A sex pheromone named after Mr. Darcy has been found to put (most) female mice in the mood.
Pheromones are chemicals produced by animals to communicate with one another. According to a study published in Nature, the pheromone darcin—named after Jane Austen's elusive Fitzwilliam Darcy—takes a different route to the brain from other pheromones, using a second system that alters the mouse's response depending on her internal state.
While most mice show an immediate attraction to the hormone, some appear to dismiss it—and this "complex" biological process may explain why.
"Pheromones have long been associated with an innate, immediate behavioral response, but here we have shown that darcin can elicit complex behaviors that are dependent on the internal state of the animal," Ebru Demir, lead author and post-doctoral researcher at Columbia University Medical Center, said in a statement.
The pheromone was discovered ten years ago, when Professor Jane Hurst and colleagues at the University of Liverpool, U.K., discovered that male mice release darcin in their urine both to mark their territory and initiate courtship.
Female mice then use darcin to identify the male and decide whether or not to reciprocate and continue the courtship.
However, darcin is processed differently to other hormones, which are detected and interpreted by olfactory receptors in the nose. It uses a second, parallel olfactory channel.
"Unlike people, mice have essentially two functional noses," said Demir. "The first nose works like ours: processing scents such as the stinky odor particles found in urine. But a second system, called the vomeronasal nose, evolved specifically to perceive pheromones like darcin."
In a new paper, researchers from Columbia University's Zuckerman Institute describe how different female mice reacted in the presence of urine that contained darcin.
The majority of mice showed an immediate attraction to the urine—in some cases, even reciprocating by offering their own urinary markings in return. Other behaviors reported in the study include singing in frequencies higher than the human ear can pick up. This also indicates a high sex drive, the researchers say.
However, this reaction to the darcin was not universal. Some mice appeared to ignore the darcin. In particular, the researchers noted, females who were lactating seemed oblivious to the presence of the pheromone.
The researchers suspect the mechanism behind these varied reactions lies in the medial amygdala, a part of the brain that plays a key role in the processing of social stimuli and, crucially, social odor cues.
The neurons specifically involved in the response to darcin appear to be a group called nNOS neurons. These turn on when the hormone is detected. The team appeared to confirm this by artificially activating and suppressing the neurons when mice were exposed to darcin.
According to Demir, these results suggest nNOS neurons "do not simply pass along information" but integrate sensory information about darcin with the internal state of the mouse. In this case, the internal state of the mouse includes whether or not she is lactating.
The study's authors suspect there could be other pheromones that are processed in a similarly complex way.
