It’s frequently said that scent is the sense most powerfully tied to memory. For mice, it turns out, that’s especially true—at least when it comes to a sniff of the urine of potential mates.
According to a study published today in Science by researchers from the University of Liverpool, female mice exposed to the potent pheromone darcin (found in male mouse urine) just a single time will repeatedly return to the exact site of exposure up to 14 days later, even after the pheromone is taken away.
“We have shown that a male sex pheromone in mice makes females . . .remember exactly where they encountered the pheromone and show a preference for this site for up to two weeks afterwards,” said lead author Sarah Roberts in a statement. “Given the opportunity, they will find that same place again, even if they encountered the scent only once and the scent is no longer there.”
As part of the experiment, the researchers left female house mice in a cage that had two petri dishes—one filled with water, the other with male mouse urine—for either one, two, or three ten minute periods spaced out over the course of a day. Then, 24 hours later, they put them back in the cage, with both dishes taken away.
The alluring memory of urine was remarkably potent: All the female mice demonstrated a noted preference for the spot in the cage where the urine had been. Even the mice who’d only sniffed the urine once lingered at the spot where they’d remembered smelling it roughly five times as long as where the water had been placed.
When they tested other mice who’d been exposed after waiting periods of 2, 3, 7, 10 and 14 days, they showed nearly as distinct a preference, indicating that their enticing memories of the pheromone lingered for some time. It was only after 28 days that the mice finally stopped returning to the site of the urine.
“This attraction to the place they remember is just as strong as attraction to the scent itself,” said co-author Jane Hurst. “Darcin, therefore, induces mice to learn a spatial map of the location of attractive males and their scents, to which they can easily return.”
The researchers determined that the important factor was the pheromone darcin because the same results occurred when a synthetic version of the chemical was put into a petri dish on its own. Additionally, when the female mice were exposed to female urine instead, there was no indication of a preference, because darcin isn’t present in the females’ urine.
Interestingly, the pheromone also produced a powerful effect on another group of mice: competitor males. When they were used in the same experiment, they also demonstrated a preference for the place where they remembered smelling other males’ urine, but they didn’t show this type of spatial memory when the urine used was their own. The researchers speculate that this is because of a motivation to linger near the site and mark the territory with their own pheromone scent, to advertise their availability to female mates.
The scientists speculate that this lingering affinity for the memory of urine is used by the mice as a mental shortcut for finding mates. In a natural setting (instead of cages), rather than having to smell the pheromones from a distance and then track them to the source, they can simply camp out by urine deposited by a potential mate and wait for their likely return.