Discovering A New Sound in the Cloud Forests of Honduras

By Ben Mirin, Safina Center Launchpad Fellow

The cloud forests in northwestern Honduras harbor many natural wonders. Resplendent Quetzals tuck their tails into nesting cavities in the tall conifers, and troops of howler monkeys awaken the dawn with roaring choruses that carry for miles. Beneath this dazzling orchestra, I sit with my friend Jon Kolby on a riverbank watching something we have never seen before: a tree frog swimming underwater.

“This is one of the highest canopy frogs living in this forest,” Jon explains. He has been studying amphibians in Cusuco for over 10 years, and this is the only frog he has observed swimming in the Cusuco River. “The last place we would have ever expected to look for it would be under the water, yet here it is.”

The frog of our affections is the Exquisite spike-thumbed frog (Plectrohyla exquisita). Named for the bone spurs on its front feet, it is one of 16 endangered and critically endangered amphibian species found in Cusuco National Park. Many of these species are found nowhere else on earth, but their home in the park is far from a stronghold. In addition to climate change and deforestation, Cusuco now harbors chytrid fungus, an infamously lethal pathogen that infects frogs’ skins and prevents them from absorbing nutrients from their environment. The fungus has been responsible for as many as 200 amphibian extinctions worldwide, and since its discovery in Cusuco in 2007 researchers have been developing a captive breeding program to protect and rebuild local amphibian populations before the fungus kills them all.

“We are building the Honduras Amphibian Rescue and Conservation Center in Cusuco to breed these animals in captivity and reintroduce them,” Jon explains. He is the program’s co-founder and executive director. “The problem is that in order to effectively conserve this frog, we really need to understand the behavior, the natural history, what they eat, and what they sound like. We really know very little about any of this.”

Being so enigmatic has made Exquisita a particularly difficult species to conserve. To this day, no one is certain what the frog sounds like. Filling that knowledge gap may be a critical step toward saving the species from extinction.

“In a captive breeding situation, calls and sounds of the environment are critical to success,” explains Dr. Ruth Marcec Greaves. She is another member of our expedition team, and director of the National Amphibian Conservation Center at the Detroit Zoo.

“Male frogs especially rely on calls in order to help stimulate breeding behavior. But it’s not just the calls. We want sounds of water rushing, we want sounds of wind in the trees and things like that, to help artificially immerse these animals in their environment and give them everything they need to be successful.”

Our team has come to Cusuco with a singular mission to collect the first recordings of Exquisita calling in the wild. We still haven’t heard anything, but with two days left in our expedition we are not giving up. Walking back up the Cusuco River, Jon and I come across another Exquisita perched on a tree above the water. Without breaking his stride Jon walks up and plucks it easily from its perch to check its health. The frog is the size of his fist, mint-green, and covered in black streaks that may be symptomatic of chytrid infection.

“We know that Exquisita can make sound,” says Jon, the frog wriggling in his grasp as he stretches its legs out to look for sores and lesions. “There have been some instances where we have captured this frog to check its health and it actually will chatter.”

Jon returns the frog to its branch and we hike back up the valley to basecamp. After an early dinner of rice, beans, and homemade tortillas, we set our alarms for midnight and prepare for a final excursion to patrol the Cusuco River during the darkest hours of the night. It’s the only time of day when we still haven’t explored the park for signs of our frog.

At 11:50pm, my cell phone alarm shatters Cusuco’s nocturnal insect chorus with brutal clarity. I drag my feet through the darkness, shuffling over loose conifer needles and wet ferns toward the center of basecamp. Jon and our filmmaker Katie are waiting for me. We pull on our boots, grab our gear, and hike back down into the valley where we skid over slippery boulders until we reach the southern extreme of the Cusuco River.

Just above the waterfall that stops us from proceeding further, the water has formed a clear still pool with sand and leaf litter on the bottom. As Jon and Katie begin looking for signs of exquisita, I listen to the surrounding scene. In the dark, the roar of the waterfall is overwhelming, and the only way I can imagine escaping it is to turn my attention underwater. Donning my headphones, I power up my recorder and lower a hydrophone into the pool.

As the microphone touches down among the leaves, an unmistakable sound punctuates the ambient gurgle of the river. Eight mechanical clicks, in rapid succession, repeated every ten seconds. I freeze. This is something new, and I have to keep my cool. I’m pretty sure no one has ever listened to the bottom of the Cusuco River, and I’m not about to blow my chance to capture something new.

After several minutes of repetition the sound stops, and I frantically wave Jon over to listen. We stand together for another few minutes, and then the sounds starts again.

“That’s a thing,” I whisper excitedly.

“Yeah, that’s definitely a thing,” Jon answers, “and whatever it is it’s very interesting.”

The next morning we return to the pool with our hydrophones, and hear the sound again in broad daylight.

“This might be mechanical, or local only in this pool,” Jon says. “The nail in the coffin for calling this a biological sound would be if we hear it someplace else.”

Walking back to basecamp, we wonder aloud whether it might be an insect, or even something geological like rocks clattering against each other. We look at the recordings we have so far using spectrograms to visualize the signal, and clearly see the repetitive clicking patterns standing out against the background of rushing water.

With one night remaining in our expedition, we decide to take our hydrophones out for a final recording session in another pool upstream. As soon as I begin recording, I hear the sound again, and this time I turn around and see an Exquisita swimming up from the depths of the pool. The frog approaches my tripod and starts to climb. I continue recording the sounds of the pool, and as soon as the frog leaves the water, the clicking sound stops.

Thank you for watching our film “Sounds of Survival” and reading this blog about our exciting discovery and conservation work with frogs in Cusuco. First and foremost, I would like to thank my expedition team members, Katie Garrett and Dr. Jonathan Kolby. Special thanks to our local guides, Miguel Mejia Barrenga and Santiago Sanchez Argueta, and the residents of Buenos Aires village in Cusuco. Thank you to Ruth Marcec Greaves, Brandon Greaves, and Ryan Marshall of the Detroit Zoo. If you want to help save exquisita and the other frogs in Cusuco National Park, please visit frogrescue.com and consider making a donation to the Honduras Amphibian Rescue and Conservation Center, or purchasing some of their frog-themed merch at redbubble.com.