Women redden around ovulation

It's not just embarrassment or blushing that makes a lady redden: women also become subtly redder when they are ovulating, a new study has shown. Hannah Rowland told Chris Smith how she went about making this discovery.

Hannah - About 10 years ago, somebody did some research on whether women changed in attractiveness across their ovulatory cycle so, their reproductive cycle. He found that when women were ovulating, so when they were the most fertile, they were rated by male participants as more attractive. And so, over the last 10 years, more and more research has come out, showing that women's behaviour changes, their voices change, their odour changes, so they smell differently and they're always more attractive when they're fertile. But in those 10 years, nobody has really got to the crux of why women are more attractive. And so, we hypothesised that perhaps it was explainable by changes in skin colour. So, chimpanzees and mandrels and other primates, when they're fertile, they have really obvious and conspicuous changes. So they get red bottoms and macaques get red faces. So, we hypothesised that perhaps women also had redder, more attractive faces.

Chris - It's like a sexual traffic light then although in this case, red doesn't mean stop. It means go.

Hannah - Absolutely. So, red is a very attractive colour to us and to other non-human primates.

Chris - Actually, I've got a big red end on my microphone so I wonder what that says about me. How did you do this study?

Hannah - We had two undergraduate students who were all data collectors and they recruited their friends to come and have their photograph taken. So, they came every weekday and we asked them personal questions about their fertility cycle and we also asked them to use an ovulation test kit, so that they could detect the hormones that signal when they're ovulating.

Chris - What was the outcome apart from taking these pictures?

Hannah - So, we designed a programme to detect cheek patches and measured the red, green and blue colour of the photograph. So, we then converted the red, green, and blue using models from psychologists of how the human eye responds to colour. We modelled how it would be perceived, how these cheek patches would be perceived by the human eye.

Chris - Can my eye tell when you're ovulating?

Hannah - Really interestingly, we found that women do get redder when they ovulate but it's just below the level detectable by the human eye.

Chris - Did you ask any real humans?

Hannah - No. That's the next step.

Chris - Doctors talk about a phenomenon called chloasma, which is when women have a high oestrogen state and they classically develop this during pregnancy but also, when they're using the oral contraceptive pill because oestrogen does change blood flow through the skin. Is this just because when a person is normally cycling, the oestrogen changes are just a little bit less than they would be in something like pregnancy. So it's there, it's just too subtle for us to see.

Hannah - You're right. Oestrogen is implicated in skin colour, but also, women with higher oestrogen levels have paler skin. Pale skin is very attractive in other studies. So actually, when we predicted that skin colour would be the causation of this attractiveness, we thought it would be tied to oestrogen. But when we map our colour change across the cycle so it goes very low, it drops significantly during menses and then rises rapidly towards ovulation and then stays high, when you map that onto the textbook oestrogen cycle, it doesn't map very well.

So, oestrogen peaks ovulation but it drops again just before menses, but our change in red actually maps basal body temperature much better. So, there's probably a combination of mechanisms causing this subtle change in redness.

Chris - If it's not detectable by the average male, then what's the point of it?

Hannah - Humans are sexually active across the month, across the year whereas other primates are only receptive and only mate when they are fertile. And so, there's a benefit to concealing ovulation in humans because it means that men are more likely to stick around and care for any offspring rather than, like you see in chimps, where males are only interested in receptive females and then go off and are more interested in the next receptive female. So, I think ovulation is concealed in women, but there are these little bits of information - voices, odour, behaviour - leaking this information.