Women change colour across the month and become subtly redder when they are at their most fertile, a new study has shown.
Several studies over the recent decade have shown that human females appear to subtly, and subconsciously announce their fertility, leading males to, on average, rate them as more attractive at times when they are most likely to conceive.
Giveaway signals include changes to a woman's tone of voice, behaviour and demeanour, posture and gait also alter, and even clothing colour, with many demonstrating a penchant for racier, red colours at the right time in the month.
Cambridge University's Hannah Rowland and her colleagues wondered whether a woman's overall skin colour might also change in sync with the menstrual cycle and serve as a further cue to advertise fertility.
Making use of the Cambridge collegiate system, which sees students living in halls and therefore within easy regular reach, twenty-two participants with regular menstrual cycles and who were not using oral contraceptives agreed to be photographed on a daily basis and also carry out biochemical tests to confirm the timings of their ovulations.
The team used a computer system to isolate the red, green and blue colour hues present in their images from a standardised part of the cheek of each participant.
This showed that, at the time of ovulation, the participants' skin tones became significantly more red.
However, when these colour values were fed into a computer model of the workings of the human eye, the changes turned out to be just below the threshold of human detection.
Critically, the researchers haven't yet tried showing the images to real human raters. "That's the next study," explains Rowland.
Nevertheless, the results, published this week in PLoS One, show that, unlike our macaque relatives, which develop pronounced colour changes to their faces and buttocks to advertise their fertility, the same is not true in humans.
"We think the red colouration is actually reflecting body temperature," says Rowland.
So why do human females not advertise their receptive periods like some other apes?
"The difference is that we are sexually active across a whole month, whereas these other primates only mate when they are fertile," Rowland explains. "It's probably evolutionarily advantageous to women not to overtly advertise fertility, because this helps to ensure that men will stick around and help to care for any offspring..."