A guy who can shed a tear really can drive females wild—among mice, at least. According to a new study, male mouse tears contain a sex pheromone called ESP1, which makes female mice more receptive to mounting. While sex pheromones are known to have similar effects in other animals, the new study shows for the first time how the interaction works "at the molecular level and also the brain level," said study co-author Kazushige Touhara of the University of Tokyo. (Related: "Pheromone in Urine Spurs Mating in Elephants.") Male mice shed tears to keep their eyes from drying out. As they groom themselves, the tears—and the pheromone—get spread around their bodies and nests. When female mice come in contact with a male or his nest, they pick up the pheromone via a nose organ called the vomeronasal, where the pheromone binds to a specific protein receptor. "She has to touch it, because this is not a volatile compound like a fragrance," Touhara said, referring to the ease with which some chemicals turn into vapor. Upon contact, the pheromone is sent to sex-specific regions in the female's brain. The female mouse is then three times more likely to engage in what's called lordosis behavior, a posture shown by many animals in heat in which they thrust their rumps and tails upward. Tears as a Captive Breeding Tool? Humans lack the gene that codes for ESP1 and its receptor, so men are unlikely to gain a sexual edge—chemically speaking—if they decide to show their more sensitive sides, Touhara noted. (Get a human genetics overview.) "But the thing is, in human society we don't use chemical communication anymore, because we have good eyesight"—for visually sizing up attractive mates—"and we use language," he said. (Related: "True Love" in National Geographic magazine.) The findings, however, may have real-world applications for mouse population control. "Most of the wild mice express this pheromone robustly, but surprisingly, most of the laboratory mice don't," he said. This has led to a decrease in lab-mouse breeding efficiency, which means that researchers may be spending more time and money than necessary to get animals genetically suited for lab experiments. By: John Roach for National Geographic News