It's a sheep! It's a bird! It's a ...

Most of my exploring in Arizona has been at lower elevations, in habitats dominated by saguaro, ocotillo, and cholla, and by now I usually know what to expect there. But one afternoon a few years ago, I went for a hike in the Santa Catalina Mountains. Although it's just a short drive from Saguaro National Park, it's a completely different ecosystem, one of Arizona's "sky islands" – small mountain ranges that are isolated from each other by surrounding low desert. As I hiked a trail in the upper part of Bear Canyon, I crossed an open area in the dry pine forest, where a powerful stream cascades down the mountainside, splashing over the exposed granite. Or at least that's what I picture happening in a rainstorm or spring snowmelt. At the time, the streambed consisted only of scattered boulders and a few downed trees. From the trail, I couldn't even see the tiny pools of water that remained in the canyon below me.

As I walked, I heard a distinctive sound – a sharp, staccato baa-aa-aa-aa-aa-aa – that seemed to come from somewhere among the rocks below the trail. I didn't know whether any desert bighorn sheep lived here, but both the habitat and the voice seemed about right. If that was a bighorn, I wanted to find it.

I slowly made my way down the rocky slope, pausing every few steps to listen for another call. The echoes off the rocks made it hard to know exactly where the sound was coming from. I had to keep changing directions, triangulating, to zero in on the source.

After about a hundred yards I seemed to be very close. I was surrounded by nothing but rocks and a few small trees. There was no place for a sheep to be hiding, and yet it had to be nearby. By this time I had begun to think it might be a bird I was hearing, so I kept an eye on the trees and the higher slopes of the rocks. Still nothing. I kept moving, as slowly as possible, until eventually I was at a point where the source of that sound couldn't have been more than two or three yards away.

Finally I saw it. No, it wasn't a bighorn. It wasn't a bird, either. It was a canyon treefrog, the first one I had ever seen – or heard, obviously. After recovering from the shock of being fooled by a frog (after all, I'm supposed to be the frog guy), I refocused my attention and found there were several more within a few yards of me.

Every species has its comfort zone – or maybe I should say discomfort zone – an invisible circle that defines the area that, if you cross into it, you will usually cause the animal to flee. In a lifetime of watching reptiles and amphibians, I've learned how to cross that line with most of them. As I got closer to the canyon treefrogs, I discovered some pretty cool things about these little critters.   

The first thing I noticed is that they can be pretty hard to see when they're not moving. The pattern on the frog's back blends in perfectly with the granite. I made a few shots showing the camouflage, and others with very shallow depth of field to isolate the frog from its background.

Even more interesting, and just as challenging to photograph, was the way the vibrations of the male's vocal sac send ripples out across the surface of the water.

Later that evening, I heard a chorus of hundreds of frogs, a truly impressive sound that echoed up and down the canyon and seemed to come from everywhere at once. (And it didn't sound anything like a sheep.) But it was that first encounter that I remember most vividly, the frog that confused me and reminded me that I really don't know all that much about frogs.

My first tropical frog

The Hotel Bougainvillea is known for its ten acres of gardens, which attract a wide variety of birds, butterflies, and photographers. Only twenty minutes by taxi from Juan Santamaria International Airport, it's a popular stopover for ecotourists on their way to the various wilderness lodges that contribute so much to Costa Rica's economy. The gardens also include three small ponds that were designed especially as habitat for Agalychnis annae, an endangered species known locally as the coffee frog. You can guess where my priorities were.

Exploring one of the ponds around mid-day, I photographed several Forrer's leopard frogs (also called grass frogs) hiding in the leaves that covered most of the water's surface. Seeing and photographing a new species is always good news, but I have to admit I was disappointed with this one. I mean, it was my first Costa Rican frog – but it was a leopard frog, almost identical to one of the most common frogs in the United States. (It was a bit like discovering that Costa Rica's national bird is a robin.) In spite of the fact that I had never seen this species before – and never mind that I was in a foreign country and a whole new ecosystem, 3,000 miles and almost 30 degrees of latitude from home – everything about this frog looked familiar. It was unmistakably a member of the genus Rana (or Lithobates, if you're keeping up with the latest in herpetology). I wanted something new.

Coffee frogs are nocturnal; I returned to the pond just before sunset so I'd be ready. But ready for what? I carefully scanned all parts of the pond and its abundant plant life. As dusk turned to dark, though, I realized I had no idea what I was looking for. How big are these frogs? Are they likely to be in the water? At the pond's edge? In the trees? I was beginning to doubt whether I'd find anything at all when suddenly I heard a new call, higher-pitched, shorter, and simpler than the weird grunting and moaning of the leopard frogs. I didn't really know what it was, but at least it wasn't a leopard frog, so I concentrated my search around where the new sound seemed to originate.

Shining my headlamp into the leaves of a huge bromeliad, I finally saw what I'd been looking for. It was a few feet in front of me, and bigger than I expected – its body was a little over 2 inches long. Now this was a tropical frog, not an old familiar Rana. Nothing about this guy looked familiar: It was a brilliant shade of green, with blue along the sides of its belly and legs, and huge eyes with vertical pupils. It could turn its head left and right, independent of its elongated body. Its movements were slow and deliberate, almost chameleon-like. Everything about this frog said I wasn't in Kansas anymore. Or California. Whatever.

Naturally, I had the wrong lens on the camera, and the frog was moving away from me. While changing lenses in the dark, I made my way around to the other side of the bromeliad and tried to get to where I thought the frog was heading. As I stepped deeper into the foliage, I was pretty sure I'd end up falling into the pond while trying to avoid one of the spiders that suddenly seemed to be everywhere. Eventually, though, I found the right spot and the frog stepped into view right in front of my camera.

I had photographed my first tropical frog. With its improbable colors, bulging eyes, and thoughtful gaze, it seemed to be welcoming me to Costa Rica.