National Geographic : 2006 Nov
Gorging on eggs, a cat-eyed snake (right) and a wasp (left, below), shake up nearby embryos, prompting them to hatch up to two days early. It's a frantic struggle out of the egg membrane and into water, but when a clutch is assaulted, some 80 percent of tadpoles (left) make the escape. 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 scien tists is that the animals can distinguish a preda tor'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 dif ferent assailants. Boston University biologist Karen Warkentin, working at the Smithsonian Tropical Research Institute in Panama, assaulted tree frog eggs 144 NATIONAL GEOGRAPHIC * NOVEMBER 2006 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?" Fantasti cally, 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,"