I know those kinds of experiments. They can't reuse them because that will complicate the experiments. Basically you want to start new. An invasive experiments will be required by law that they will be put down. The non-invasive one is optional. I suppose some kind soul don't want them killed. That is the part of science that I hate, but it is a necessarily evil thing to do. Many animals are sacrificed for our human benefits and to increase our knowledge of things.