Mice give clue to why sexes are different

New Zealand scientists have suggested that a hormone other than testosterone may explain some common differences between boys and girls - and why boys behave differently from one another.

A study by University of Otago neurobiologists Associate Professor Ian McLennan and Dr Kyoko Koishi has found evidence that male mice lacking a hormone named mullerian inhibiting substance (MIS) may behave in a way that is less typically "male" than mice with the hormone.

Their theories, which are now being tested with humans, were published in US journal Proceedings of the National Academy of Science.

Professor McLennan said most parents knew boys and girls tended to behave differently but tended to put the difference down to testosterone. However, boys did not have any testosterone between about 6 months and puberty.

"But they have very very high levels of MIS, and no one has known the reason why."

Until recently it was thought that MIS was there to stop males developing a uterus. But the study found it may also influence behaviour by contributing to a range of "sex biases" - subtle physical differences that can affect animal behaviour.

The study found male mice without MIS tended to be less eager to explore a new environment than other males.

Results of a study of the effect of MIS on humans will be known in about six months. If humans are affected by MIS, the hormone would form just another part of the complex range of social and genetic factors that determine how people behave.

Biologists stress that there are greater differences in behaviour within the sexes than there are between them.

"Men on average are taller than women but if you think about the Silver Ferns, Irene van Dyk is just slightly taller than Colin Meads," said Professor McLennan. "Almost every part of the body is slightly different between males and female but with huge overlap."

However some behaviour is considered more common in one sex than the other.

Professor McLennan said sex biases meant boys tended to engage in more rough-and-tumble play, throw balls differently, sit less quietly in class, and draw using different colours and shapes compared with girls.

MIS levels varied widely between boys, which could explain the huge differences in how much individual boys displayed these behaviours.

"Until very recently science has only been able to explain why we've got men and why we've got women and not why men are variable and why women are variable."