2 female mice?

Question
I got 2 mice from a pet store today for my daughter.  I looked and so did the pet store associate and we assume that the mice are both female.  Daisy is a plain white mouse with red eyes and KiKi is a black mouse with some streaks of white and a white belly, black eyes.  I've been looking at them trying to better determine whether or not they are both indeed females and I noticed that Daisy is heavier than KiKi.  I am hopeful that neither are pregnant but I know it's always a possibility when males and females are together.  Is KiKi a fancy mouse and just built different?  I also noticed that KiKi was attempting to (for lack of better wording) hump Daisy.  It was short lived however I'm not sure if that's common with female mice as a dominance characteristic or if KiKi is actually a He.  I appreciate your help with this, thanks :)

Answer
Hi Brittany,

While there is no difference between white mice and fancy mice (just an albino gene), some mice are indeed just built differently.  Some mice are naturally chunky, while some stay slender all their lives.  It's simply a genetic issue.  Females also may mount other females as a sign of dominance like you mentioned.

However, with all of the clues together it might be a good idea to start comparing KiKi's rear end with those of known males, or pictures online.  If you go back by your pet store and ask to see males, you should notice a large distance between the anus and the penis, which when standing normally or hunched over a food bowl will show testicles.  When lifted by the tail the testicles may fall into the body for a short while, which might make identifying more difficult without a comparison.  Females, on the other hand, will have very close genitals and anus and frequently don't have any fur between them.  You can see pictures of male and female mice here:  http://www.fancymice.info/sexing.htm  and  http://www.fancymice.info/sexingcait.htm

If the pet store isn't very helpful and you still don't have a good feel for it from the pictures, you can also try looking for nipples.  These will be easiest to see when lifting the mouse by the tail so that the front feet still touch your other hand.  There will be two rows of nipples in females only (males have none), but they may be difficult to spot through the fur.  Until you're sure of their genders, it might be a good idea to keep them apart for a little while so no surprises come along.  If you'd like you can write me a "follow up" question and include clear pictures of the genital area and I may be able to tell for you.  :)

-Tam