New Delhi, June 30: Women's faces acquire extra redness during ovulation, but it's not this subtle change in complexion but their behaviour, clothes or other factors that men seem to find attractive, a study exploring the biology of facial attractiveness has suggested.

The study by a team of scientists in the UK is the first to show conclusively that women's faces gain extra shades of red during ovulation, but it has also found that these variations in facial colour are unlikely to be detected by human eyes.

The scientists believe the changes in facial colour are among involuntary physiological or behavioural cues that women display without really knowing it to highlight ovulation - the release of a mature egg from an ovary into a fallopian tube where it waits for about 24 hours to be fertilised.

Over the past decade, independent research groups have established that females of several primate species signal their ovulatory status through physiological features such as changes in skin colour or swelling in genital areas.

Earlier studies have also suggested that women in their fertile phase of the ovulatory cycle are more flirtatious and their pupils dilate more readily when they are thinking about or interacting with attractive men. Six years ago, a team of US scientists had shown that women also make greater efforts to augment their beauty during ovulation, wearing fashionable or red clothing.

"We wanted to determine whether facial skin colour is an outward signal for ovulation in women - we find that women's faces do get redder, but these changes in redness are probably not perceivable by humans," said Hannah Rowland, a zoologist at the University of Cambridge and the Institute of Zoology, London.

"But we know from earlier studies than women are seen by men as more attractive when ovulating - so there must be other factors that contribute to that attraction," she told The Telegraph.

Rowland and her colleagues photographed 22 women for a month using a scientific camera modified to accurately capture colour and used a computer program to analyse changes in an identical zone of their cheeks.

They found that redness increased during the second week of the menstrual cycle, remained elevated during the days just ahead and during ovulation, and decreased rapidly after the onset of menstruation. Their study's findings were published today in the journal PLOS ONE.

Although the colour change is too feeble to be noticed by men, the scientists say they are now closer to finding out what it is that changes in women's faces over the cycle.

"We can rule out colour changes, but there are probably still some effects of the ovulatory cycle on skin and face appearance," said Robert Burriss, a psychologist at Northumbria University and member of the study team. "It is possible that other cues such as differences in head posture or tiny facial expressions may be at work."

Rowland said the next phase of the study would seek to determine whether skin colour changes more significantly over one part of the face than another. "We only looked at the colour of the cheeks. Perhaps lips change colour more dramatically," she said.

The scientists say the patterns of changes in facial redness they observed are not explained entirely by the hormone oestrogen, which peaks in the days preceding ovulation but is relatively low during the rest of the ovulatory cycle.

"Research published earlier this year indicates that oestrogen and a red face go together," said Burriss. "But we found that redness more closely follows changes in expected basal body temperature."

The UK scientists and other biologists caution that the findings are primarily based on Caucasian women and subtle changes in redness might not matter at all in women with darker skin tones.

"The nice thing about this study is that they have tried to test whether the colour variation visible to software is also visible to the human eye," said Anindita Bhadra, a zoologist at the Indian Institute of Science Education and Research (IISER), Calcutta, who was not associated with the study. "But I would have given the researchers more cookies if they had used the same photographs to score attractiveness to real men," she told this newspaper.

Burriss also cautions that makeup can mask physiological changes. "We know that when women wear makeup, there are no differences in how attractive they appear to be from the most to the least fertile stage of the cycle - any changes in skin colour are probably overwhelmed by the makeup."

Some scientists suspect that women have shifted from involuntary physiological signalling to conscious signalling, although others are sceptical about such suggestions. "This is a tricky argument," said Bhadra.

"Evolutionary theory predicts that such signals should be honest to be evolutionary stable. If someone can provide convincing evidence against this in humans, it would be path-breaking, but this has not been proven conclusively yet."