When a mouse catches a whiff of a cat or a rat, it cautiously steps away from the direction of the scent, generally backward so it can keep an eye on the predator.

Then it stands on tiptoe, ears perked up, ready to make a dash if need be.

Researchers report that what triggers this fear is the scent of urinary proteins excreted by the predators. The scents are picked up by neurons in the vomeronasal organ inside the mouse’s nose.

Previously, the communication of information to one species through the excretion of a chemical from another was only fully understood in insects, said Lisa Stowers, a researcher at the Scripps Research Institute in California who led the study.

In their research, Dr. Stowers and her colleagues first identified the urinary proteins as the source of fear for the mice. Then, they disabled the vomeronasal organs of lab mice and placed the mice in a cage with a live but anesthetized rat. Rather than expressing fear and caution as they typically would, the mice instead showed no signs of being scared.