It appears all really is fair in love and war for women - scientists say they are programmed to be critical of rivals when they are looking for a man.
Women criticise each other more at certain times of the month
Researchers at York University in Toronto, Canada, say women are prone to be at their cattiest when they are at their most fertile.
They are more likely to criticise other women's appearance during the days around ovulation.
The aim is to attack rivals to boost their own chances of finding a mate.
This extra-critical period may last for up to 10 days a month.
Dr Maryanne Fisher, who led the research, told BBC News Online: "When women are at their most fertile, they'll pay more attention to each other's appearance.
"They are more likely to criticise, and to do it in a more pronounced way.
"There can be more catty behaviour, there will be more gossiping, nit-picking and spreading of nasty stories.
"You might see two women in a pub and one might say to the other 'Oh God, look at her, she's so ugly' or 'your hair is such a mess'.
"That is an example of a competitive strategy."
The researchers asked around 100 male and female college students to look at photographs of 35 women and 30 men and assess how attractive they were.
The women were also asked what stage of their menstrual cycle they were at.
It was found women rated men as equally attractive regardless of what stage they were at in their cycle.
But women who were ovulating rated the female faces as less attractive than those looking at the photos who were not in the fertile stage of their cycle.
The researchers, led by Dr Fisher, say this is probably only one way in which women compete with each other for the attentions of men.
Writing in the journal Biology Letters, published by the Royal Society, they say: "Women may derogate other women's fidelity, promiscuity or maternal aptitude for example, in addition to, or as an alternative to derogating their attractiveness."