I’m probably not supposed to be bothered by this Why not? I'd argue it's better to be bothered by it than just exclaim "ok it's for science" without any critic

I’m probably not supposed to be bothered by this Why not? I'd argue it's better to be bothered by it than just exclaim "ok it's for science" without any critical view. Anyway: it's extremely common especially with mice. You'd be suprised by the total numbers (which might be hard to acquire having to sift through animal-rights but the order of magnitude should be correct; I'm guessing a couple of tens of millions rodents per year in the US is ballpark correct). A bunch of reasons like what a sibling commenter says but also because you can usually not use mice which were used for experiment A in experiment B exepcting A had no influence.