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.