It seems it’s not just humans who find the opposite sex stressful, too many male mice can affect the sex of a mother’s pups, a study has found.

The research, led by UWA evolutionary biologist Renee Firman, found that when female mice are surrounded by too many male mice, they became stressed and produced more female pups.

“In a number of different species, it has been shown that maternal stress can sort of lead to a bias in production of female offspring and no one has really looked at it in terms of measuring stress,” Mrs Firman says.

The researchers set up two areas, one where female mice were exposed to more male mice, and another where the ratio was skewed the other way for 12 to 14 weeks.

“What I found was the females that were reared around a lot of males did have higher stress levels in terms of corticosterone levels and they produced female biased offspring,” Mrs Firman said.

“It’s a hard one whether to know if it’s an adaptive strategy in terms of there’s something going on or if it’s a physiological mechanism.

“What I do know is that my study suggests that a selective method is happening during fertilisation or during the plantation of embryos,” she said.

While it is known that the maternal diet can affect the sex of other animals’ offspring such as deers, this is the first time that psychological factors such as stress have been observed to have a similar result.

University of Tasmania biology researcher Amy Edwards could not respond to questions by the time this article went to print.