Shakers and movers

Natural History, July, 2005 by Stephan Reebs

Eggs, whether from amphibians, birds, insects, or reptiles, cannot move on their own, and few carry noxious chemicals. So when a predator chances on a clutch of eggs with no parent in sight, it's breakfast time! Case in point: a mass of red-eyed treefrog eggs glued to a leaf above a tropical pond. But as soon as a snake chomps clown on that clutch, breakfast is likely to change from lavish Sunday brunch to basic continental.

Karen M. Warkentin, a biologist at Boston University, has discovered that if the frog embryos are in the final third of their development, a predator attack on their egg clutch leads to quick group action. The embryos wriggle frantically, break out of the capsule that detains them, and drop into the water below. Because the escapees are not yet fully developed, they may soon fall prey to other predators--but any chance of survival is better than certain demise in the gut of the snake.

What triggers the embryos' response? It's the precise pattern of vibrations caused by the snake's first few bites. The soon-to-be frogs react by hatching a day or even two days earlier than their usual six-to-seven-day stint on the leaf. By recording and then mimicking the shaking of an egg clutch under attack by a snake, Warkentin has shown how well the embryos can distinguish the telltale signs of an important predator from the tremors caused by innocuous disturbances such as rain--and how well-developed an embryo's decision making can be. (Animal Behaviour doe 10.1016/j.anbehav. 2004.09.019, 2005)