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SCIENCE OBSERVER

Shake an Egg

Fenella Saunders

An egg might seem to be its own little world—encapsulated and cut off from its environment, at the mercy of the forces of nature. But some eggs are very aware of what's going on around them. Not only that, certain eggs can also respond to their circumstances and hatch up to 30 percent prematurely in order to better their chances of survival.

A red-eyed treefrog tadpole...Click to Enlarge Image

Karen M. Warkentin, a biologist at Boston University, and her colleagues have been studying the red-eyed treefrog, Agalychnis callidryas, an inhabitant of Central American tropical forests. The frog lays gelatinous clutches of eggs on leaves overhanging ponds, so that when the tadpoles hatch, they drop into the water. However, the eggs' arboreal location makes them vulnerable to consumption by snakes and wasps.

The usual gestation period for red-eyed treefrog eggs is six to eight days. Warkentin found that after four days of gestation, if the clutch is attacked by a predator, the tadpoles will suddenly start to hatch and fall into the water. A predator can do nothing but watch as its prey literally slips away while the predator chews its first mouthful.

But how do the eggs know that they are under attack? It doesn't seem to be chemical cues or signals from doomed sibling eggs. The eggs, it turns out,  sense vibrations from the attack. As the eggs are secured in a rather tough gelatinous coating, a predator such as a snake has to really tear into the clutch in order to pull free a bite, jarring the entire clutch in the process.

To prove that this was the signal that the eggs cue into, Warkentin inserted a small accelerometer into clutches of eggs and recorded the vibrations from snake attacks. She then replayed these vibrations, using a device called a mini shaker, to other egg clutches. These tadpoles hatched in response to the vibrations, with no actual snakes present.

The treefrog eggs are subjected to a number of other, harmless vibrations, such as those from wind and rain. In the same type of playback experiments, few eggs would hatch in response to vibrations from these benign events. Somehow, the eggs can tell the difference between dangerous and safe vibrations. As Warkentin reported in June at the Acoustical Society of America conference in Providence, Rhode Island, the eggs seem to pick up a combination of  attributes of the signal.

Both snake attacks and rain have low-frequency components in their vibrational signals. However, rain has additional higher-frequency components that are not found in snake attack signals. In addition, the duration and spacing of vibrational events are different for the two signals. For instance, during a rainstorm, longer-duration vibrations occur when multiple drops fall in rapid succession, but the interval between drops shortens. A longer snake attack would likely have increasing intervals between bites.

Warkentin modified the recording of rain and snake attacks so that each sounded like the other: She clumped the rain vibrations into short bursts that were widely spaced and evened out the intervals and durations of snake-bite vibrations. The eggs hatched more to the "snakeified" rain signal than to actual rain or to the "rainified" snake signal.

The investigators then created entirely artificial signals from white noise with various burst and interval lengths. They found the highest rate of hatching—up to 75 percent—from signals that had a half-second duration and intervals of 1.5 to 2.5 seconds. Such stimuli with intervals longer than their durations are consistent with the patterns of snake attacks, but not with rain. Either signal on its own was not sufficient to induce hatching. Neither was continuous vibration. "More vibration is not more scary to the eggs, it's not that simple," says Warkentin.

It is still unknown how fetal tadpoles sense vibrations, but Warkentin suspects that they may be using their lateral lines. These hairlike receptor cells on the skin of tadpoles and fish are sensitive to water movement.

But why wait until a snake attacks before hatching, instead of sensing, say, the vibration from a snake approach? "A snake can sit there looking at the egg mass for an hour and nothing would happen," says Warkentin. The problem may be that once the tadpoles hatch, they are not out of the woods. In the water, they face a host of new predators, such as shrimp and insects. Preemie tadpoles are not only three-quarters of the size of a fully-developed tadpole, but have less-developed respiratory, gastrointestinal and possibly nervous systems, all of which make it harder for them to escape predators.

The eggs therefore need to be absolutely certain that they are under attack before they hatch, trading this off with the risk of being taken in the first mouthful. The eggs begin to hatch on average 16 seconds after the first bite, taking their time to sample the signal for all the components that confirm that this is a deadly event. A snake attack is several minutes long, so most tadpoles still have time to escape.

Warkentin says that more than a dozen amphibian species show induced hatching, as do some fish, one species of spider and possibly some crustaceans. "We never used to ask these kinds of questions about eggs," says Warkentin. "Over the last 10 years, we have discovered a number of ways that embryos can detect information about their environment and respond to it, and we still don't know the full range of the kinds of things eggs can do."


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