Passage II

What's Shaking, Tree Frog?

Red-eyed tree frogs (Agalychnis callidryas)lay their eggs in clutches, gelatinous sacs of dozens of eggs, on tree leaves that dangle over ponds and swamps. Mature tadpoles hatch in six to eight days, gently dropping into the water. Though the egg clutches are safer when stuck to a leaf, than they would be drifting in a pond snakes can find and eat them. Fortunately, the clutches have a defense. As a snake attacks an A. callidryas egg clutch, the tadpoles that haven't been harmed hatch prematurely. As early as four days into development, the tadpoles can burst out of their eggs to swim away.

Biologist, Karen Warkentin, wanted to determine why the egg clutches react to being shaken and tugged during a snake attack they rarely react to being jostled during a rainstorm. She inserted devices called accelerometers, which measure and record the frequency of vibrations, into egg clutches at Ocelot Pond, near Gamboa, Panama. Her recordings revealed that a snake attack and a rainstorm each cause sustained low-frequency vibrations of the egg clutch. However, it also showed that a rainstorm usually produces short periods of significantly higher-frequency vibrations. While these interruptions are nearly absent from the vibrations caused by a snake attack.

Bringing the recordings back to her lab at Boston University, Warkentin's simulation of the snake-attack vibrations and the rainstorm vibrations on several four-day-old egg clutches. Most eggs in the clutches that occupied only low-frequency vibrations hatched. But few eggs in the clutches that were exposed to low-frequency vibrations combined with periods of high-frequency vibrations hatched.

The periods of high-frequency vibrations that occurs during a rainstorm, however, seem to signal safety to A. callidryas egg clutches. Given that premature tadpoles risk predation by fish or shrimp, needlessly hatching early is dangerous. Warkentin has shown that the egg clutches have a complex, nearly perfectly controlled way to avoid false alarms.