My '08 normal male is reluctant to switch to rats. Currently, he is taking 2 F/T adult mice a week and he's gaining weight on them (he's around 700 g). He should be on rats, but he has no interest. Recently, he was on a fast, so I figured that would be the perfect time to introduce a rat. After a month of not eating, he wouldn't take the rat but was all over the mouse.

I've tried all of the tricks with rats...even exposing the brain, but to no avail. If anyone has any tips I'd be most appreciative. Thanks!