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A beloved symbol of biodiversity, the red-eyed tree frog, shown here in Panama, has evolved a flexible strategy for survival. (Christian Ziegler)

How the Tree Frog Has Redefined Our View of Biology

The world’s most charismatic amphibian is upending the conventional wisdom about evolution

“I’m ready,” Vonesh says. Vonesh tips the tank as Jimenez holds a net under the gushing water. The guys watch the net for any tadpoles that Vonesh missed. “See anybody?” Vonesh asks. “Nope,” Jimenez says. It takes almost 30 seconds for the water to flow out. Most of the researchers wear tall rubber boots to protect against snakes, but they’re useful as the ground rapidly turns to mud.

A flock of grackles wanders nonchalantly through the grass. “They like to eat tadpoles,” Vonesh says. “They like to hang out and pretend they’re looking for earthworms, but as soon as you turn your back, they’re in your tub.”

Vonesh takes his tub of tadpoles to the shed where Warkentin photographs it. A student will count the tadpoles in each picture. Insects and birds sing from the trees. Something falls—plink—on the metal roof. A freight train whistles from the train tracks that run alongside the canal; a group of howler monkeys barks a raucous response from the trees.

To scientists like Warkentin, Gamboa offers a bit of rainforest about an hour’s drive from an international airport. “Oh, my god. It is so easy,” she says. “There’s a danger of not appreciating how amazing it is. It’s an incredible place to work.”

During the day, the iconic red-eyed frogs aren’t hopping about. If you know what you’re looking for, you can find the occasional adult male clinging to a leaf like a pale green pillbox—legs folded, elbows tucked by his side to minimize water loss. A membrane patterned like a mosque’s carved wooden window screen covers each eye.

The real action is at night, so one evening Warkentin, Vonesh and some guests visit the pond to look for frogs. The birds, insects and monkeys are quiet, but amphibian chirps and creaks fill the air. One frog’s call is a clear, loud “knock-knock!” Another sounds exactly like a ray gun in a video game. The forest feels more wild at night.

Near a shed, a male red-eyed tree frog clings to the stalk of a broad leaf. Tiny orange toes outspread, he shows his white belly and wide red eyes in the light of multiple headlamps. “They have these photogenic postures,” Warkentin says. “And they just sit there and let you take a picture. They don’t run away. Some frogs are, like, so nervous.” Maybe that’s why the red-eyed tree frog has gotten famous, with its picture on so many calendars, I suggest—they’re easier to photograph than other frogs. She corrects me: “They’re cuter.”

Scientists think the ancestors of modern frogs all laid their eggs in water. Maybe the red-eyed tree frog itself could have evolved its leaf-laying habits as a result of phenotypic plasticity. Maybe an ancestor dabbled in laying its eggs out of the water, only on really wet days, to get away from aquatic predators—a plastic way of dealing with a dangerous environment—and that trait got passed on to its descendants, which eventually lost the ability to lay eggs in water at all.

Nobody knows if that’s how it happened. “That was a very long time ago and no longer amenable to those kinds of experiments,” Warkentin says.

But intriguing experiments on another kind of frog—one that might be still navigating the transition between water and land—are underway. Justin Touchon, a former PhD student of Warkentin’s, studies how the hourglass tree frog, Dendropsophus ebraccatus, lays its eggs, which are less packed with jelly and more prone to drying out than red-eyed tree frogs’. A female hourglass tree frog seems to choose where to lay eggs based on dampness. At ponds shaded by trees, Touchon found, they’ll lay eggs on leaves above the water, but at hotter, more exposed ponds, the eggs go in the water.

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