American Fancy Rat & Mouse Association

This article is from the Spring 2000 AFRMA Rat & Mouse Tales news-magazine.

Breeding & Stuff

Mouse Sexes; Sterile Rat?; Attacking Mouse

By Nichole Royer

Mouse Sexes

Jennifer Lebendig, e-mail
QThe time to sex my 14 baby mice has come and I have only one male from my larger litter. Am I doing something wrong or is this not uncommon?

AWhile having only one male is not the norm, it also isn’t all that unusual. It happens now and then that you get a litter with more of one sex than the other.

Male mouse side

You are lucky . . . with me it’s always the other way around and I get a litter of mostly males. You aren’t doing anything wrong, the genetic dice just fall strange ways sometimes.

Sterile Rat?

QI am new to the fancy aspect of breeding rats and I just got a new fancy Blue Agouti male rat a few months ago. I would very much like to get babies out of him, but after being with two of my females for several months now, there are no babies. I have a friend that has had this happen on occasion to his rats and mice. What am I doing wrong? I thought rats were very prolific breeders.

AThis happens now and then and it can be very frustrating. There are a number of possibilities.

Male rat side

On occasion you will run into a male that is sterile. The only way to test for this is to put him in with other females (and put the females you had with your Blue Agouti with another male). If the females in with your Blue Agouti never take, and yet they do with another male, he probably will never be usable for breeding.

Females are at their most fertile between 3 and 6 months of age. Often, they have a very difficult time becoming pregnant after they are 6 months old. They also may have difficulties giving birth if bred for the first time over that age.

Condition has a lot to do with fertility. Both males and females become less fertile if they are underweight or malnurished. Any amount of pudginess or obesity can prevent a female from becoming pregnant.

Cage size and space can also affect fertility in rats. There is a decrease in fertility if rats are overcrowded. Some rats will happily breed in a very small space. Others require more room.

The final reason is very unscientific, though it appears to be true. On occasion, rats just don’t seem to like each other. They will each breed and produce litters with other rats, but not with one another. No one knows or can explain why this happens.

Attacking Mouse

Lynn Lehman, Racine, WI
QI have 12 mice, 11 of which are female. I would like to breed a few but am not sure exactly what is all involved. I have always bought my mice at pet shops, but would like to breed a few of my own. I read in a mouse book that the mice should be put in a cage new to all individuals for breeding, but the male was attacking the two females so I quickly put them back in their cages. Is this normal for the male to attack them? I was also concerned that since these two females are in the same cage as four other sisters and the mother of all of these that if I did breed only the two, that the others may attack or kill the babies?

ABreeding mice can be fun, but is often frustrating. I have a method that seems to work well for me. When I want to breed a female, I set up a “maternity” aquarium (a 5-gallon tank works well, as does a 10). I then put both the male and female together in it. I have never had the male hurt the female, but sometimes he can be rather rough in his “handling.” Usually, the male pounces on the female, mounts her, and often even grabs the scruff of her neck with his teeth. The female always lets out a squeak each time this happens. This can be rather loud, so much so that one of our members frequently will say, “I heard the mice in the next room eeking last night, so I know babies are on the way.” If this is what you have happening, don’t worry abut it, it’s normal. If your male is actually drawing blood, then there is something wrong. I would recommend not breeding him (since he could pass this trait on to his sons) and get another male.

I leave the male in with the female until she starts to thicken around the middle, usually about 2 weeks. I then return the male to his cage, but leave the female in the maternity cage. On the few occasions I have put the female back into the cage with her “girlfriends,” there have been no problems with the others hurting the babies. I have, however, noticed that it seems to create a lot of stress on the mother mouse, and the babies never do very well. I prefer to leave a single mother mouse in the maternity tank and let her raise the babies to 4 weeks in peace and quiet. I like to breed only one mouse at a time, that way I don’t have to find so many homes for babies all at once. Other people will put two females together (or more in a bigger maternity tank) and let them raise their babies together. This does cause the complication of not knowing whose baby belongs to who. When the babies are 4 weeks old, I remove the mother and her female babies and put them in with my other females. The males I can usually leave in the maternity tank for another 2–3 weeks, or until they start fighting. Once they start fighting, they each get their own tank. I hope this helps with your breeding plans. *

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Updated February 14, 2014