Penis shape matters more than size when it comes to the mating rituals of mice, Australian scientists have discovered.

Researchers from the University of Western Australia say male mice who face competition from other males when it comes to mating with females tend to have thicker penis bones.

“Our research findings support the idea that penis shape and not size is under sexual selection and begs the question how this structure influences a male’s ability to father offspring when females mate with several males,” lead researcher Gons’alo Igreja Andre said.

The findings were published in the journal Proceedings of the Royal Society B: Biological Sciences on Wednesday.