At the edge of Ocelot Pond, Panama, red-eyed tree frog embryos still in their eggs are about to make a life-or-death decision. The egg clutch, a gelatinous blob clinging to a leaf overhanging the water, has been spied by a bright green parrot snake. In a twinkling, the snake tears a few eggs from the clutch.
With that bite, the embryos start to wiggle frantically. As the snake moves to take another morsel, the embryos rupture their egg capsules, drop into the water, and, as tadpoles, swim away to safety -- hatching one to three days earlier than they would have if left undisturbed.
What cues prompt the embryos to hatch early when shaken by a hungry reptile but not when buffeted by another outside force such as rain, is a question that has now been answered in part by Boston University researcher Karen Warkentin. According to her findings, reported in an upcoming issue of Animal Behaviour, it's a particular characteristic of the vibrations that shake the clutch -- not only the speed of these vibrations, or how hard the clutch is shaken, but the length of and time between the movements that signal the embryos to hatch.
Undisturbed, red-eyed tree frog eggs usually hatch six to eight days after fertilization, but can hatch up to 30 percent earlier if attacked by animals such as snakes or wasps.
At her open-air laboratory at the Gamboa Field Station in Panama, part of the Smithsonian Tropical Research Institute, Warkentin, an assistant professor of biology at BU, used parrot snakes and cat-eye snakes, to find which vibrational cues red-eyed tree frog eggs use to trigger early hatching.
By inserting a minature accelerometer, a device like a microphone that records vibrations instead of sounds, into clutches of eggs she had collected, Warkentin was able to record vibrations that occurred when the snakes attacked the eggs. As comparison