This article is from the WSSF 2016 AFRMA Rat & Mouse Tales news-magazine.

By Karen Robbins

Katherine Brown, FL, e-mail
Q Both of my female mice are pregnant. I’m separating the male mouse but should I leave the other female there in the cage? One of them is going to give birth I would say a week before the other. I heard keeping the other female would help as a nanny.

A I separate my females into their own cage and don’t use nannies. This way you can see how she is at milking, raising them, how they do, what is produced from that pairing, etc. Some breeders do this but others have found it doesn’t work. While female mice will normally do well helping raise all the kids together, there are times in groups of pregnant females where all the babies are killed when each female has hers until the last mouse has her kids, then everyone helps raise the last mouse’s babies. Other times if you have only 1 pregnant mouse in a group of non-pregnant females, the babies are killed. Also, having a week difference in age isn’t good as the smaller ones sometimes get pushed away from the feedings by the bigger, older kids, especially if there are too many in each litter. The only advantage I can think of to having another female in with a mom is if the mom dies, the babies have someone to be with them even though she may not be able to nurse them.

Katherine Brown, FL, e-mail
Q I think my white mouse is going to give birth soon, she is in the corner of the cage, eyes slightly closed, and breathing very heavily. Should she be giving birth soon? I am scared the other female is still in the cage but I do not want to disturb the soon-to-be-mother, that is if she really is in labor. Is there any specific way to tell when they are in labor? Also, she is making clicking sounds and throwing her head up sometimes. Is this just because of labor breathing or is she bruxing?

A Take out the other mouse and leave the white one in the cage. No, it is not a good idea to disturb her if she is getting ready to have her babies. Give her some nesting material and keep her in a quiet area.

She should not be breathing very heavily and with the clicking noises—it sounds like she has respiratory issues. Clicking, chirping, wheezing, sneezing, rattling are all sounds of respiratory problems. Not sure what the throwing up the head is from—respiratory problems or something else. Have a vet look at your mouse for the respiratory symptoms. You wouldn’t hear bruxing in mice because they are so small (see the article Bruxing Mice).

Mice normally don’t have issues having their kids—you might see some stretching when she is delivering them (if you see her have them) and she will make a nest beforehand. If she is sick, that is not good as the kids will more than likely be sick as well and she may have problems nursing and caring for them. One of the problems with buying feeder mice to use as breeders—they aren’t meant to live more than a few days after they go to the pet shops, so health/temperament isn’t important to the commercial breeders.

Update: The vet fixed her up with some antibiotics, thanks so much for your help!

Katherine Brown, FL, e-mail
Q Can a mouse get pregnant even when she is not in heat?

A No. Mice come in season every 4–6 days and it is only during this time she can get pregnant. Mice go through four phases of their cycle: 1. Proestrus – when a new batch of eggs reach maturity within ripe, large ovarian follicles; at this point you can see she has a swollen vulva with an open vagina that is more pink/purplish in this area, the female is receptive to the male; 2. Estrus begins with the ovulation of fully mature oocytes (an immature egg cell) and will last 6–8 hours; the vulva is not as swollen but the vagina is more open (still pink) and the female is most receptive to the male during this time; 3. Metestrus – is when the mature eggs move through the oviducts and into the uterus; the vulva is no longer swollen and the vagina is now closed and normal color in the area; 4. Diestrus – this is when a pregnancy does not occur and the unfertilized eggs are eliminated, the vagina and vulva are at a minimum size and normal color, and new follicles begin to undergo a rapid growth for the next ovulation. You can see this on the Jackson Lab poster Visual Guide to the Mouse Estrous Cycle. They also make Mice Pups Appearance by Age and Coat Color Guide posters.

Also, if a female is left with a male when she has her kids, she can get bred after delivering them and be pregnant while nursing the first litter, thereby having another litter when the first one is still with her.

You can read more in the online Mouse Genetics book Mating And Pregnancy section of the Reproduction and Breeding chapter or the University of Kentucky Mouse Reproduction.

The AFRMA Breeding book, Genetics book, and Color Standards book would be helpful for you to get and answer a lot of your questions.

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