Scent of a Man

Associate professor Vosshall and research associate Andreas Keller of The Rockefeller University, along with their collaborators at Duke University, are the first to do just that, with a component of male sweat. The substance they studied, a hormone called androstenone, is a derivative of the sex hormone testosterone.

Inside the Nose

Odor molecules fit specific smell receptors. image courtesy Leslie Vosshall

The lining of the nose has millions of smell receptor cells (olfactory sensory neurons). Each of these cells has one of about 400 types of differently shaped receptors. Each receptor will accept one odor molecule with a matching shape. In the artist's rendition to the right, you can see a receptor molecule inserting itself into a receptor. This triggers signals to the brain allowing a person to smell it.

The Smell Test

Vosshall and Keller recruited almost 400 volunteers to do a smell test with 66 different odors, such as cedar wood, banana and vanillin. The test measured how the volunteers responded to each odor--what it smelled like and how strong it was. The participants would pick up an odor vial identified only with a bar code, scan it, smell it and record their impressions on a computer.

At the same time, researchers at Duke University were studying all the genes in human DNA that were responsible for the creation of the receptors on smell receptor cells. They found that people had a gene responsible for making the receptor that androstenone fits into.

The Discovery

The collaborators merged their data. They found that people with a functional copy of the gene, called OR7D4, made properly shaped receptors that accepted androstenone. But thirty percent of people had a variation of the gene (OR7D4 WM) that made a receptor into which the androstenone could not fit. The difference in the receptor was very small — just two amino acids differed — but the effects were pronounced. Those with the functional copy were eleven times more sensitive to androstenone than their counterparts. And that marked the first proof that a particular gene was responsible for a person's ability to smell a certain odor.

"I find it most exciting about the study---that it explains how the world is perceived is subjective between different people and that there is actually a reason for that," Keller says, "And an explanation that something may smell one way, but it may smell very different to another person. And that has nothing to do with us interpreting it differently, but it's just that it really does smell different to us and I think that's very fascinating."

The Question of Sex Pheromones

Androstenone is well known in the pig farming industry. It is a sex hormone found in the saliva of male pigs. The smell of it causes female pigs in heat to assume mating position. Companies sell cans of androstenone to farmers. And other companies market it to men, claiming it can attract women.

Regarding the claim that androstenone is a human pheromone, Vosshall says, "There's some data to support that but I think now that we have a gene that determines whether or not you can smell it, we can really go in there and try to evaluate the claims — figure out whether women who are super sensitive to these compounds behave differently with men who produce a lot of these substances versus women who can't smell it at all."

In addition to doing research on the rest of the odors they tested, the researchers plan to study ovulating women and their response to this and other odors.

"We still have to figure out down the road--what are the consequences of smelling these things on men and women who do or don't have a functional copy of this gene" adds Vosshall.

In the meantime, Vosshall and Keller field emails from people who hear about the study. One man wrote to Vosshall about his sensitivity to odor: "The smell of human sweat is really repellant to him…he's married but it's a troubling thing in his relationship with his wife so it seems like there's a lot of sociology and psychology to be delved into with the supersensitive people."

On a lighter note, Keller shares another communication he received. "We got an email from a woman who told us a story that she has a female pet pig and that one day the pig was in heat and it disappeared. And she went searching for it and found it among a group of sweaty construction workers at a nearby house and she told us that she thinks it's the androstenone odor of these men that attracted her pig."

But unless you know your mate digs your post-workout scent, don't count on androstenone as an aphrodisiac. Just shower.

This study was published early online in Nature, September 16, 2007
Study co-authors: Hanyi Zhuang, Hiroaki Matsunami and Qiuyi Chi, Duke University
Research funding: National Institutes of Health