COMPOUNDING THE misery wrought by the rain was an overarching sense of isolation and uncertainty, a feeling that was magnified by strange noises that shattered the forest's silence and set the men's nerves on edge. That afternoon, as Roosevelt and the men in his dugout paddled quietly down the river, a long, deep shriek suddenly ripped through the jungle. It was the roar of a howler monkey, one of the loudest cries of any animal on earth. The sound, which can be heard from three miles away, is formed when the monkey forces air through its large, hollow hyoid bone, which sits between its lower jaw and voice box and anchors its tongue. The result is a deep, resonating howl that vibrates through the forest with strange, inhuman intensity, and echoes so pervasively that its location can be nearly impossible to identify.SOURCE: The River of Doubt: Theodore Roosevelt's Darkest Journey, by Candice Millard (Doubleday, 2005), pp. 156-158
Worse even than the noises they could recognize were those that none of them could explain. These strange sounds, which disappeared as quickly as they came and were a mystery even to those who knew the rain forest best, had made a strong impression on the British naturalist Henry Walter Bates fifty years earlier. "Often, even in the still hours of midday, a sudden crash will be heard resounding afar through the wilderness, as some great bough or entire tree falls to the ground," the naturalist wrote. "There are, besides, many sounds which it is impossible to account for. I found the natives generally as much at a loss in this respect as myself. Sometimes a sound is heard like the clang of an iron bar against a hard, hollow tree, or a piercing cry rends the air; these are not repeated, and the succeeding silence tends to heighten the unpleasant impression which they make on the mind."...
The Amazon's sudden, inexplicable sounds were especially terrifying at night, when they were all in the pitch-black forest, with no way to see a potential attacker and no sure means of escape. While the jungle in daylight could sometimes appear completely devoid of inhabitants, the nightly cacophony left no doubt that the men of the expedition were not alone. Even for veteran outdoorsmen like George Cherrie, the setting of the sun came to mark an unnerving threshold between the relative familiarity of a long day on the river, and sleepless nights in the jungle, spent trying to imagine the source of the spine-chilling noises that echoed in the darkness around him. "Frequently at night, with my camp at the edge of the jungle," he wrote, "I have lain in my hammock listening, my ears yearning for some familiar sound—every sense alert, nerves taut. Strange things have happened in the night."
The screams, crashes, clangs, and cries of the long Amazon night were all the more disturbing because they often provoked apparent terror among the unseen inhabitants of the jungle themselves. In the fathomless canyons of tree trunks and the shrouds of black vines that surrounded the men at night, the hum and chatter of thousands of nocturnal creatures would snap into instant silence in response to a strange noise, leaving the men to wait in breathless apprehension of what might come next.
"Let there be the least break in the harmony of sound," Cherrie observed, "and instantly there succeeds a deathlike silence, while all living things wait in dread for the inevitable shriek that follows the night prowler's stealthy spring."