It’s easy to stereotype mice: They really can all look alike. In fact, even armed with genetic knowledge about their DNA, it can be hard for biologists to tell apart different species. They’re just so similar. Well, not every bit of them is. A group at biologists recently found a new way to tell apart their murine specimens — by looking at mouse sperm.

Their findings appear in the Journal of Mammalogy, and could mean not just improved games of Guess Who in the lab, but also shed light on