A question for the men out there: ever find yourself becoming mysteriously aggressive towards rugged, super-masculine men once a month? The reason could lie with your wife or girlfriend - scientists have found that men become jealous of other dominant-looking males when their partners are at their most fertile.
The research, published today in Evolution and Human Behaviour, suggests the feelings are a protection strategy to prevent partners being stolen away by rivals at a time when they are most likely to be able to pass on their genes to offspring.
Rob Burriss, a biologist at the University of Liverpool, who led the study, said the behaviour reflected that of other primates: "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."
His study supports the idea that women would naturally prefer to reproduce with dominant-looking males as they get closer to becoming fertile, because they would have higher levels of testosterone, pointing to better genes to pass on. Dr Burriss's work shows men have developed compensatory tactics to thwart infidelity when their partners are most fertile.
In the experiment, scientists asked 64 volunteers to rate the perceived dominance of a series of men with neutral expressions. The images they were shown were photographs that had been rated on a seven-point scale ranging from "not at all dominant" to "very dominant".
Men whose partners were near ovulation tended to rate the faces as more dominant than men whose partners were at a less fertile stage.