Field Mice Sexing

Question
I have looked at photos to sex 3 1/2 week old baby field mice which where born of a captured mouse that was in my house. Separated what I think to be the girls with the mom and the one boy in his own cage. But, still very unsure, because it looks like what I thought was the girls have a piece of tunnel like long dangling thing that to me looks very much like a penis. Help!! Want to make sure we have no more babies. Pictures would be very helpful.  Thank you, Janene

Answer
Dear Janene,

Both boy and girl mice pee out of what looks like a little penis. There are three differences between the sexes. Firstly, only girl mice have nipples. Unfortunately it can be a little hard to see them when they have all their fur, and certainly if they don't hold still and cooperate. Secondly, the distance between the genitals and their anus is larger on males. This is the easiest way to tell on babies. And thirdly, when they get a little older, the boys do develop quite prominent testicles, which can look like a butt.

Good luck sexing them! You have about two weeks to figure it out, since you don't have the dad. Although girl mice can be sexually mature at 4 1/2 weeks, the boys are about a week behind.

squeaks n giggles,

Natasha