Following Our Noses

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."