Men become jealous of dominant-looking males when their female partners near ovulation, British researchers say.
Previously, scientists have found a woman's preferences for male physical appearances varies depending on her fertility status.
Now they've found a man can pick up on the shift in his female partner's preference and use a counterstrategy.
During ovulation, women seek out masculine-looking men with features linked to high testosterone levels thought to show high quality genetic traits that can be passed on to children.
"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," said biologist and study author Robert Burriss of the University of Liverpool.
"Similarly in humans, rated dominance increases when the female is most fertile. What is interesting here is that male behaviour is determined by that of the females; men become more wary of masculine-looking men only when the females facial preferences begin to shift prior to ovulation."
Strong jaw lines versus rounded chinsFace shape and structure are considered good indicators of dominance.
Men with large eyes, rounded chins and full lips are viewed as more feminine when choosing long-term partners, but they aren't seen as dominant, Burriss said.
Women are also more likely to have an affair with a masculine-looking man during their fertile phase, studies suggest.
Burriss and co-author Anthony Little made composites images of male faces that were either high or low in dominant features, such as strong jaw lines and thinner lips.
They found a man seems to sense his partner's shift in preference for more masculine males during her most fertile phase, that is, before ovulation.
Men respond by finding masculine men more threatening, according to the ranking results of dominance perceptions. Dominance was defined as someone who looked like they "could get what they wanted."
Participants were asked about whether their female partners used oral contraceptives and the date of her current or previous period.
Males whose partners did not take the pill and were near ovulation rated masculine faces as more dominant compared to others, the team reports in this week's online issue of the journal Evolution and Human Behavior.
"We conclude that males have evolved counterstrategies to deal with female infidelity that include an overattribution of dominance to those rivals most likely to present a threat at times when that threat is greatest," the pair wrote.
"This overattribution is likely to lead to increases in jealousy and mate-retention behaviours."