Well I can't say it hurt Stubby at all or at least I've never noticed it did. She's not genetically tailless though, it was nipped off accidentally when she was a day old. They do use their tails for balance while climbing and for heat regulation. I know Stubby doesn't do as well as some of the other rats if the rat room gets overly warm but since they've been in the new room which is much better regulated I haven't noticed her having any issues that way. The funny thing is she holds her stub off to one side, always has and rarely moves it.