Women's faces get redder during ovulation but this colour change is so subtle that it is undetectable by the human eye, according to a new study.
Previous studies have shown that men find female faces more attractive when the women are ovulating. The new study rules out skin colouration as a driver of the attractiveness effect.
Researchers said the findings suggest that facial redness in females was once an involuntary signal for optimal fertility, but has since been "dampened" by evolution as it is more beneficial for females to hide or control outward signs of peak fertility.
Involuntarily signalling ovulation can prevent longer-term investment from males. In primate species that advertise ovulation, males only express sexual interest in females when they appear to be fertile.
In humans, ovulation is less conspicuous and sexual behaviour is not restricted to the period of peak fertility.
The research, published in the journal PLOS ONE, is the most complete objective study of female faces during the ovulatory cycle.
Twenty-two women were photographed without make-up at the same time every working day for at least one month in the same environment and using a scientific camera modified to more accurately capture colour (usually used for studying camouflage in wildlife).
A computer programme was designed to select an identical patch of cheek from each photograph. The participants also self-tested for hormone changes at key times dictated by the research team's "period maths."
A surge in luteinising hormone showed that ovulation would occur in roughly the next 24 hours, so researchers knew which photographs were taken when the women were most fertile.
The team converted the imagery into red/green/blue (RGB) values to measure colour levels and changes.
They found that redness varied significantly across the ovulatory cycle, peaking at ovulation and remaining high during the latter stages of the cycle after oestrogen levels have fallen. Skin redness then dips considerably once menstruation begins.
The research suggests facial redness closely maps fluctuations in body temperature during the cycle.
However, when running the results through models of human visual perception, the largest average difference in redness was 0.6 units. A change of 2.2 units are needed to be detectable to the naked human eye.
"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 Dr Hannah Rowland, from the University of Cambridge's Zoology Department.
"We had thought facial skin colour 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," she said.