Men are more aware of potential love rivals when their partners are at their most fertile, a study suggests.
A composite image of a dominant face (l) next to a submissive one
A Liverpool University study of 64 men suggested those with ovulating partners were more wary of the dominant-looking men who attract women at this time.
Those with partners not at risk of conception are less aware, the Evolution and Human Behaviour study suggested.
Earlier research shows ovulating women are more attracted to dominant men.
Arnie facial type
Researchers in the university's School of Biological Sciences showed their subjects, all of whom were in long-term relationships, a series of 66 pictures of two facial types - dominant and submissive.
They were then asked to rate the pictures for dominance, with a dominant person being defined as someone who "appeared as if they could get what they wanted".
Those with partners in the most fertile stage of their menstrual cycle were more able to spot classic masculine face types - ie men with strong jaw lines, thinner lips and smaller eyes.
But those with partners who were not at risk of getting pregnant at that particular time were not.
Researcher Rob Burriss said many previous studies show that women's preferences for male physical appearance change according to their fertility status.
He said during the fertile stage of their menstrual cycle they are more likely to be attracted to, and have an affair with, a masculine-looking man.
"Earlier research has shown that women are more interested in men who have a high testosterone face type - a jaw line like Arnold Schwarzenegger for example - for a short term relationship.
"They have better genes and are more likely to have strong children.
"This is because high testosterone levels create stronger immune systems which make them more resistant to disease."
He surmised that men with fertile partners were sub-consciously aware of this preference change from their female partner and were reacting to it.
He said: "Groups of animals, such as chimpanzees, can live quite happily together, but when a female is ready to mate the two dominant males within the group become rivals and fight for her attention.
"Similarly in humans, rated dominance increases when the female is most fertile."
He said it was interesting that the male behaviour appeared to be determined by that of the females and could not see another reason for it.
"Men become more wary of masculine-looking men only when the female's facial preferences begin to shift prior to ovulation," he added.
Dr George Fieldman, principal lecturer in psychology at Buckinghamshire Chilterns University College said the paper was interesting because it suggested men perceived greater dominance when women were fertile.
"The issue is about having a strategy to prevent or preclude infidelity of the female partner.
"Whether they consciously know it or not, women do tend to have affairs with dominant males during their most fertile phase.
"That's independent of whether they want to get pregnant or not."