Women are meaner towards other females when they are at their most fertile, research has suggested.
Academics claim a woman can tell when a rival's odds of pregnancy are high - just by looking at her. Her competitive nature then kicks in and she tries to see off her 'rival', a study at Chicago University found.
They studied 127 women while they played a psychological game that measures generosity. Volunteers were given a set amount of money and told they could share it with a second woman, whose photo they were shown. The pictures had been taken at different times in the women's menstrual cycles and some were of women who were ovulating and so highly fertile.
Others were taken when the women could not conceive.
The volunteers were not told at what stage in the cycle the photos were taken but could still sense whether the women in them were a 'threat'. As a result, when a volunteer was at her most fertile herself, she gave less money to one who was also highly fertile than to one who had no chance of conceiving.
In other words, her mean streak came out. Non-fertile women weren't affected. The US researchers said fertile women may have an inbuilt way of sensing potential love rivals.
Writing in the journal Evolution and Human Behavior they said: 'Our results suggest fertile women may compete differently with other fertile women, who potentially challenge their reproductive potential.'