This article is from the Mar.–June 1993 AFRMA Rat & Mouse Tales news-magazine.

By Troya Duncanson

Ann Quinn, Twin Peaks, CA
QSomething occurred this week with one of my mice which completely surprised me, and I am wondering if this is a recognized phenomenon in mice or is it a unique event.

I had three female mice living together, a mother and her two daughters. They were bred by the same male but apparently not at the same time. On February 25 the mother had nine babies. On March 1 one daughter had seven babies. The second daughter did not yet appear pregnant. Due to cage shortage, the male remained with the females during this time. Eventually, daughter number two appeared pregnant. On March 10 I removed the male. On March 12, in the evening, I found eight new babies, and daughter number two no longer looked pregnant. The first 2 litters were, at the present time, 15 and 11 days old respectively. I knew the new babies would not have much chance with so many older ones but didn’t have time that night to move them. The next morning only three were left. I put the mom and babies in a separate cage and added four foster babies that were in a similar predicament. Mom accepted the foster babies and appeared to be taking good care of them all. However, they did not grow, grew gradually thinner, and by the third or fourth day they had all died.

Now I get to the interesting part. I put daughter number two, whose babies had died, with three other young females, two of whom were obviously pregnant. After a day or two, I noticed that daughter number two looked kind of plump. The plumpness increased daily until she looked more pregnant than her cage mates. I thought she must have some disease or disorder that bloated her although she looked pregnant, not bloated.

On the evening of March 21 I found ten fully developed newborns in her cage, and she looked quite slim. Her cage mates looked just as pregnant as before. Unfortunately, her cage mates decided to eat the new babies and only six were left when I got them moved. This time they are doing fine and at three days old they look big and plump.

The other two mice that were pregnant with her in the second cage had their babies this morning—16 between them.

How could this happen? I can understand a female “holding” embryos and then developing them when the time is right. I have had a couple of instances this year where babies have been born to females who have been away from any males for up to seven weeks. But how could her second batch develop to birth size within nine days? Before her first litter she looked definitely pregnant and then after the babies appeared she didn’t look pregnant at all. Within nine days of the first birth (actually less because some of the first babies lived at least three days and she was not plump looking for a couple of days after) she produced another ten full-sized newborns.

I have tried to figure out if the mother or daughter number one could have given birth to the first eight, but the mother had more babies herself a few days before daughter number two did, and neither of them had looked pregnant.

My speculation is that the first babies died because she did not produce milk because the second batch were already developing. But why should they start immediately upon her giving birth rather than wait until the first litter was weaned? And they must have been partially developed at the time the first litter was born. Otherwise, how could they have been born so soon after?

You will probably tell me that this is quite common in mice, but since I have never seen it before, it seemed truly amazing.

AWell, I’ve puzzled over this at length, and have no useful information whatsoever. I even have a Bachelor of Science degree in Biological Sciences and can come up with no intelligent guesses. You have a truly unusual situation.

I will share one similar puzzle that I had when I first started breeding mice. I had two Long Haired Spotted Agouti littermates, a pair. They each carried several recessive traits (mother was a Satin Champagne, or something like that). For some reason the male sickened and died. I thought that he died before the first litter was born, but the doe had a second litter—all Spotted Long Hairs, with some Satins, Lilacs, Champagnes and what-have-you.

Now this male was the only mouse that existed who carried all of these traits, so he was the only one who could have fathered this litter. A friendly “escapee” could not have done it. Okay, fine, it must be simple memory lapse, he must have died after the first litter was born. The problem was that this doe had a third and a fourth litter. Each of these litters were Long Haired Spotted mice, with the assorted recessive traits. I had been separating the previous litters as each new litter appeared, so we’re not talking precocious Oedipal mice.

I called U.C.L.A. and couldn’t locate anyone in the life sciences who would venture a guess as to what happened (but they were all quite interested). So yes, some mice can do some weird things. I decided that mice must be able to store sperm (a few other animals do), but this was just a convenient guess to prevent me from going mad thinking about it. (Your comment about arrested embryonic development is equally valid, if the embryos are stored). That might be why I “colony” breed now. If this is happening again in any of my current cages, there would be no way for me to find out, so I don’t have to go crazy.

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