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, she also recorded the vibrations of the clutches caused by tropical rain storms.
To determine whether the frog embryos were hatching in response to vibration and not to some chemical or visual cue, Warkentin played the recordings back through a device that mechanically shook the clutches. Sure enough, the eggs hatched more often in response to snake-attack recordings than rainstorm recordings.
"They hatch when the snake starts biting the clutch, not before," she says. "It's not because there are snakes in the neighborhood or snakes there looking at it."
Warkentin examined the recordings to see whether she could tell how rain and snake-attack vibrations differed. "Two of the most obvious differences between vibrations caused by rain and vibrations caused by snakes are elements of the temporal pattern," she says. "Snake bites, in general, last longer than raindrops, and the spaces between snake bites are generally longer than the spaces between raindrops."
To test the hypothesis that embryos use these features of vibrations to assess danger, Warkentin clumped the rain recordings together into tightly spaced segments, making them more snake-like. Conversely, she spliced bits of stillness into the snake vibrations, making them more rain-like. When snake-like rain and actual rain recordings were played to a clutch of red-eyed tree frog eggs, the eggs hatched more often to the snake-like rain recordings. When the rain-like snake recordings and actual snake recordings were played to the clutch, the embryos hatched less often in response to rain-like snake recordings. Both instances, therefore, provided more evidence that temporal patterns serve as an important cue for the frogs.
Warkentin created similar temporal patterns out of computer-generated nonspecific sound, or "white noise." Again, the snake-like pattern of white noise caused more eggs to hatch prematurely than did the rain-like white noise. "These experiments don't rule out the possibility that the frog eggs use other cues," she says. "But clearly, differences in temporal patterns are enough to affect the perception of how dangerous a disturbance is."