TUESDAY, June 30, 2015 (HealthDay News) -- Women's faces are redder than usual during ovulation, but this subtle change isn't detectable to the human eye, new research shows.
This change may be a possible signal of peak fertility, and it may have evolved to become less noticeable since controlling or hiding ovulation has greater benefits, the researchers suggested.
In primates, males only express interest in females when it's apparent they're ovulating. Among humans, however, ovulation is not obvious and sex is not limited to the period of time when women are ovulating.
Researchers investigated changes in the color of women's faces over the course of a month. The study involved 22 women. They were photographed daily without makeup using a scientific camera able to capture color more accurately than a typical camera.
"We were able to recruit undergraduates in a number of colleges and photograph the women just before they had dinner in the college hall every evening. The collegiate routines and networks were vital to collecting data with such regularity," the study's leader, Hannah Rowland, from the zoology department at University of Cambridge in England, said in a university news release.
A computer program then selected the same patch of cheek from each woman's photo. The images were converted into red, blue and green values to assess changes in color levels.
Meanwhile, the women tested themselves for hormonal changes at specific intervals during the month. The researchers identified when the women were most fertile based on these levels. Those who experienced a spike in their luteinizing hormone level were expected to ovulate within 24 hours.
Redness in the women's faces changed significantly throughout the month, but peaked at ovulation, the study published online June 30 in PLOS One found.
The women's faces remained extra red until after estrogen levels fell. The largest average difference in redness, however, wasn't a level high enough to be detected by the human eye.
Once menstruation began, the redness in women's faces dropped to much lower levels. The researchers said this redness closely matches changes in body temperature throughout an ovulation cycle.
"Women don't advertise ovulation, but they do seem to leak information about it, as studies have shown they are seen as more attractive by men when ovulating," said Rowland. "We had thought facial skin color might be an outward signal for ovulation, as it is in other primates, but this study shows facial redness is not what men are picking up on -- although it could be a small piece of a much larger puzzle."
Since people -- and other primates -- are attracted to red, women may subconsciously enhance this naturally occurring facial redness with makeup or red clothes, the researchers suggested.
"As far back as the 1970s, scientists were speculating that involuntary signals of fertility such as skin color changes might be replaced with voluntary signals, such as clothing and behavior," the study's co-leader, Robert Burriss, a psychologist from Northumbria University in England, said in the news release.
"Some species of primate advertise their fertility through changes in the color of their faces. Even if humans once advertised ovulation in this way, it appears that we don't anymore," he said.
Another theory is that women blush more often when they are in the company of attractive men, the study authors said.
"Other research has shown that when women are in the fertile phase of their cycle they are more flirtatious and their pupils dilate more readily, but only when they are thinking about or interacting with attractive men," said Burriss. "We will need to do more research to find out if skin redness changes in the same way."
The American Pregnancy Association has more about ovulation.