Frogs of the Cloud Forest

After a week in Monteverde, I probably know more about frogs than about any other animal. We spent approximately two days talking about frogs, listening to recordings, and meandering through the forest in the dark looking for them. I know now that most reptiles in the order Anura do not fall into either the “frog” or the “toad” family and that there are colorful toads and warty frogs. Some species also have incredible medicinal uses, as painkillers or tumor-markers for example. Other frogs are see-through and you can classify them by bone color. And still other species shriek when you pick them up, a noise that may be intended to shock the captor but also attracts crocodiles and other large predators.

            As with most species of plants and animals, Costa Rica hosts an amazing diversity of frogs. We got our introduction to the frogs of Monteverde on the very first day when, walking through a cow pasture, we heard a clacking sound off in the distance.

            “It’s an elf hitting stones together,” Pablo explained with a smile. We listened. It really did sound like two rocks smacking against each other. After a couple minutes he admitted it was a frog and the next night we spotted the culprit sitting on a fern leaf. As with a lot of frogs, I was amazed that something so small could make such a loud noise.

            A couple days later, we had our first official frog lecture by a visiting scientist (who had written the Mammals of Costa Rica book- we were all a little star struck). We talked about amphibian taxonomy, which sounds dry, but the natural history of each order and family can make it fascinating. There is an incredible amount of diversity in Anurans in terms of development. In fact, many frogs don’t even lay their eggs in the water and some go from egg to tadpole to fully developed frog entirely within the egg, thus skipping the vulnerable aquatic tadpole stage. We also touched upon frog mating rituals; some species leave sperm packets for the female to pick up and others have complex adaptations for holding onto a female such as spikes on their arms or a sticky glue-like substance on their chest.

            In the evening, we all put on raincoats (in Monteverde we learned not to leave the station without a raincoat) and headed towards the trails. The moonlight filtering through the clouds backlit the canopy, leaving just black shadows of trees against an eerie grey sky. After a couple of minutes with 10 flashlights shining along the ground and into the undergrowth, we spotted our first frog. Mark bent down and quickly scooped it up, holding the small creature by its hind legs so that we could see it and so that the warmth of his hand had minimal impact on its body temperature. It was a small frog with brown markings on top and a white belly. He told us the species and then showed us the identifying features- a subtle orange coloration along the backs of the thighs, a white stripe down the chin, and the lack of spikes on top of the eyes. These little brown frogs in the leaf litter are called, collectively, rain frogs. We found a couple more of them along the walk as well as one other species- a green frog who liked to perch on leaves a couple feet off the ground.

Although we only caught five frogs, we heard hundreds of them. We stopped along the trail at one point and switched off our flashlights and headlamps. Mark told us to listen and try to count the layers of sound in the night chorus. I captured the high-pitched chirp of the dink frogs, the peeps of glass frogs, the plop of rain onto leaves, the buzzing of cicadas, and the rustling of leaves in the wind, among a variety of other sounds I couldn’t place.  Mark told us that different species of frogs distinguish from each other by calling at a specific frequency, with very little overlap. They even use frequencies not covered by ambient noise such as the sound of a nearby stream so that their song stands out. Within a species, individuals will coordinate so that all are heard. Glass frogs, for example, have been recorded sending a call down a river, a dominant male beginning the call and each adjacent frog chirping in turn. But frogs don’t just call to attract mates, they make a large range of noises with different purposes and meanings: to repel competitors, scare predators, or tell a prospective suitor they do not want to mate. Frogs have their own complex language that we have just begun to understand.

             I realized during my time at Monteverde that most people (myself included) know very little about frogs but that these creatures are much more complex and diverse than they appear, especially for those of us who have spent most of our lives in the temperate zone, where comparatively few types of Anura live. Even for scientists, many aspects of the lives of frogs are still a mystery. As a group that has seen some drastic declines in recent decades and may be especially vulnerable to climate change, these animals deserve more attention. Personally, I look forward to learning more about them at the next two sites we visit, where I am sure the night hikes will bring new frogs and new choruses.

Nicole Oppenheim

Washington University at St Louis