The eggs of red-eyed tree frogs usually hatch between six and eight days after fertilization but the animals can emerge up to 30 percent sooner if necessary. Karen M. Warkentin of Boston University and the Smithsonian Tropical Research Institute in Panama studied egg clutches located on the edge of Ocelot Pond in Panama to identify the circumstances under which embryos leave their clutch (the gelatinous mass deposited on a tree leaf) prematurely. She observed eggs that were exposed to two predators--parrot snakes and cat-eye snakes--and analyzed the vibrations caused when they attacked the clutch and captured some of the developing frogs. "They hatch when the snake starts biting the clutch, not before," Warkentin explains. "It's not because there are snakes in the neighborhood or snakes there looking at it."
Warkentin recorded vibrations in the clutch set off by snake-attacks using a device called an accelerometer and compared them to vibrations linked to natural events such as rainstorms. The major differences between the two samples were related to timing: snake bites tend to last longer than raindrops do and there are longer spaces between snake bites than there are between raindrops. When Warkentin tweaked the recordings and generated vibrations using unrelated sounds, she found that the more snake-like patterns caused a greater number of eggs to hatch early and swim to safety as compared to rainlike recordings. "These experiments don't rule out the possibility that the frog eggs use other cues," she notes, "but clearly differences in temporal patterns are enough to affect the perception of how dangerous a disturbance is." A paper detailing the work will be published in the July issue of the journal Animal Behavior.