I don't know, I'd never sexed baby rats until the litter I had in February, and at 4 weeks of age (still a week from date of separation) all of the males had very distinctive testicles. I can understand female as male more readily than male as female since you can even look at rats from the top and see that they're male!
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry