The low roar thundering through the undergrowth grew closer. Much closer. It was first light, just after 5am, on our first hike of the day out from the Tambopata Research Center (TRC), a lodge deep in the Peruvian Amazon, near the Bolivian border.
Suddenly, Yuri, my guide, stopped and pointed into the dense canopy at the source of the intimidating rumble. "Don't move," he whispered urgently. But instead of some magnificent specimen of the Amazon's apex predator, the jaguar, Yuri was waving at a small, brownish lump of fur. Gazing nonchalantly down at us, the howler monkey began scratching one of the more intimate parts of his anatomy.
Despite their diminutive size, this species is reputed to be the loudest land animal on the planet. The roar of the male, produced via a large, hollow bone in the throat, is thought to mark territory between rival groups, and can travel up to three miles.
During my three days at the TRC, I managed to see all seven of the local primate species. The large spider monkeys made spectacular leaps between branches 90ft above the ground, while the tiny squirrel monkeys were the most playful. On more than one occasion, as our paths crossed in the sweltering jungle, they descended almost to head height to check us out.
One of the Amazon's most remote lodges, the TRC lies eight hours upriver from the sleepy regional capital of Puerto Maldonado. It normally requires an overnight trip to get here, although there are lodges much closer to town, including two run by the Center's owners, Rainforest Expeditions, where visitors stop over en route.
"Our 'why' is conservation. That is what drives the business. Tourism is a way to achieve that end," Kurt Holle, Rainforest Expeditions' general manager told me as he explained how the company was created in 1989 with the initial purpose of preventing loggers accessing what is now the Tambopata Nature Reserve.
Since then, the company has co-founded the Tambopata Macaw Project, dedicated to studying the region's spectacular birdlife in partnership with Texas A&M University. The lodge normally hosts four or five researchers at a time, mainly from the project but also including primatologists, conservation ecologists and other scientists from around the world. Their board and lodging is heavily subsidised by the tourism operation; visitors to the lodge have the chance to mingle with the experts and pepper them with questions.
It is easy to understand why tropical ornithologists would want to use the TRC as a base for their studies. Parrots love to eat clay, possibly for the salt it contains, or because it binds with toxic alkaloids in their diet. And the largest clay lick in the world, a red cliff stretching some 500 yards along the Tambopata River, is a short journey from the Center.
Here, with the Andean foothills jutting out of the clouds on the horizon, macaws, parrots and parakeets gather most mornings for an earthy breakfast amid a cacophony of guttural squawking and a rainbow palate of flashing blue, green, red, and yellow wings.
But the Peruvian Amazon, three times the size of the UK and home to numerous biodiversity records, is not just a twitchers' paradise. Along with its estimated 806 bird species, it also has a staggering 293 types of mammal, 2,500 butterfly species and more than 7,000 kinds of flowering plants.
Many species are hard to glimpse; visitors make their luck by spending long hours tramping through the sweltering rainforest. With humidity extremely high and long trousers recommended to minimise mosquito bites, the TRC is for those seriously motivated to see wildlife. Returning to the Center was always a treat.
A simple, airy wooden structure, the TRC has a thatched roof, built along indigenous principles, absorbing the din of the rainfall when the skies open spectacularly to dump on the jungle. Energy use is kept to a minimum; there is no hot water, not that you would want it anyway.
The rooms are all open on one side, allowing visitors to fully experience the pulsating crescendo that the insects, frogs, birds and other animals make during the night.
The nearest I came to a dicey moment was when we were surrounded by a dozen white-lipped peccaries, an Amazonian version of wild boar that, in numbers, have been known to kill jaguars. Clicking his tusks, their leader eyeballed us while the females and adolescents charged playfully at each other through the undergrowth. As so often with wild animals, quietly standing your ground, making it clear you present no threat while equally refusing to behave like prey, prevented the encounter turning dangerous.
Despite its size, the Peruvian Amazon is under pressure. Oil drilling, logging, gold mining and poaching are taking their toll. A new road connecting Brazilian raw materials with Peru's Pacific ports passes just 30 miles from the Tambopata Nature Reserve. With it come development opportunities for isolated rainforest communities but also the potential for widespread deforestation.
While I was staying at the Center, in a nearby part of the rainforest, a 14-year-old boy from a local community was shot with an arrow, apparently by a member of one of Peru's – and the world's – last uncontacted indigenous groups, no doubt bewildered and scared at the intrusion on his ancestral lands. The boy survived, which may be more than we will be able to say for his assailant's tribe.
Yet it is possible to enjoy this far-flung corner of the Amazon in a responsible and safe way. And few things are as satisfying as knowing that your tourism dollars are helping to save this fragile wilderness, especially as you sip an icy beer on the TRC patio while wild parrots scatter against the sunset.
There are no direct flights to Peru from the UK. Options include Iberia (0870 609 0500; iberia.com) and Air Europa (0871 423 0717; aireuropa.com) via Madrid, Air France (0871 663 3777; airfrance.co.uk) via Paris and KLM (0871 231 0000; klm.com) via Amsterdam. Flights from Lima to Puerto Maldonado are offered by LAN (0800 977 6100; lan.com).
A four-night trip to the Tambopata Research Center, including stopovers at another lodge, the Refugio Amazonas, costs US$905 (£566)pp, based on two sharing, with all-inclusive board, guides, transfers and the entrance fee to Tambopata Nature Reserve. Two nights at Rainforest Expeditions's other lodges (00 51 1 719 6422; perunature.com) starts at US$375 (£234).