You'd know the face of your Valentine anywhere--those twinkling eyes and crooked smile are only two of the features that help you pick out your one-and-only from the faces in the crowd. Although a sweet voice and special scent may complete the package, humans depend largely on vision to identify their partners. Mice, which live mostly in the dark and have poor vision, do things a bit differently, relying on chemicals known as pheromones to find their mates.

Scientists have known for some time that a number of mammals use pheromones to determine among other things the social status and reproductive "readiness" of newly encountered individuals. Pheromones are detected by the so-called vomeronasal organ, which sits in the nasal cavity and sets off a cascade of nerve signals that ends in the accessory olfactory bulb, the organ that processes pheromones. But exactly how these chemical messages are deciphered was not well understood. Now researchers report that by inserting ultra-fine electrodes into the brains of live mice, they have identified which neural cells in the accessory olfactory bulb fire when one mouse checks out another's pheromonal fingerprint. The findings were presented yesterday at the annual meeting of the American Association for the Advancement of Science in Denver, Colo.

Lawrence Katz of the Duke University Medical Center and his colleagues discovered that once signals reach the accessory olfactory bulb they are mapped out into distinct patterns of nerve cell activity that vary according to the traits that enable a mouse to identify desirable mates, such as species and sex. The animals thus form what are essentially pheromonal snapshots of other mice. "The most exciting thing we found was that individual neurons were responsive to individual animals. Each type of animal encountered set off a unique pattern of neural excitation or inhibition," Katz comments. "We did not see any neurons that responded to all male mice or to all female mice. They responded to the male mice of a specific genetic identity, but not to male mice of other genetic backgrounds."

Whether or not humans possess an accessory olfactory bulb or have tacked pheromonal signaling onto the list of the main olfactory system's chores remains a matter of debate. "If it's there, it has atrophied over the course of human evolution," Katz says. "But it's clear you can get pheromone-like responses in human brains that are different from standard olfactory response." Indeed, some researchers believe that the French kiss evolved as a pheromone-sensing act and that other intimate activities humans engage in closely resemble the ways in which animals seek out pheromonal clues from strangers. "We show a profound drive to do the same investigative behaviors. Consider kissing and oral sex," Katz quips. "I can't believe that it's just coincidental."