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.