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March 12, 1998
Study Finds Signs of Elusive Pheromones in Humans
By NATALIE ANGIER
hey may be odorless and colorless and their function may be mysterious, but human pheromones at last have the zest of scientific truth.
Researchers at the University of Chicago have demonstrated that compounds swabbed from the underarms of young women at different times of the month can alter the length of other women's menstrual cycles, compressing or expanding the cycles in predictable fashion.
The experiments offer the first solid proof of the existence of human pheromones, compounds produced by one individual that can influence the biology or behavior of another. Over the last few decades, scientists have identified a wide variety of insect and animal pheromones, but until now, the evidence for their relevance to human affairs has been circumstantial, equivocal and bitterly contested.
The new results, being reported Thursday in the journal Nature, show that people are indeed capable of being led by the nose.
"This is a very exciting study that is going to make a lot of researchers sit up and take notice," said Dr. Charles Wysocki of the Monell Chemical Senses Center in Philadelphia. "It has the experimental safeguards and controls that have been lacking in the past."
The findings also cast new light on the widely discussed but much disputed phenomenon of menstrual synchrony, in which women who live together supposedly find their monthly cycles coinciding over time. Dr. Martha McClintock, the senior researcher in the new study, first reported her observations of menstrual synchrony in 1971. Since then, some researchers have confirmed her finding, while others have failed to find supporting evidence for it.
The latest data reveal that women release a complex bouquet of pheromones throughout the month, with the result that synchronicity among female roommates is possible, but by no means guaranteed.
"Finding menstrual synchrony was like finding a fossil tooth," said Dr. McClintock. "Thirty years later, we've excavated the whole beast, and it turns out to be a whole lot more interesting than that first tooth alone."
She and her co-author, Kathleen Stern, have determined that compounds taken from the underarm secretions of women who were in the early, or follicular, phase of the menstrual cycle can shorten the cycle of women exposed to the extracts.
By contrast, compounds extracted from the women at midcycle, when they are ovulating, can have the opposite effect on recipients, lengthening their menstrual cycle. Underarm effluvium taken from women after ovulation, in the luteal phase of the cycle, has no detectable impact on the timing of other women's periods.
The researchers have yet to isolate the actual pheromones, but they said the work might eventually yield new birth-control methods and treatments for infertility.
Other researchers predicted that the discovery would throw wide the door on pheromone research. They said that subliminal chemical cues very likely underlay many human behaviors, including mate choice, nepotism, dominance struggles, even xenophobia.
"Now we can stop horsing around and go after these chemicals to find out what they are," said Dr. Thomas Eisner, a chemical ecologist at Cornell University in Ithaca, N.Y. "There should be no barrier to the isolation and elucidation of these molecules."
Scientists emphasized that humans were unlikely to be as rigidly beholden to pheromones as are certain other creatures. When an ovulating female boar is exposed to a pheromone from a male boar's saliva, for example, she immediately freezes into a spread-legged mating posture, a fact that pig farmers use to facilitate artificial insemination.
In the new study, the scientists asked nine healthy women to wear cotton pads under their arms for eight hours at a stretch during distinct phases of their cycles. The pads were then cut into sections, treated with alcohol, and wiped under the noses of 20 other women every day for a month. The recipients said they could smell nothing from the pads except the alcohol used as a suspension.
The effects of the applications, however, were quick and significant. In 68 percent of the women, the follicular-phase extracts shortened their current menstrual cycle by anywhere from one to 14 days, with an average abridgment of 1.7 days. When the same group of women were later swabbed with ovarian-phase extracts, a different 68 percent of them were affected, this time by a lengthening of their cycle by from one to 12 days, with an average expansion of 1.4 days.
Because the pheromonal compound either stretched or condensed a recipient's cycle depending on its origin, the new study circumvents complaints about previous studies, when critics pointed out that the random fluxes of women's periods could make coincidental convergence look like pheromonally orchestrated synchrony.
What remains unknown is why women are sensitive to one another's menstrual cycles in the first place. The McClintock laboratory has shown that communal ovulation occurs in rats, and that the female rats benefit by being able to conceive, give birth to and nurse their pups en masse. Through a sharing of lactational duties, the females rear larger offspring more quickly than they do when nursing on their own.
Thus it makes sense for rats to have a method of communicating their reproductive status to their peers, and for dilatory cyclers to speed up or fast cyclers to slow down in order to coordinate their timing of ovulation and conception. In theory, ancestral women might also have benefited from group cycling, group mothering, and the ability to breast-feed one another's young.
But Dr. Jane Lancaster, an anthropologist at the University of New Mexico in Albuquerque, said human gestation and lactation were too drawn out for the rodent form of communalism to apply.
"Great apes don't have that kind of group synchronicity, so why should we?" Dr. Lancaster said.
Instead, Dr. Lancaster hypothesized that a young woman who is entering puberty might benefit from being responsive to the cycles of her mother and older sisters.
"In this case, competent ovulators help their female kin during development to become regular cyclers and ovulators themselves," she said. Alternatively, women might use pheromones aggressively, as part of female-female competition. Co-wives of a polygamous man might seek to suppress one another's fertility, for example.
"Reproduction is a very time-consuming, risky business," Dr. McClintock said. "The more cues a female has to her social setting, and whether this is the right time to try getting pregnant, the better off she'll be."
In other words, nosiness pays.
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