversations. Someone had seen a television documentary featuring an Amazonian arachnid so large "the jungle tribes make omelettes out of its eggs". Fico fiddled a large hairy spider from its hole in the ground with a long piece of grass. The spider reared up and bared its fangs - meanwhile distracting us from its more dangerous weapon, allergenic stomach fur - then scuttled back in the hole.
Morning brought us a three-toed sloth clinging in a high tree. Even through binoculars it looked wizened and flea-bitten, with the glazed eyes and the fixed smile of a game-show host. Exactly once a week, explained the guide, it would slowly climb down the tree to defecate on the ground. An added entry to my anti-sloth list: anally retentive.
After four days, tropical torpor started to take its toll. No one looked up when a squadron of enormous jabiru storks flew low overhead. Few cameras clicked for the log-load of turtles drying out like stacked dishes in the sun. Not a head turned for the raucous call of the hoatzin, an ancient breed of bird with claws on its wings.
What did wake us up, though, was the "huff, huff, huff" of two pale-pink river dolphins circling the boat to offer glimpses of their bulbous heads, angular fins and long thin snouts. It was strange to see such large marine mammals 4,000 miles (as the fish swims) from the sea. But they seemed quite at home.
Three of us slipped over the side to join them in the river. We were a bit nervous - not of caiman or piranhas, but of the 650-volt, 6ft electric eels which also swim these waters. No way could Alberto and Fico persuade us they were harmless.However, the river dolphins ignored us so we gratefully clambered back on board and motored down river. There, some old Ford pick-up trucks waited to drive us back over the rutted tracks to the relative civilisation of Rurrenabaque.
Six hours and a cold beer later we counted our mosquito bites and our blessings: four days of dry weather and enough close encounters with wildlife to use small writing on postcards for weeks to come.
Arrivals There are no direct flights from Britain to the Bolivian capital, La Paz. South American Experience (0171-976 5511) has a fare of pounds 614 on Aerolineas Argentinas via Buenos Aires. From La Paz, microbuses take around 24 hours to reach Rurrenabaque, along the so-called "Death Road". The alternative is to fly on the airline run by country's air force TAM, which has flights from La Paz for about pounds 30 one-way.
Getting organised Agencia Fluvial runs trips into the jungle. You can contact this company by sending a fax to the only machine in Rurrenabaque, on 00 591 832 2205. The number is frequently engaged.
How to navigate The American military Tactical Pilotage Chart N26D covers the area. It costs pounds 8.50 from Stanford's, 12-14 Long Acre, London WC2E 9LP (0171-836 1321).