By David N. Leff

Try this simple experiment.

Let a young mother sniff a T-shirt from her newborn infant and one from a different new baby. Chances are she'll unerringly recognize which belongs to her child.

Yet neither garment gives off a detectable odor. The olfactory message to her nose is subliminal. It strongly suggests that she and her own offspring share a chemical message.

In the subhuman animal world, from primates to insects to yeast cells, these airborne chemical social signals are attributed to pheromones. By definition, they affect the physiology or behavior of other members of the same species.

"For example," pointed out neuroendocrinologist Martha McClintock, "pheromones influence mating preferences in hamsters, dominance relationships among male elephants in musth [their annual period of frenetic sexual and aggressive behavior], timing of weaning in rats, how rat pups learn to distinguish edible foods from poisons . . . and the level of stress experienced by a mouse in a new environment on the basis of the emotional state of the previous occupant."

And pheromones mobilize fireflies to flash in synchrony.

In 1971, McClintock, a professor of psychology at the University of Chicago, showed that women, such as roommates, living together tend to synchronize their menstrual cycles. This suggested a role for pheromones in humans, but it remained unproven.

Scientists presumed that the pheromonal pathway from nose to brain is atrophied in Homo sapiens. Mammals as a rule receive pheromone messages via a small receptor-lined tubular structure, the vomeronasal organ, next to the nasal cavity. It transmits incoming information to brain regions, including the hypothalamus, which manages much of reproductive physiology and behavior.

In fact, the 1995 edition of a leading medical dictionary defined the vomeronasal organ as "a fine vestigial horizontal canal, ending in a blind pouch . . . which usually regresses after the sixth month of gestation."

McClintock set out to prove that odorless pheromone-broadcast information is alive and active in humans. Her research report clinching this finding appears in today's Nature, dated March 12, under the title: "Regulation of ovulation by human pheromones."

In a commentary accompanying the Nature article, endocrinologist Aron Weller of Bar-Ilan University, in Ramat-Gan, Israel, questioned McClintock's pheromonal interpretation of menstrual synchrony. "Although vomeronasal organ olfaction has been proposed as the most likely candidate," he wrote, "it is also possible that menstrual synchrony is mediated by visual or auditory signals, through mutual social activities, similar daily schedules or exposure to similar stimuli."

Armpits Record Daily Changes In Pheromonal Activity

The Chicago psychologist enlisted 29 young women, ages 20 to 35, from the university staff and student body. These volunteers all had histories of regular and spontaneous ovulation.

Nine of the women, the donors, wore 4-by-4-inch cotton pads in their armpits for at least eight hours every day, to collect the odorless pheromone compounds. The sequential pads were like tape recordings of pheromonal activity day by day throughout their menstrual cycles.

McClintock explained: "As in other species, human pheromones might be produced by four potential sources: apocrine glands (active only during reproductive maturity); eccrine glands (which produce sweat that contains compounds found also in saliva and urine); exfoliated epithelial cells; or bacterial action. We collected compounds from axillae [armpits] because they contain all four of these potential sources."

For four consecutive menstrual cycles, the 20 recipient women had pieces of the donor-collected pads wiped under their noses. And each donor provided a urine sample every evening, which detected the onset of the luteinizing hormone surge that triggers ovulation. Moreover, it clearly distinguished the follicular from the ovulatory phase of their menstrual cycles.

Can Pheromones Feel Your Pain?

The responses to the controlled and blinded experiment reflected the action of two separate pheromones, one for each phase.

"We found," McClintock reported, "that the recipients had shorter cycles when receiving axillary compounds produced by donors in the follicular phase of the menstrual cycle, and longer cycles when receiving ovulatory compounds — which represent significantly different opposite effects.

"Although a significant proportion of women in this experiment responded to the pheromones with changes in cycle length in the expected directions (68 percent of women responded to follicular pheromones, 68 percent to ovulatory pheromones), some women did not," the Nature paper added, reporting that some cycles were shortened from one to 14 days, while others lengthened from one to 12 days.

In her paper, McClintock pointed out a potential payoff from her findings: "From this initial test of human ovarian-dependent pheromones we do not know whether the phenomenon is fragile — that is, limited to ovulation timing in healthy young women — or robust, and so capable of modulating ovulation in a diverse population for either contraception or treatment of infertility."

Israel's Weller went further. "Because odors have well-known influences on emotion, perhaps human pheromones complement other sources of interpersonal information," he suggested. "This may result in feelings such as emotional contagion (experiencing another person's feelings), sympathy, empathy and their accompanying physiological reactions."

A recognized authority in the pheromone field, Weller also proposed that, "To test whether humans actually do communicate by pheromones in modern society, we could examine whether a phenomenon such as menstrual synchrony exists in women with no sense of smell."

McClintock said well-controlled studies of humans are needed "to determine whether there are other types of pheromones, with effects that are as far-reaching in humans as they are in other species."

She concluded: "Further work in this area may well reveal that, as in rats, social interactions mediated by ovarian-dependent pheromones affect age of puberty, interbirth intervals, age at menopause and level of chronic estrogen exposure throughout a woman's lifetime." *