It’s a classic high school conundrum: Women want female companionship, but definitely don’t want a so-called friend to try and steal their partner. Or, in scientific parlance:

For women, forming close, cooperative relationships with other women at once poses important opportunities and possible threats—including to mate retention. To maximize the benefits and minimize the costs of same-sex social relationships, we propose that women’s mate guarding is functionally flexible and that women are sensitive to both interpersonal and contextual cues indicating whether other women might be likely and effective mate poachers.

In a paper published on Thursday (Jan. 14) in the Journal of Personality and Social Psychology, psychologists from Arizona State University studied women’s efforts to guard their mates from other ovulating women.

The team conducted four studies with a total of 478 heterosexual engaged or married women. All participants were recruited from Amazon Mechanical Turk, a crowdsourcing online marketplace. In each study, the women were shown photographs of a series of women, and were asked how willing they would be, on a seven-point scale, for the women to befriend their partner.

Participants were significantly more likely to want to create distance between the photographed woman and their partner when the woman shown in the photograph was ovulating.

The women were not told which women were ovulating and, in all likelihood, they didn’t consciously consider the idea. But humans seem to be subconsciously aware of various physical cues that indicate when women are more fertile, as found by several studies.

“Research across species demonstrates that social perceptions and behaviors do temporarily shift in response to ovulation.”

“Research across species demonstrates that social perceptions, cognitions, and behaviors do temporarily shift in response to ovulation, and that these shifts may enhance individuals’ reproductive fitness,” write the authors. “Similarly, psychological research on humans has demonstrated that (a) women’s perceptions and behaviors shift across their own cycles and (b) men respond to these cyclic shifts.”

As the tests continued, the psychologists found that women were especially protective when subjects viewed their mate as desirable, and when the woman in the photograph was physically attractive.

Besides creating physical distance, the authors note that women employ other tactics designed to keep their partners close: “Specifically, women with desirable partners reported that they would show increased sexual interest in their partners after viewing a high-fertility target, regardless of how attractive that target was.”

According to the researchers, however, there’s no evidence from the study that “mate guarding” is all that effective. Another caveat: the study relies on composite photos of strangers. In real life, when socializing with friends, colleagues, and acquaintances, women may well choose to trust their friends and worry less about ovulating threats. After all, even if this mate-guarding is strategic, it sounds pretty exhausting.