Here’s what it’s like to follow behind the herpetologists as they work their way down the trail in the early evening. The sun set an hour ago and the air is starting to lose some of the afternoon’s heat, but your forehead and arms are still damp with a cool film of sweat. The full moon is rising through the trees ahead of you: a bright spot in the canopy that’s not strong enough yet to throw patches of moonlight on the forest floor. Everything in the understory is black, and everything in the midstory is black, and up in the canopy the leaves and branches are black against a night sky that is almost blue. In the upper strata of the forest legions of stridulating insects are making a scritch-scritching chorus; to the right a far-off frog croaks once and falls silent; from the left comes an anxious-sounding hooting; a bat flutters past almost noiselessly, raising a tiny breeze; and ahead on the trail comes the rustling sound of the herpetologists searching through dry leaf litter.

As they work, the beams of their headlamps go sweeping restlessly through the forest, back and forth, up and down, pausing a second and then moving on. Sometimes both of them look up at the same time, illuminating the forest from the ground to the canopy, and for a few seconds that stretch of forest stands in perfect cross section, backlit by their searchlights. Then their lights fall back down to a shrub, or go running up the trunk of a tree, or pause on a bromeliad, or return to the leaf litter, flitting this way and that to show you all the different places you might find a frog in this part of the world. Jonh’s light has a yellowish cast and is concentrated in a beam like the headlight of a train; Rudolf’s is whiter and more diffuse, like a bright lamp. Whenever you see the yellow light and the white light stop their sweeping, come together and focus on the same place, you know they’ve found something.

Nigel Pitman Herpetologists Jonh Mueses-Cisneros and Rudolf von May during a nighttime survey along the Rio Cotuhe.

It happens about every 10 minutes tonight. It has been especially dry in much of the Amazon the last few months, and in recent days the herpetologists have been muttering about how scarce amphibians are and how much they’d like to see some rain. For the last few days it has been “wanting to rain,” as they say in Spanish, but not much has fallen and tonight the sky is clear and thunder-free. So they go on working through the leaf litter with their snake hooks, turning over rotting logs, searching around the bases of buttressed trees, and wading off the trail now and then to investigate some intriguing eye shine.

Jonh Jairo Mueses-Cisneros is a herpetologist at Corpamazonia, a branch of Colombia’s Environment Ministry that focuses on the Amazon region. (His first name, which is going to give some copy editor a scare, is spelled correctly — that’s how the notary chose to write it on his birth certificate.) Rudolf von May is a Peruvian herpetologist who recently finished a doctorate on Peruvian frogs at Florida International University. Jonh was part of the team that set a world record in amphibian diversity at Leticia, a Colombian town just across the border from us, while Rudolf has published a flurry of recent papers on herpetofauna ecology and conservation. Both of them are under 40, and both are part of a new generation of South American biologists that promises to turn local universities and museums into the powerhouses of biodiversity science that they’ve always been destined to become.

In spite of the dry conditions, Rudolf and Jonh are doing pretty well. That world record from Leticia — which appears to have been superseded more recently by a site to the west of us, in Yasuní, Ecuador — was 97 amphibian species in three months of sampling. At our two campsites, in just eight days of sampling, Rudolf and Jonh have recorded 61 species. In the five-hour walk tonight they’ll add two more. And they’ve got six more days ahead of them.

Whatever their number turns out to be in the end, it seems clear that there are something like a hundred ways to be a frog in these forests (or a toad). You can be a tiny little animal that creeps high into the canopy looking for a bromeliad where you can lay your eggs, or you can be a warty giant who oozes bright yellow toxins when you’re feeling stressed, or you can be a meaty, nervous-looking fellow who sits all night outside your burrow keeping an eye out for local hunters who regard you as a convenient alternative to chicken, because you have no feathers to pluck. Maybe you spend all night perched on a leaf in the understory, doing your best to look invisible to bats. Or maybe you find a little depression on the forest floor where you can sit and pretend you’re a leaf and gobble up any unsuspecting mice that happen along. And if none of those sound attractive, you can choose one of the 95 other lifestyles.

Tonight the herpetologists end up recording 13 amphibians, three by song alone, as well as three reptiles: two geckos and a harmless, wiry little snake that for reasons of its own is dressed in the tan and brown patterns of a pit viper. The most entertaining moment of the night is when Jonh reaches into a small bromeliad on a fallen tree and plucks out three blue and yellow poison dart frogs, one after another, like clowns from a car.

It’s morning now as I type this up, and the herpetologists are almost done photographing their specimens. The sky is gloomy and it’s beginning to thunder, and if it starts raining really hard they’re going to head out to hunt for caecilians.