Q Recently we bought two mice for watching (since our current rat is 2 years old and very tired). The female had nine pups 12 days ago and seems to be pregnant again already, even though she’s lactating. Is this possible? How frequently will the female get pregnant? Steve

A Both female rats and mice experience something called a postpartum estrous cycle. This means that they will come into heat within hours of giving birth. Because of this it is very important to remove the male before the female gives birth.
Not all females left with males will get pregnant after they give birth. It varies, but usually only about 30% of female rats and 50% of female mice will have the postpartum breeding “take” and actually become pregnant. Most breeders advise against rebreeding females immediately after they give birth because the resulting pregnancy, combined with nursing the current litter, will put huge demands on the female’s system. This will cause her quite a bit of stress and might result in her actually becoming ill. It is possible for both rats and mice to produce a litter of babies every 3 weeks for much of their lives.

She has also tried to mate at least five times with two different males, but she doesn’t get pregnant. I notice that she doesn’t go into lordosis . . . which is the arched back position while the male mounts her. Tailless rats do have less vertebrae at the “tail” end. Her butt is round like a guinea pig, so I don’t know. I’d just love to have more Tailless babies!!! Colette Theriault, Canada

A The urine problem can be a Tailless thing. One of our members had a male that did that. I wouldn’t try breeding her, but rather her parents again and a pair of siblings. I know of a couple of breeders of Tailless that have bred the females and have had successful matings and births. With the female Tailless you never know which ones are capable of reproducing and there is a higher incidence of birthing problems. Most people that breed Tailless use the tailed females (out of Tailless) bred with a Tailless male or a tailed male (out of Tailless) rather than chancing the Tailless females. You need to be careful breeding Tailless as urinary problems, skeletal problems, or deformities may be passed down into the line.

A Your mouse is most likely a Waltzer. PLEASE DO NOT BREED HER. These mice are born with a genetic defect which causes damage to their inner ear. The degree of waltzing can vary from animal to animal, ranging in severity from a slight sideways staggering from time to time, to constant high speed spinning in tight circles. Animals like yours who exhibit only a mild case of waltzing can lead relatively normal lives. Others will waltz more and more as they get older until they are unable to stop to eat, drink, or sleep. These die of starvation, dehydration, and exhaustion. Many who do not “waltz to death” are weak, sickly, and die at a young age. Because many of these animals are destined to die such horrible deaths, most fanciers consider it highly unethical to intentionally reproduce them. I am very happy to hear that yours has a good home and that you enjoy her. It’s not the waltzing mice that fanciers object to, just the breeders who purposefully produce them.

A I usually don’t breed my males until they are 3 or 4 months old. There are two reasons. Before 3 or 4 months it’s hard to really be sure of a male’s full potential. Type and color often are not clear until then. Also, the one time I did breed a male young, he became very aggressive. I have no proof that the early breeding caused it, but I have had several other people tell me similar stories, so I figure it’s better to be safe.

As far as how old to use him—I usually figure that until he starts showing signs of being very old (thin, difficulty moving, etc.), he can be used. For some males, this is 2½ or older! Nichole Royer

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