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By Amanda Gardner

WEDNESDAY, February 8, 2012 (Health.com) — The elaborate courtship displays found in the animal kingdom—a peacock spreading his feathers, the hissing of the Madagascar cockroach—aren't always appropriate in an office or classroom. Male humans seem to have devised other, less obvious ways of showing off.

A new study suggests that when young men interact with a woman who is in the fertile period of her menstrual cycle, they pick up on subtle changes in her skin tone, voice, and scent—usually subconsciously—and respond by changing their speech patterns.

Specifically, they become less likely to mimic the woman's sentence structure. According to the researchers, this unintentional shift in language may serve to telegraph the man's creativity and nonconformity—qualities that are believed to attract potential mates.

"At least a part of the interpersonal dynamic that exists when men and women are getting to know prospective romantic partners is being governed by the biology of a woman's body and men's sensitivity to these biological factors," says Michael Kaschak, PhD, the senior author of the study and an associate professor of psychology at Florida State University, in Tallahassee.

This idea isn't new to evolutionary psychologists, who have long known that males of various species will change their behavior if they're trying to find, or hold on to, a mate. In humans, these displays can include risk taking (rashly choosing to "hit" in blackjack, say), writing a romantic poem, or using big words.

Moreover, previous research has found that female fertility cues tend to trigger this type of behavior in men. With that knowledge in mind, Kaschak and his coauthor designed a pair of experiments to test whether a woman's fertility affects male speech. Their findings appear in the journal PLoS One.

The first experiment included 123 male and five female college students, all of them heterosexual. The researchers tracked each woman's fertility by marking the beginning and end of her menstrual cycle. At various points in the cycle, they paired off a woman with one of the men in a laboratory.

Next page: Effect seen only in male-female interactions

The man and woman sat together at a table and were given a few minutes to interact, so the man could discern fertility cues, if there were any. To make cues as noticeable as possible, the women refrained from using makeup, perfume, or scented shampoo.

Researchers then gave each partner a stack of line drawings depicting simple scenes—one child giving another child a toy, for example. The woman was asked to provide a one-sentence description of each picture ("Meghan gave Michael a toy"), and the man responded by doing the same with one of his own pictures. As he did so, the researchers compared the structure of his sentences to that of the woman's.

When the women were less fertile, men copied their sentence constructions 62% of the time. But the rate dropped to just under 50% when the women were at peak fertility.

What's more, when the researchers repeated the experiment with women only, fertility had no discernible impact on sentence structure. "It didn't show the same effect at all," Kaschak says. "The effect was specific to men."

There is a catch, however: Men in the first experiment who thought their partner was being flirtatious were actually more likely to match their sentence structure to hers. In this scenario, Kaschak says, the male's priority might be to reciprocate the female's interest rather than draw added attention to himself.

"If the woman seems noncommittal, then maybe the correct strategy is to do something to try to stand out a little bit, to try to get her attention," he says. "Maybe you can drum up some interest."

The findings will likely be of interest to language researchers, especially those who study so-called "linguistic alignment" in relationships, Kaschak and his coauthor suggest. For the rest of us, they're a reminder that conversations between men and women are often more complex than they might seem on the surface.

"A lot of the behavior that we exhibit when we interact with other people happens on an unconscious level," Kaschak says.