Sexing baby field mice & mounting

Question
Hello Natasha,
I apologize in advance for my wordiness. Three months ago, 5 baby mice entered my house one at a time from the pipe outlet attached to my dishwasher.They were so tiny and slow moving that my 2 cats did not attempt to pounce on them and my German Shepherd just laid down next to them and watched. My wife and I originally thought that our cats were carrying the babies inside until we found the fourth and fifth ones coming out of the space around our dishwasher pipe. Unfortunately, the first 3 babies were the smallest and they did not survive the night. The last 2 seem to be in perfect health and after deciding that to release them back into the wild in our neighborhood would simply be sacrificing their lives, we have decided to keep them as pets. After reviewing your sexing advice, I think that they are of opposite sex. At three months, the smaller baby has noticeable testicles while the larger one simply has a mound where the testicles should be. I want to be sure of the larger one's sex before separating them since they are good companions. However, last night I did notice that the larger and apparently female mouse is attempting to mount the smaller mouse. Is this behavior designed to run the male out of the nest area before getting ready for birth (although he/she is not showing any apparent pregnancy) or could the larger mouse be a male that does not show the large hairless testicles like the smaller mouse? Please advise and I will attempt to get some pics later tonight. Thank you. Regards, Don Bishop.

Answer
Dear Don,

A "mound" can be where he is holding in his testicles. The best way to tell is to put them in a glass dish and compare their genitals from below. As you will read at this very excellent post, there are several other ways to tell does from bucks besides testicles.

1. Girls have nipples; boys don't. Presence of nipples means a girl; not being able to see them might mean you are not looking hard enough, or might mean it's a male. Check another method if there seem to be no nipples.

2. The uro-genital distance is longer for males than for females. This is why you have them both in the dish, to compare.

3. The females have a subtle but visible vaginal opening between the urethral opening (looks like a male organ) and the anus.

http://www.thefunmouse.com/info/sexing.cfm


Mounting behavior can be seen in any sex pair, so this isn't indicative of anything in particular except dominant behavior. Of course it is only a problem if the male mounts the female.

Just as in humans, all kinds of behaviors can accompany pregnancy, from nesting to hostility to other personality changes.

If you have two males, you can try keeping them together. Although that is usually problematic for fancy mice, from what I have heard wild mice are better able to remain friends.

If you have to separate them, be sure the two cages are close so they can chat. They communicate in high pitches that we can't hear. I would love to hear them!

Enjoy your little critters : ))

squeaks n giggles,

Natasha