Published: November 2006
It's a Frog's Life
Born on the run, hiding in plain sight.
By Jennifer S. Holland
National Geographic Staff

They look like spilled candy, these tropical frogs with the red eyes and outsize orange feet. You'll be tempted to scoop one up and cup it in your hands. But let it go, because the red-eyed tree frog's life is an extraordinary journey.

It's the wet season, and a Central American rain forest hums with life. Chock, chock, chock, the love song of Agalychnis callidryas plays in overlapping notes around a pond. The frogs have left their tree canopy home to mate; males wrestle one another for territory, then pile on females, vying to fertilize their eggs. The females wander all night, bush to bush, leaf to leaf, stacked with one or even two suitors, in search of good spots over water for spreading their jelly-encased eggs. The next morning, hundreds of shiny clutches, each housing up to a hundred frogs-to-be, smudge the landscape —and attract predators.

A. callidryas eggs, which are laid throughout the rainy season, make easy prey. They hang exposed for six days in sacs that shimmy wetly at the slightest disturbance. Snakes attack entire clutches, and wasps pluck out and carry off single squirming embryos. In all, the two predators take well over half the eggs. Related frog species such as A. saltator may be less vulnerable because while they breed less often, they breed explosively, producing so many eggs at once that snakes and wasps barely make a dent.

But here's the elegant twist: A. callidryas embryos have evolved a safety net. If attacked, they can hatch within seconds, and up to two days prematurely, dropping to safety in the water below. And what most astonishes scientists is that the animals can distinguish a predator's attack from a shiver of wind or a wash of rain through the vibrations in the egg jelly. Embryos judge whether the threat is real by how often the vibrations come and how long they last. The eggs even react differently to different assailants.

Boston University biologist Karen Warkentin, working at the Smithsonian Tropical Research Institute in Panama, assaulted tree frog eggs with various forces to study their reactions. "We had a window on the embryos' minds and could ask them questions: Is this scary? Can you discriminate between this and that?" Fantastically, they could. It turns out that when a snake bites into a gooey mass, all the embryos try to wiggle free. A wasp's more focused attack prompts only neighboring eggs to hatch. And a rainstorm triggers nothing at all.

All the Agalychnis species Warkentin and collaborator Ivan Gomez-Mestre have studied so far also hatch early if the eggs are submerged—as when an egg-heavy leaf falls into a pond—which can drown the embryos. Reacting to a lack of oxygen "is clearly an ancient survival response that's preserved in many egg-laying vertebrates," says Gomez-Mestre. But premature hatching under predatory threat wasn't known until Warkentin observed it. Now, other scientists report the behavior in various amphibians, a spider, and a fish, suggesting that the ability has evolved independently many times. But how the embryos sense danger and make their Houdini-like escape is still a mystery.

What happens to the embryos after the fall? No pocket of rain forest is benign, and having squeezed from egg membrane into waiting pond and dropped to the bottom, the premature tadpoles face new threats: invertebrates such as shrimp and giant water bugs, and, at some sites, fish. But many endure and complete their development, in coming weeks sprouting legs and growing the lung power they'll need on land. A gantlet of new predators awaits them there—large spiders, birds, snakes —but the fittest survive yet again to master another novel environment, climbing to safety in the tall trees.

Creative biology aside, the red-eyed tree frog, of the thousands of known frog species, is singular in charm. Nearly 200 million years of evolution has crafted a creature that grows vibrant and bold, a beautiful tree nymph—the literal meaning of the Latin callidryas—but also a clown content to walk across another's head, foot to eyeball. With a broad smile that opens into a night predator's maw, it tags insects with a sticky lick and swallows them whole. Blinking its big eyes helps force the prey down. Toes tipped with grippy pads fan from those gangly limbs; the reach is expansive, and each step seems to follow a long, hard thought. As a frog sleeps away the midday heat, its hunkered-down form hardly seems like flesh—bulbous eyes cloaked, glistening body pressed to leaf, a dewdrop vanished against the green.