A smooth, long whistle, the pitch rising throughout and ending shrilly, eclipsed all sounds of the jungle. As my guide, Gerardo, stopped to identify it, it sounded again. Suddenly Gerardo hissed, "Get down. Wild dogs!"

We threw ourselves into the muck on the river's edge. We waited, peering upstream with binoculars, scanning the boulders barricading the riot of foliage from the river. We were a three-day hike from the nearest footpath, four days from the nearest road. The Amazon had peeled back a tiny corner of its veil to me.

It was August 2006, and I had been exploring jungles in Peru and Bolivia for six months, finishing one trip and leaving the jungle long enough to travel, arrange a new trip, and buy supplies. I became progressively more experienced and bold, and it was paying off in rare sightings and experiences.

I arrived at Amboro National Park in Bolivia intending to walk to an observation station listed in my guidebook, a nine-day round-trip hike. In Buena Vista, a tiny town bordering the park, I asked for a local guide at the ranger station. The rangers called a guide, and I studied park maps in the tiny cement shack while I waited.

When Gerardo arrived, he told me that all tourists stay in the bungalows just outside the park; no one had ever stayed more than three nights. Sleeping in the park was prohibited, he said.

"But you can't see anything in a day hike," I protested. "You barely enter the jungle."

He nodded. He told me the observation station was long gone and that no trails penetrated the park anymore. Smirking, he suggested we follow a river upstream, as far as possible, into the heart of the park.

On the morning of our third day, we lazed on a pebbly beach, fishing for breakfast. Suddenly Gerardo yelled, "Look, an agouti."

A terrier-size, bulbous-eyed rodent with sepia-toned, white-spotted fur was crawling downstream, submerged in the knee-deep water. Gerardo splashed toward the creature, and it stopped abruptly. It did not surface or exhale bubbles. It stood motionless.

I approached with my camera while Gerardo fashioned a vine lasso. I was astonished that the animal did not flee. Gerardo gently slid the lasso around the neck of the agouti, still submerged, and walked it out of the river.

"It's being hunted. It's exhausted," he explained. Agoutis are nocturnal and terrestrial, but they take to the water when they're pursued to destroy their scent. This rodent's hunter was upstream and coming our way. I felt nervous with excitement. Could it be a jaguar, a puma, an anaconda?

Gerardo shrugged, uncertain. We crept upstream, observant and alert, till Gerardo spotted dog prints in the heavy mud. The prints were not from feral dogs, but a species native to the jungle, he said.

The piercing whistle, like the call of a massive bird, sent us diving for cover. Through binoculars, we watched four squat, shorthair, blunt-nosed dogs with pointed ears and stubby tails - all brown but with black heads - appear from behind boulders lining the river. They paced and swung their heads back and forth, scanning downstream.

Suddenly they bolted, blending into the jungle. Gerardo smiled broadly, astonished, and said he had not seen dogs in his last 10 years as a guide.

I felt the beauty and mystery of the Amazon and understood that to become rewarded with experience and knowledge, you must sacrifice, plunge in, and go beyond the perimeter.

Todd A. Levins, who grew up in Kennett Square, Chester County, lives in Durham, N.C.