How Wild Male Mice Decide to Mark Their Territory: Unlocking the Secrets of Pheromones

Pheromones are produced by most animals. These chemical signals can help trigger some of the most basic behaviors in the animal kingdom. Now, scientists have decoded some of these cues in mice, learning a bit more about how pheromones function.

In this case, the researchers focused on an important mouse behavior, called countermarking. In the wild, a male mouse will mark his territory with splashes of urine. This is to advertise his presence to females while at the same time scaring away subordinate males. When a male mouse encounters another male mouse's mark, though, he may leave a countermark-assuming the other mouse is subordinate.

"How does an individual respond differently to the environment based on experience?" asked Lisa Stowers, one of the researchers, in a news release. "How does it distinguish itself from others? These are some of the fundamental questions that a study like this one helps us address."

In order to answer these questions and find out how mice make countermarking decisions, the researchers examined a special set of 21 molecules in mouse urine. These molecules, known as Major Urinary Proteins (MUPs) are linked to testosterone production and are found in high concentrations in the urine of adult male mice. Since they're pheromones, MUPs are detected by the vomeronasal organ (VNP) instead of the main olfactory system.

So what did the researchers find? It turns out that mice each have their own distinct combination of several MUPs. Not only do male mice learn to distinguish their own self code, but can tell it apart from the codes contained in urine from other mice.

"When a urine mark contains a set of MUPs that are similar to the mouse's own repertoire, he won't respond to it, but if we vary the set of MUPs in the mark, he will countermark," said Stowers in a news release.

That's not all the researchers found. They also noted that a mouse won't countermark if it has previously learned to associate the sensed set of MUPs with a dominant mouse. This means that the behaviors are less hardwired, and more like other sensory systems found in humans.

The findings are published in the journal Cell.