A woman's tendency to be catty about another may actually be traced to evolution.

And they apparently unleash most of their scorn against other women at the peak of their fertility each month as a weapon in the mating game, according to Canadian researcher Maryanne Fisher, whose study is published in Biology Letters, an online journal of the Royal Society.

By belittling the competition, fertile women are better poised to fight for a mate.

Fisher asked 57 women and 47 men to look at pictures of female and male faces and to rate their attractiveness.

The men were fairly consistent in their voting. But women's votes on female faces were "significantly" affected by the stage of their menstrual cycle. Yet their rating of male facial attractiveness remained identical during periods of high and low fertility.

Fisher said poor ratings on attractiveness in the study likely translate to disparaging remarks between women in the real world.

"The theory of intrasexual competition in women has been controversial, but this study shows that womens' perceptions of potential rivals' attractiveness is considerably altered depending upon the state of their own fertility," Fisher said in a statement. "This demonstrates a potential intrasexual competitive process."