Amoeba betrayed by anticannibal defense.

Although substantially smaller than Tyrannosaurus rex, Amoeba proteus terrorizes the cell-eat-cell jungle of a drop of pond water. That dangerous predator, however, shows one weakness in its hunting performance. Some of its prey detect a chemical that the amoeba releases and thrash backwards out of range. In a rare insight into predation, scientists may have figured out why the hunter keeps giving itself away.

The amoeba's one-celled snacks, members of the genus Euplotes, are just a few of the many organisms that mount a defense on demand--such as fleeing or growing spines or thickening a shell after picking up a chemical cue. These so-called inducible defenses raise a difficult question. What evolutionary benefit could possibly accrue to the predator?

For amoebas, the answer lies in a vital function of the telltale chemical, proposes Jurgen Kusch of the University of Kaiserslautern in Germany. In the March Ecology, he presents data indicating that the peptide, called A-factor, prevents cannibalism by clone mates.

Some theorists have speculated that evolution would have removed tattle-tale chemicals unless some great benefit to the predator constrained that tendency. Researcher Drew Harvell of Cornell University, coeditor of The Ecology and Evolution of Inducible Defenses (1999, Princeton), comments, "This is the first experimental study that I know of that suggests a real constraint."

Kusch tested his idea by feeding beads to amoebas. They readily engulfed plain beads or ones coated with albumin. However, amoebas ate few beads treated with A-factor and rejected them entirely when concentrations of the peptide became too high.

It could be a self-recognition cue, Kusch says. Amoebas multiply by splitting--no one has observed sex--so clone mates with the same A-factor accumulate.

"It makes no sense to multiply and afterwards to eat each other," Kusch says. "It's a waste of time and of energy."

Harvell says she wants to see more evidence before accepting self-recognition as A-factor's main function.

Kusch's proposal is "really an interesting idea," says Stanley Dodson of the University of Wisconsin-Madison. He notes that some theorists have taken a different tack, suggesting that predators benefit from revealing themselves and thus avoiding wiping out their food supply in a single feast. "That's never been demonstrated," he says.