The scent of a man, at least among mice, can reveal the state of his health and determine whether a female gets pregnant, a new study shows.
The research suggests that other animals, perhaps even you, choose mates in part based on the strength of their immune systems.
Previous research had shown mice prefer to breed with mates whose immune-system genes -- which produce chemicals that help the body fight invading cells -- are different from their own. Such selective sex leads to healthier offspring.
The new study shows how the selection occurs.
Researchers at the University of Maryland examined molecules known as peptides that come from the immune system and end up in urine. Each mouse's disease-fighting peptides are unique, like fingerprints. A female records and remembers the scent of a mate's peptides using its vomeronasal organ, inside the nose.
"Exposure, during a critical period, to urine odor from another male, will prevent embryo implantation, leading to loss of pregnancy, while exposure to the familiar odor will not," said Frank Zufall of the university's School of Medicine.
Spiking the punch
"We can trick this odor memory and the outcome of the pregnancy-block test by adding peptides to urine," Zufall told LiveScience. "In other words, we can switch an unfamiliar urine odor to a familiar one (and vice versa) by spiking the urine with only a few peptides."
Other studies have shown that vomeronasal organs in many animals detect pheromones and other molecules that pack information on sexual and social status. Pheromones were first discovered in the 1950s to be sex attractants in insects.
"We believe that detection of [immune system] peptides via the nose may be of general significance for social behaviors in all vertebrates," Zufall said.
The study was led by Trese Leinders-Zufall and will be detailed in the Nov. 5 issue of the journal Science.
Similar peptides exist in human immune systems. But our vomeronasal organ has apparently been rendered defunct by evolution, many scientists believe, though there's some uncertainty about this. In fact the question of how and whether scent affects a woman has been widely debated in recent years.
Since discovering powerful sex pheremones in silkworms decades ago, scientists have been hot to learn whether humans could be similarly stimulated. The investigation has proved frustrating.
"Compared to insects, whose behavior is stereotyped and highly predictable, mammals are independent, ornery, complex creatures," notes writer Maya Pines of the Howard Hughes Medical Institute.
Like any animal, we humans are picky. And that provides a line of investigation.
In 1996, Claus Wedekind, a zoologist at Bern University in Switzerland, conducted what's become known as the stinky T-shirt study. Wedekind had 44 men each wear a t-shirt for two nights straight, then tested how women reacted to the smelly shirts.
Like mice, women preferred the scent of men whose immune systems were unlike their own. If a man's immune system was similar, a woman tended to describe his T-shirt as smelling like her father or brother.
Since then, companies have developed pheremone-based perfumes and cologns, with promises of increased sexual attraction. Researchers don't agree on their effectiveness.
More research is needed to figure out how and to what extent a woman's nose leads her to sex, and how adept she is at picking a healthy partner.
"We cannot rule out that other parts of the human nose are able to detect the peptides," Frank Zufall said. "We can now ask whether these peptides are present in human secretions such as sweat and saliva, whether they can be detected by the human nose, and if so, whether they have any influence on our own social behavior."