Besieged tadpoles send chemical alert.

Tadpoles have a stinky way of warning each other to hunker down when a predator looms, according to a new study.

Like many aquatic species, tadpoles use their keen olfactory sense to identify danger, locate good food, and recognize family. Scientists also know that the amphibians, when captured, send out chemical distress signals. Now, researchers have learned that tadpoles that are merely harassed also release such distress signals. The new findings show that the signal's chemistry includes ammonium.

"This is something that, in my opinion, it's amazing people haven't discovered earlier," comments Lee B. Kats of Pepperdine University in Malibu, Calif.

In the recent study, Joseph M. Kiesecker of Yale University and his colleagues put two groups of tadpoles of red-legged frogs in an aquarium partitioned by a screen that blocked visual and acoustic communication between the groups. Water, however, flowed freely through the partition.

When a wooden heron stalked the tadpoles in one compartment, those on the other side slowed down, moved away from the divider, and ducked under a shelter, the researchers report in the June ANIMAL BEHAVIOUR. Undisturbed tadpoles did not elicit these defensive behaviors in their neighbors. The researchers conclude that the beleaguered tadpoles released a chemical signal that penetrated the screen.

"If [such signaling] is a common occurrence, which I think it is, it may be an effective way that prey animals can avoid predation," says Kiesecker.

Several other water-dwelling animals, including crayfish, hermit crabs, and a fish called the Iowa darter, also release a chemical distress signal. Researchers have speculated that the agent carrying such signals may be ammonium, a foul-smelling compound that is the animals' chief metabolic waste.

In further work, which bolsters that idea, Kiesecker's group observed that disturbed tadpoles release more ammonium than undisturbed tadpoles do. What's more, when the researchers added ammonium to an aquarium, tadpoles displayed the same defensive behaviors seen in the earlier experiment.

"More and more, we're becoming aware that tadpoles are sensitive to very, very subtle chemical cues in the environment," says Richard J. Wassersug of Dalhousie University in Halifax, Nova Scotia. The importance of the new study, he adds, is that it asked the question, "What are the chemicals?"

Kats says that Kiesecker's findings illustrate the animals' sensory sophistication, although he notes that the tadpoles' ability to release signals does not mean the creatures are acting with intention. "Just in the process of being stressed, their metabolism changes and they release ammonium, and then those nearby can pick that up," he says.

The researchers haven't yet investigated whether different aquatic species share the same chemical language, whether ammonium combines with other chemicals to send more complex messages, or if signals vary depending on the environment.

"Those are all really, really interesting questions," Kiesecker says, "but at this point, they're all the great unknown."