Three or four weeks ago my girlfriend and I bought another rat, a young girl who was living in a cage with her brothers.
The petstore employee informed us that Pig (our new girl) was indeed a female, and we examined her ourselves. She was the same age as all of the others in the cage, who had quite prominent and obvious testicles. She very clearly had a vagina.
Just today, however, we noticed that our girl appears to have sprouted a pair of testicles.
We have noticed that she smells strongly like a male and appears to have a lot of the social tendencies of a male rat. Though she can be very active and curious, she also enjoys lazing in our laps or on our shoulders (for hours at a time) and she is not as adventurous as the other girls. She's not interested in exploring.
The picture included is one we've just taken of what looks clearly to us to be a vagina and testicles. It's not as clear in the picture so we increased the contrast.
She also has nipples, which males do not have as far as I have heard.
The other notable thing is that while all of her brothers had very large testicles at six weeks, she is now about 9 or 10 weeks old and the testicles she has are still smaller than the ones her brothers had.
The main dilemma is that if she turns out to be an intact male, we cannot keep her, we have all females. But she does not appear to have a penis.
I'm sure there is somebody who is greater educated in rat anatomy and can give me some information based on the picture below.
Sorry if it's a bit graphic for some, but we're all rat-parents here. You've seen it! (well, half of it at least).