They’re not mice studies, they’re mouse studies, and I’ll show you why.
If they were studies conducted with dogs, you wouldn’t call them dogs studies. You would call them dog studies. We conduct a fruit fly study, not a fruit flies study.
In English, we use the singular form of the animal as the adjective when we’re talking about the study or studies we’re subjecting them to. Likewise, we have a mouse problem, not a mice problem.
(By the way, if you have a mouse problem, I’m sorry about the scientists trying to make them live longer. What could these Einsteins be thinking?)
English Is A Complicated Language 