It's mostly a euphemism. You could replace the "cu" with a "ki" and it would still be an accurate sentence.

But yes, I do some selective breeding: I try to keep a mixture of different colors (to make educational programs with the mice more interesting) and I kill any mice that bite humans or are overly aggressive to their cagemates. This week I found a black mouse with a white blaze on its head. I saved him and put him into a breeding tank, to see if we can have patterned mice!