BioEd Online

It may not be considered manly for humans to cry. But when male mice shed a tear, they seem to be trying to prove their masculinity.

So say Japanese researchers who have discovered that male mice release pheromones in the fluid that moistens their eyes.

"Nobody expected that sex-specific pheromones would exist in tears," says Kazushige Touhara of the University of Tokyo in Chiba. Pheromones, the chemicals that convey messages about everything from fear to sexual desire, are most common in sweat in humans, and in urine in mice.

It is not clear whether mice ever cry for the same reasons as humans; in this study, their tears were just the result of a basic physiological response that keeps a mouse's eyes wet and comfortable.

Touhara says the pheromones in these secretions are probably picked up by females when they groom the faces of their fellow mice. These sexy cues may help females to work out which of their companions are male and therefore potential mates, Touhara and his team report in Nature¹.

The crying game

In most vertebrates, pheromones seem to trigger nerve cells in the vomeronasal organs, which are situated in the hard palate between the nose and mouth. Some studies have found evidence for such an organ in the developing human fetus, but the presence of a functioning one in human adults remains controversial.

Touhara's group set out to explore how various pheromone compounds affect mouse vomeronasal organs, which are thought to influence mating behaviour. But when they picked through the chemicals in mouse urine to find the pheromone responsible for triggering a specific receptor in the organ, they came up empty handed. After testing naturally produced chemicals from various mouse glands, the team finally hit upon the right chemical... in mouse tears.

The researchers note that humans are not likely to release pheromones in their tears. The mouse-tear pheromones are produced by a family of genes for which humans don't seem to have an analogue, says Touhara.

But he adds that the work highlights a mystery of human tears: scientists do not fully understand why we cry when hurt. "I think that to cry means more than people have thought," Touhara says.