Many nonhuman primates display the fact that they’re ovulating for all the world to see. A female mandrill’s perineum swells, while female chimps and baboons bring red and bulging rear ends into play. But in humans, ovulation is far more subtle. A woman’s temperature rises slightly around ovulation, for example. And there may be some behavioral changes: Some studies show heterosexual women become more flirtatious with men they find attractive, or their voices rise slightly in pitch. Most of these signals are not something that the average passerby would notice.
Now a study shows that a sign of ovulation might be written all over a woman’s face. But, the researchers conclude, it doesn’t have any biological importance. It’s refreshing to see this sort of negative data get published, because it can help inform future research. But the results also raise a question: What signs should we look for instead?
“When you visit the zoo, you’re told how chimps clearly advertise ovulation,” says Robert Burriss, an evolutionary psychologist at Northumbria University in Newcastle Upon Tyne, England. “Why are humans different? Did we lose those cues? The evidence is that we didn’t lose them, instead chimpanzees gained them.”
Could women have gained signs of ovulation as well? In a quest to find out, Burriss and his colleagues investigated flushed skin. “In a nonhuman primate, [sexually aroused] skin gets redder,” he explains. “We also know facial redness is seen as more attractive.” So while women might not walk around with violently red nether regions like chimps do, “they could leak cues,” Burriss says. “Not a signal, but something people could pick up on.”
The scientists recruited 22 women from the University of Cambridge to be photographed once daily for a month. The women kept track of exactly where they were in their menstrual cycles. Each day, the subjects were photographed without makeup and with their hair pulled back, a black cloth covering their clothes. The researchers analyzed the photos to see how the redness in each woman’s cheeks varied over her cycle.
The women had subtle variations in redness, with redness increasing just before ovulation, and staying high until menstruation. They were less red at the beginning of the cycle, before ovulation occurred. The changes were subtle but significant, the researchers report July 2 in PLOS ONE.
But the change is redness is so slight that the human eye has no chance of detecting it. “I think it’s kind of interesting in putting us in context of [other primates],” who experience reddening to a much greater extent, says Emily Barrett, a reproductive epidemiologist at the University of Rochester Medical Center in New York. “But scientifically we’re never going to use facial redness to measure whether someone is ovulating.”
It’s not necessary for every physiological change — even ones associated with fertility — to have a broader meaning in the context of finding a mate and reproducing. “There could be physiological explanations for both feminine appearances and redness at ovulation — it could just be a by-product of high estrogen at that time,” says Kate Clancy, an anthropologist at the University of Illinois at Urbana-Champaign. “And estrogen's main target tissues are the brain, mammary glands and reproductive organs, but it's not like it doesn't go everywhere.” Or the slight flush could be a by-product of increased blood flow or temperature.
Even if the results — as in this case — turn out to be unimportant, Barrett notes that scientists still need to get the information out there. In this case, the reddening was real, but if a tree falls in the forest — or a face reddens at ovulation — and no one can detect it, it’s still pretty meaningless. “When you get a null result, often people don’t publish,” she says. “It’s hard, it’s not sexy or exciting.” But, she says, “If you don’t publish, people just do [the same experiment] over and over.” For research to be productive, other scientists need to know what doesn’t work as well as what does.
But why, Clancy asks, is there so much emphasis in research on passive changes such as redder skin? Why isn’t there more focus on what women do and the choices they make? “Females across primate taxa make decisions about who to mate with, overtly and covertly, all the time,” she notes.
Behavioral studies that examine active behavior would be a start. But, as Burriss notes, the field of attraction is still wide open for studies from many different angles. “The most important decision many of us make is who we’re spending our lives with and making a family with,” he says. “We know very little about how that happens. A lot of questions haven’t been answered yet.”
Editor's note: This article was updated on July 13, 2015, to correct that redness increased until (not through) menstruation.