So far the only animal(s) I have killed were 2 mice. I had transferred the young 3 week old mice to another cage, and didn't pay attention to the type of water nozzle that was attached to the automatic dispenser. I didn't notice it was the "adult mouse only" kind, and that the babies were dwarf sized due to a problem with their mother's milk. It was about the saddest thing I've ever seen, and knowing it was my fault was pretty rotten...