The first spatters of warm tropical rain fall on our party of five as we cross the marsh that separates the beachside resorts of Drake Bay from the nearby hamlet of Agujitas. We are embarking on the famous Night Tour, a two-hour walk through the coastal wetlands and primary rainforest of the Osa Peninsula, led by a local naturalist who will point out numerous critters as we go.

Rolando, our guide, has told us to dress for rain and provided everyone with waterproof flashlights. I swing my light back and forth lazily, hopping between the cement-filled tires that form a path through the marsh. Rolando's flashlight motions are somewhat more erratic; his beam darts in all directions, stopping occasionally for a split second to linger on an overhead branch here, a patch of marsh grass there. His light doesn't spend any time pointed forward; he's walking the trail from memory as he rapidly scans the forest for interesting fauna.

Halfway across the marsh, he motions for our single-file line to stop. "Listen," he says. "Do you hear that sound? It is the mating call of a tree frog."

A muted clicking buzz with an almost machinelike quality emanates from the marshland off to our right. Rolando flicks off his flashlight for a second, orients on the sound, and opens his mouth: "Kik-kik-kik-kik-kik" he repeats several times, making a passable imitation of the sound. He is answered by a chorus of several voices and his light stabs out to find their source.

In the center of the beam is a pale grey amphibian half immersed in a pool of water, barely visible against the sandy soil of the marsh. Its vocal sac is fully inflated, a diaphanous sphere that vibrates in time with the buzzing sound. "The males use this noise to attract the attention of females," Rolando explains. "They come to pools of water in the marsh to copulate with the female. Once they have copulated, the females lay their eggs in a foaming nest of mucus, which floats on the surface of the water. The purpose of this nest is to conceal the eggs from predators, and also to nourish the tadpoles when they are born."

Rolando's dry style of narration requires some adjustment, but this is our second tour with him and I'm beginning to appreciate the depth of his knowledge and his thorough, if slightly clinical, command of English. After answering a few questions about the foaming-nest tree frog, he guides us toward the edge of the marsh and onto the dirt road that runs through Agujitas.

The rain is falling in a steady, annoying sprinkle now; I pause occasionally to wipe my eyeglasses on my shirttail. Rolando is tapping on an iPhone with one hand while he scans the treetops bordering the road, looking for iguanas. "Next we'll go to my friend's back yard," he explains; "I want to make sure I have his permission first because sometimes there's a guard dog. This yard is the best place for red-eyed tree frogs, so we don't want to miss it!"

An iguana may have been spotted a few trees back from the road; half of our party detours to get a closer look at it. Anthony and I walk briskly onward, more interested in reaching the presumed shelter of the friend's house than in stalking lizards. After a few minutes we reach a break in the shrubbery lining the road. Rolando leads us through the shrubs and into his friend's front yard; I'm glad to see a covered verandah attached to the house.

We huddle under the verandah, joining a superannuated black labrador that's curled into a ball, asleep, in a dog-sized rut in the ground. Rolando raises his voice and slips back into lecture mode, regaling us with frog factoids: The red-eyed tree frog can be found throughout America, mostly in the equatorial and southern regions. The frog's padded feet let it stick to smooth surfaces, even upside down. Its beautiful colors may be an adaptation for the purpose of startling predators. It is an icon of rainforest preservation.

The rain has steadily increased to a full downpour, pounding against the corrugated tin roof of the verandah loud enough that Rolando is shouting his frog facts. The dog's nose twitches as she finally catches our scent. She opens an eye halfway and laboriously rises, joints stiff with arthritis, hobbling over to Rolando for a friendly snuffle. It occurs to me that she's probably deaf, or at least deafened by the rain. Some guard dog!

Rolando beckons us out into the night to see the frogs. Anthony and I make eye contact with Barbara and Olivier, the Belgian couple we met at the hotel who comprise the other half of our tour group. We all shrug and grin, confirming that, yes, we're all willing to get drenched to see the frogs.

We hurry around the side of the house heading for the back yard, but Rolando abruptly stops us halfway. He grabs a twig and begins poking forcefully at an earthen embankment that parallels the house. He clambers several feet up and down the hillside, then side to side, to poke at the dirt in different areas. Occasionally he stops to lift a patch of moss or thump his hand against the earth. After thirty seconds of this startling behavior, he turns to us, nods, and says "Trapdoor spiders."

Some of us look confused; he elaborates. "Trapdoor spiders make a house by digging a hole in the dirt, several inches deep. They create a door from moss and attach a piece of web to their door. They can feel the vibration when anything walks on top of the door, and they open the door to pull their prey into the house. This is how they eat."

Rolando pokes around for another thirty seconds; failing to spring any trap doors, he continues around the side of the house and into the back yard, where we find … a large white bathtub embedded in the lawn! The tub is almost overflowing with murky rainwater, but is otherwise empty except for a huge bullfrog swimming around its perimeter.

"Red-eyed frogs like fresh water sources," Rolando explains to us. "They lay their eggs in puddles like other tree frogs, but the eggs have no coating; other frogs can eat the eggs. By giving them a protected water source, my friend has created the best habitat in the area for these frogs."

The bullfrog in the tub has grown tired of bathing and is hopping at the sides, trying to get out. Its weight and relatively puny legs give it too little purchase to clear the lip of the bathtub. It occurs to me that the tub doubles as a trap for frogs that would otherwise eat all of the eggs; the delicate tree frogs with their sticky feet can get in and out, but larger frogs must swim until they drown.

Rolando begins combing the undersides of large leaves and eventually comes up with a stunning specimen: a tiny frog about an inch long with huge, flame-red eyes, acid green skin, and spindly toes. It's skitterish at first, hopping out of Rolando's grasp several times and ending up on Barbara's shirt once; eventually it stops struggling and stares at us. The frog's eye structure - black pupils, circular eyes, big eyelids - give it an almost human stare, and I begin to anthropomorphize. It cocks its head inquisitively; it sticks out its tongue hungrily; it scurries around on Rolando's palm nervously. After a minute, the frog is returned to the topside of its leaf and Rolando grabs everyone's cameras to take close-up shots for us.

At this point my t-shirt is sticking to my chest; water is streaming down my pant legs; the first seeps of moisture are wicking into my socks, coming in through the top of my Gore-Tex lined shoes. Rolando hurries us out of the back yard and we begin a brisk walk up a hillside, heading back toward the town center and its handful of street lights. Halfway there, we make a sudden right turn and begin trudging up a muddy game trail that is beginning to swell with runoff. Water sluices around our feet and splashes our ankles. The Gore-Tex begins to work against me, turning my shoes into watertight buckets.

We reach the top of the hill and find ourselves parallel to a riverbed. Rolando motions for us to wait and zig-zags down to the riverbank, his flashlight swinging wildly. His light recedes down the river perhaps 50 feet; he shouts something unintelligible. Olivier, at the front of the line, gets the gist. "HE SAYS WE SHOULD COME DOWN!" he shouts over the din of the rain and the river.

"WHERE?" I say. "ALONG THE RIVERBANK?"

"NO; INTO THE RIVER!" I realize I'm already so thoroughly soaked that wading through ankle-deep water isn't going to make a difference. We all zig-zag down and begin splashing through the riverbed.

(From this point onward, assume that all of our speech is shouted because there is no other way to communicate given the ambient noise level.)

As we slosh up to Rolando, we find him once again combing the undersides of high-hanging leaves over the riverbank. Suddenly he grins and cups his hands together. "Got one! Does anyone have something made of glass, maybe eye glasses?" Anthony and I are both conspicuously wearing our glasses; Anthony's are larger, so he hands them over. Rolando diverts some water into the concave side of one lens and sets something tiny and glistening onto the glass. He shines his light through the lens from below.

Resting on the lens is a tiny frog no larger than the first two joints of my pinky finger. Its skin is translucent milky-white, almost transparent, and we can see internal organs working in its core. "This is a glass frog," Rolando says. "It only lives in high-moisture environments, in the shade."

He points out the frog's heart, a tiny dark red structure that is throbbing incredibly fast as the frog attempts to cope with the stress of being in a strange new environment. Finally, he gingerly transports the glass frog back the leaf where he found it. Anthony retrieves his eyeglasses, which are so steamy and streaked with water that they've become useless. I remove my own glasses and squint through the rain, trying to see where I'm going.

Rolando sprints up the riverbank, calling out behind him: "Come quickly, we do not have much time." We squidge along behind him, heading upstream.

Shortly we arrive at a large obstruction in the riverbed: an enormous mound of mud and dirt, perhaps 20 feet high, has slid down the hillside and blocked the river. Erosion has already begun to cut a channel through the mound, and someone has slung a makeshift foot bridge of 2x4's across the channel.

Rolando gestures at the mound. "This is a recent land slide; it happened during the first big rain of the season. There has been no activity since then, because none of the storms have been as big as this one!" He hops onto the foot bridge and crosses to the other side of the riverbank.

I stop to do a quick risk assessment. The riverbed is narrow, the trail along the bank is only a few feet above the water level, and the water is shallow; it's unlikely we'll be trapped on the wrong side of the river. Rolando is a registered guide who has lived in this town his entire life and does these tours all the time – I decide I can trust him. We hop across the bridge and continue down the opposite side of the riverbank.

Rolando is in the lead, doing his searchlight routine against all of the plants along the riverbank. He points out an interesting spider but does not linger; he's clearly looking for something specific. We follow close behind him, using our lights to illuminate the narrow footpath.

After three minutes of walking downstream, he comes to a sudden stop. With a dramatic flourish he draws back the fronds of a bushy fern to reveal a tiny, bright red frog slightly larger than my thumbnail, with green highlights around its hind legs. Unable to make it out clearly without my glasses, I move closer to the frog until it's about two feet from my face.

"This is the green pants frog," Rolando announces proudly. "Sometimes it's called the poison dart frog because the natives once used its venom to create poisonous darts for warfare. It is one of the most toxic frogs in Costa Rica; a small amount of its poison can kill very quickly when introduced to the bloodstream."

I take a respectful step back, shifting my weight nervously between feet. "Is it dangerous to touch?" I ask.

Rolando shrugs. "Only if you have a cut or sore on your hands. Of course, it's not advisable to lick it, either."

I make a mental note not to lick the wildlife and we all step gingerly past the tiny deadly thing. We continue down the riverbank for another 50 feet or so, when suddenly Rolando stops dead in his tracks. He turns around to address us. "Guys, to be honest I have to say that it is too dangerous to continue beyond this point. There is a risk of flash flooding and the trail gets closer to the river. We need to turn back now."

None of is is surprised by this news and I am secretly relieved; thoughts of flash floods had been gnawing at me ever since we crossed the bridge at the land slide. We turn around and Rolando sidles past us to begin the walk back. Almost before we've started again, he stops and points his flashlight down at the base of the trail. "Look!" he cries triumphantly; "it's another tree frog, like the ones we saw back in the marsh!"

He stoops over and begins to pick up the tree frog, but has second thoughts; after a second of hesitation he straightens and begins a brisk double-time walk back up the riverbank. This is a man who is clearly obsessed with the natural world.

We quickly arrive back at the bridge, cross the river, and begin the long and increasingly muddy trek back down the embankment toward the road. The game trail that we ascended has turned into a small brook; rather than stepping down the hillside, I sort of slip-slide down it using my shoes as mud skis, occasionally falling back onto my hands.

The numerous potholes along the town road have turned into large puddles; at first I try to avoid the puddles, but I quickly realize that my shoes are a muddy mess inside and out. I begin to splash gleefully through the puddles, scuffing my soles against the decaying asphault and feeling like a child again.

As we walk back to the hotel, Rolando strikes up a rambling conversation. We cover hobbies (he spends all of his free time birdwatching, to the extent that his wife and friends complain); footwear (river waders are very bad for people with flat feet, an issue we share); and his family's history (originally cattle ranchers from Guanacaste province, who decided to seek prosperity on the frontier shortly after he was born). His mother is afraid of snakes after 30 years of living on the Osa peninsula. He and his brother are both guides, but they also help run their father's boat business and occasionally act as drivers for local businesses.

An idea that has been swirling about my head begins to solidify: this town of 200 households and businesses would barely count as a village in most countries, yet it has 200 or more hotel rooms. At the peak of the high season the tourists probably outnumber the work force two-to-one. To cope, the entire town operates as a sort of business collective: hotels pool their tours, borrow boats and guides from one another, and share supplies or staff when times are tight. The town lives in equilibrium with the surrounding jungle but must close ranks against the tourist onslaught. This is the wild frontier in the truest sense of the word.

We arrive at the hotel and trudge wearily to our room. I dump a half pint of water from each shoe and strip down to my briefs on the porch. After a quick shower, I'm in bed by 9:00 pm – by far the latest I've been up since we arrived. I fall asleep wondering what adventures the Osa Peninsula has in store for tomorrow.