New research shows that pheromones, biochemical secretions produced by almost all mammals, can tell a mouse a whole lot about another mouse -- including whether it's male or female, its social status, whether it's a relative or whether it's a good candidate for mating.
Mice have poor vision, explains Lawrence Katz, study author and a professor of neurobiology at Duke University Medical Center. So, the information they receive from pheromones is critical for mice to tell each other apart.
Do humans use pheromones in the same way?
Probably not, Katz says. Our eyes are the most helpful for learning things about other humans. However, studies have shown pheromones may play a role in physical attraction and menstrual synchronicity between women residing in close quarters.
"Humans, while we respond to smell and probably have remnants of this machinery, rely more on visual cues," Katz says.
Katz presents his findings Feb. 13 at the American Association for the Advancement of Science annual meeting in Denver. The study will also appear in an upcoming issue of Science.
Still, the findings do reveal intriguing new insights into the way not only mice but other mammals navigate their world, Katz says.
A wide range of animals, from dogs to elephants, detect pheromones using a specialized sense organ, called the vomeronasal organ, located in the nasal cavity. Receptors in the vomeronasal organ send information to the brain.
Katz and his colleagues fitted 12 male mice with hair-thin electrodes in the "accessory" olfactory bulb, the portion of the brain that processes signals from the vomeronasal organ. The "accessory" olfactory system is separate from the main olfactory system, which enables mammals to smell all the other scents of the world, from flowers to fire to food.
By remote control, researchers retracted or extended the electrodes by one-micron increments (100 microns equals the width of a human hair). The electrodes let them record the activity of specific neurons during various behaviors.
Researchers then added another mouse to the cage. When the male mouse nuzzled the visitor's anogenital area with its snout, neurons in the accessory olfactory bulb went wild. Researchers were surprised to find the same reaction in the brain when the mice sniffed the other mouse's head.
"That tells us the head must also be a rich source of pheromones," Katz says.
Vestiges of this behavior possibly have something to do with the reason why humans kiss, he says.
Researchers used several strains of mice in the study. A strain of mice is a breed in which all the mice are almost genetically identical.
Researchers found the mice were able to distinguish their genetic twin from those of other strains. When mice met their genetic twin, certain neurons fired. When they encountered mice from a different strain, different neurons activated.
"What we found were the neurons in that part of the brain were exquisitely sensitive to certain genetic makeups," he says.
Since the mice aren't exactly reporting their feelings, how do researchers know what information the mice are getting? By observing their behavior, such as whether they try to mate with the other mouse or if they try to attack it. Second is by observing what areas of the mouse's brain are activated and comparing that with what's already known about how a mouse brain functions.
One final note: While mammals detect pheromones through the nose, it's unclear about whether they perceive any odor.
"In a way, it is no more like smell than taste is like smell," he says. "The two senses happen to reside in the same physical region in mammals, but the processing pathways don't directly talk to each other in the brain."