This article is from the Spring 1998 AFRMA Rat & Mouse Tales news-magazine.
By Virginia Pochmann
(editor of no longer existing Mouse Review and author of
(Article from the Dec. 1996–Jan. 1997 NRMCI Journal)
Litter size at birth can vary greatly. Different varieties can vary in litter size. Different strains of the same variety can differ in litter size. Many factors can influence the size of a litter. If a doe has a litter of four, on a number basis the litter may not need to be reduced. However, other factors may require a litter of four to be reduced and if breeding to increase litter size, the entire litter may be culled. If a litter is not culled within the first week after birth, the doe herself may reduce the size of the litter. This natural cull is more likely to happen if the litter is large. Sod’s Law operates, in that the doe will probably kill the one mouse in the litter that you really wanted. There is always a chance that once a doe starts a natural cull, she will not stop but rather kill all the litter. If the fancier reduces the litter, then the doe is less likely to cull the ones remaining.
Quite often in litters you get one or two runts. If left, these runts may not survive, but if they do survive, you find they are always behind the other mice in the litter and when adults, would not be used for breeders. Fanciers will cull the runts. In a litter, there can be other defective mice; kinked tails, pied tails, nipped tails and feet are some of the faults you can see shortly after birth and such mice can be culled. These can never be either shown nor used in a breeding program.
If you think about a typical litter, out of that litter you may eventually keep one buck or a couple of does for breeding. The rest of the litter will not be required as adults. Each breeding buck will require a breeding box of his own. In order to keep the stock breeding, you will need a ratio of at least three adult does to each breeding buck. Adult does can be kept together in the same breeding box without a buck, but pregnant and nursing does will require extra breeding boxes. A young litter will occupy just one breeding box, but when grown to adults, several boxes would be needed. In order to avoid a constantly expanding mousery (and work) litters need to be reduced in size.
If you compare the progress of a litter of four to that of a litter of eight, you will usually see that the mice in the smaller litter grow quicker and larger. The mice in a smaller litter will generally get a better start in life. You will not usually be able to see a difference in full grown adults from small and large litters.
Before or when a litter is born, quite often a fancier knows what is wanted from the litter. From a good buck and a good doe, it may be a replacement stud buck. With marked mice it will be the best marked in the litter. If the litter has been bred for a show, then it may be just does that are required if does are better on the show bench than bucks for that variety. Using the knowledge of what is required from the litter, the fancier can cull the unwanted.
In culled (and unculled) mixed sex litters you can usually see after a few days, that the bucks are larger than the does. This unequal feeding/growth can be to the detriment of the does. Quite a few fanciers, therefore, cull the litter to a single sex. An extra breeding box will not then be needed. A mixed litter would be separated by sex at 5 weeks.
A fancier can be tempted to raise large litters with the thought “any surplus I can pass on to the pet shop.” At present, the pet shop prices do not make it worthwhile to do this. In the weeks after the initial cull, mice in the litter not up to the fancier’s standard or requirements can be passed on to the pet shop. If the litter has initially culled to four, a fancier may keep just one or two for breeding when adulthood is reached.
Finally, I prefer to cull litters within a week of birth, because I find it both physically and emotionally easier to cull a pinkie than to cull a healthy but unwanted youngster.