Sweet smell of sex

Scientists have eavesdropped on the brains of mice as they sniff out sexual partners.

The researchers monitored the rodents to see how they reacted to the subtle chemical cues, known as pheromones, which are excreted from their bodies.

" Pheromones are chemicals animals use to communicate with members of the same species "
Dr Lawrence Katz

The study showed how the rodents' brains went into a frenzy of activity when they smelt the right mate.

It is said to be the first time that anyone has been able to watch in-action what is going on inside animals' heads when they use what is sometimes called their "sixth sense".

A wide range of mammals can detect pheromones. They have what is known as a vomeronasal organ in their nasal cavities which can pick up on subtle chemical signals.

"Pheromones are chemicals animals use to communicate with members of the same species," said Lawrence Katz, a co-researcher on the study which is about to be published in the journal Science.

"They are poorly understood in terms of their identity and how they are perceived and how they are understood in the brain but they are very important for regulating reproductive behaviours for communicating identity and for advertising the reproductive status - the receptivity - of individuals."

'Blizzard of activity'

The Duke University Medical Center researcher, together with Minmin Luo from the Howard Hughes Medical Institute, devised a system to "listen in" on the brains of mice as they reacted to these chemical cues.

They inserted motorised hair-thin electrode probes into the heads of mice in a region of their brains called the accessory olfactory bulb (AOB), which processes the information sent from the vomeronasal organ.

The electrical activity in the neurons in the AOB was put through a sound system.

"It sounds a bit like a Geiger counter going off. When some of these nerve cells received input from a very specific stimulus animal you got a blizzard of activity and it would sound like a Geiger counter that had gone wild," Dr Katz said.

From this work, the researchers were able to show how the animals' pheromone-processing machinery formed, in essence, very specific "pheromonal images" of other mice.

It is these "images" that influenced a range of mating, fighting, maternal-infant bonding and other behaviours, the scientists said.

The specificity witnessed in the neurons that processed pheromonal signals was akin to the "face neurons" in the visual areas of primate brains that are specifically triggered by facial features of other animals, the team added.

The degree to which humans can pick up pheromonal signals remains controversial. It is not certain they have the brain structures to process such information.

However, there was good evidence humans did react to pheromone-like substances, Dr Katz said. Unlike for mice, though, visual cues are far more important in humans, he added.