Just a whiff of their predators’ urine is enough to send mice and rats scurrying off. Now researchers report that 2-phenylethylamine is the chemical that triggers such aversive behavior (Proc. Natl. Acad. USA, DOI: 10.1073/pnas.1103317108). A team led by Harvard Medical School’s Stephen D. Liberles studied urine samples from 38 different mammals and found that carnivores, such as lions, tigers, and raccoons, have up to 3,000 times as much 2-phenylethylamine in their urine as herbivores. The chemical activates mouse olfactory receptors and sensory neurons, suggesting that 2-phenylethylamine triggers aversion circuits in the rodent’s brain. Liberles’ team also conducted behavioral studies in which mice and rats actively avoided 2-phenylethylamine but showed no such aversion to lion urine that had been stripped of the compound. “These data show how a single, volatile chemical detected in the environment can drive an elaborate danger-associated behavioral response in mammals,” write the researchers. They hope that understanding the molecular basis of predator odor recognition will give researchers tools for studying the neural circuitry that underlies innate behavior.