Following Our Noses

By Jeffrey Kluger

If you're an animal, there are few things as valuable as a good nose. In a world without speech, it's often scent alone that tells you if a stranger is in the mood to mate or in distress, is preparing to attack or about to retreat in fear. The chemicals that carry these odorless messages are called pheromones, and while most animals produce them, the highest animals--humans--were thought to be above such crude olfactory signals.

Last week all that changed. In a paper published in the journal Nature, psychologist Martha McClintock of the University of Chicago reported what may be the best evidence yet of human pheromones. In an elegantly straightforward experiment, she was able to speed up and slow down the monthly cycles of a group of women by exposing them to a whiff of sweat from other women. The ovulatory command, she believes, was carried by pheromones.

If McClintock is right, the implications could be sweeping, offering not just new insights into human communication but practical medical applications as well. "Once you establish that pheromones exist," McClintock says, "the question becomes how far-ranging they can be."

For most scientists, pheromones are nothing new. In the 1930s entomologists first noticed that female moths are able to excite males even when the males can neither see nor hear them. The males, they discovered, "smell" the females, grabbing their fragrance out of the air with exquisitely sensitive antennae. Once that fragrance was isolated, it was found to be powerful indeed, able to stimulate millions of moths with concentrations of less than one 300-millionth of an ounce.

When substances this potent hit the sensory systems of a relatively unsophisticated animal, they pack a big behavioral wallop. Pheromones emitted by queen bees prevent other females from maturing sexually, ensuring that the queen's genes remain dominant. Among fish, scent markers released by females cause male sperm counts to quintuple overnight. When injured by a predator, some amphibians emit a compound that warns others of their species to keep out of harm's way.

It is in mammals that the pheromonal chatter climaxes. Countless species--from wolves to musk ox--claim territory by urinating around their borders, an olfactory keep-off-the-grass sign if there ever was one. Male voles use urine as a potent aphrodisiac, excreting a chemical that causes females to ovulate within 48 hours. "Identify anything that's of biological significance to animals," says Rachel Herz of the Monell Chemical Senses Center in Philadelphia, "and it's usually mediated by scent."

Uncovering similar tendencies in humans wasn't easy; McClintock began looking nearly 30 years ago. As an undergraduate at Wellesley College, she noticed that the women in her dormitory often developed remarkably similar menstrual patterns. In other animals, this kind of synchrony has survival advantages. "When you see others successfully rearing young," McClintock says, "it means it's a good time for you too."

The phenomenon intrigued McClintock, and she and co-researcher Kathleen Stern recently revisited it, hoping not just to observe ovulation but manipulate it. They recruited a group of 29 women and asked nine of them to wear pads under their arms for several hours, either before ovulation or just after. When the pads were wiped under the noses of the other women, the results were remarkable. Pre-ovulation pads shortened menstrual cycles by as many as 14 days in 68% of the women. When exposed to ovulation-phase pads, a different 68% experienced cycles that were as many as 12 days longer. Clearly, something was bringing the group into synch.

Among other scientists, the reaction to the study has been mostly positive--but questions remain. Even if human pheromones exist, it's not clear how the body processes them. Mammals and reptiles detect pheromones with a tiny nasal cavity called a vomeronasal organ, or VNO. Anatomists don't think humans have a VNO and aren't sure we would need one to perceive pheromones. Though a few researchers think they've found telltale pits just inside the nostrils, these may do nothing at all.

McClintock, meanwhile, is pushing ahead. Pheromone treatments designed to regulate ovulation, she says, could serve as fertility enhancers for couples who want to conceive and as contraceptives for those who don't. Other researchers think mood-altering pheromones could alleviate depression and stress. Still others think the chemicals might even control prostate activity in men, reducing the risk of cancer. New insights into how the body works, it seems, aren't right under our noses, but inside them.

--Reported by David Bjerklie/New York

With reporting by David Bjerklie/New York