Learning to Adapt
 Karen Warkentin: Escape Artists
As a graduate student in the early 1990s, biologist Karen Warkentin set out to “be paid to look at cool frogs in cool places.” Now, she regularly travels with students to the rainforests of Panama to study the adaptability of red-eyed treefrogs. These frogs live in trees and lay their eggs in a gelatinous clump, or clutch, stuck to leaves overhanging water. The eggs face many risks, including predatory wasps, a pathogenic fungus, drowning, and the four snake species that consider them dinner.
 In her graduate research, Warkentin showed that treefrog eggs can hatch up to 30% early if necessary by wriggling within the egg and dropping into the pond below to become a tadpole. “Before that, people didn’t think that eggs responded behaviorally until they hatched,” says Warkentin. A new interest in the behavioral ecology of eggs was triggered by her discovery that these embryos are not merely passively awaiting birth, but actively responding to their environment.
The time of hatching is a crucial switch point between the frogs’ egg and water stages. An early hatch, even two days, is a very risky decision, Warkentin explains: it’s like a human being born three months premature. Choosing between getting eaten by a snake or facing predators in the pond is not taken lightly by vulnerable embryos. “A snake could stare at the clutch all day and the embryos wouldn’t budge,” she says; however, early hatching can be induced by vibrations made by an attacking snake. And, according to Warkentin, red-eyed treefrog embryos can distinguish between recordings of vibrations from a snake attack and those produced by harmless rain.
To learn how the eggs use vibrational cues to distinguish between signs of danger and a benign disturbance, Warkentin teamed up with Michael Caldwell, a graduate student in biology, and mechanical engineer Gregory McDaniel, an expert in acoustics. Working with synthetic white noise, they varied both the duration of the vibrations and the spacing between them. They found that the embryos responded only to vibration patterns similar to those produced by snake attacks.
 But snake attacks are only one of the risks that treefrogs face. If they make it into the water and survive a vast array of predators under the surface, they face another risk when attempting to leave the pond as juveniles. Fishing spiders patrol the surface of the pond, just waiting for the froglets to “run the gauntlet to the forest,” Warkentin says. Working with post-doctoral researcher James Vonesh, she recently demonstrated that tadpoles can delay metamorphosis in response to a perceived risk in the next stage of life: in this case, fishing spiders. She hopes that learning more about risk-sensitive hatching and metamorphosis will lead to an understanding of how such plasticity affects the overall ecology of treefrog populations.
This research was published in the April 15, 2006 issue of The Journal of Experimental Biology and the March 2006 issue of Ecology.
For more information visit http://people.bu.edu/kwarken/.
To see a video of early hatching, see http://people.bu.edu/kwarken/KWvideo.html
— by Leah Eisenstadt |
