London, July 1 (IANS): Revealing why men find women's face more attractive when they are ovulating, an interesting research reveals that redness increases in the faces of females during the peak fertility period but human eyes cannot pick up on it.

"Women do not advertise ovulation but they do seem to leak information about it. Previous studies have shown they are seen more attractive by men when ovulating," said Dr Hannah Rowland from University of Cambridge.

For the study, 22 women were photographed without make-up every working day for at least one month in the same environment and using a scientific camera modified to more accurately capture colour.

The participants also self-tested for hormone changes at key times dictated by the research team's "period maths".

The team found that redness varied significantly across the ovulation cycle, peaking at ovulation and remaining high during the latter stages of the cycle after oestrogen hormone levels have fallen.

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 is needed for it to be detected by the naked human eye.

"Facial redness is not what men are picking up on although it could be a small piece of a much larger puzzle," Dr Rowland said.

Skin redness dips considerably once menstruation begins.

This may mean 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.

"It may be that, during ovulation, women have a greater propensity for blushing when around men they find attractive."

Other research has shown that when women are in the fertile phase, they are more flirtatious and their pupils dilate more readily, but only when they are thinking about or interacting with attractive men.

The research was published in the open-access journal PLOS ONE.