difficulties in breeding my mice

Question
I am having a very difficult time breeding my mice I have 1 male and 6 females and 2 of which both were prego and had there litter both on easter day and are very healthy ... however there other females are still not prego and I have had all of them for 6 months. what can I do to fix this problem?

Answer
Hi Heather,

Congratulations on your two successful litters!  Would you happen to know the histories of the females who did not breed?  Behavior around males as well as physical factors affecting conception/childbirth can sometimes be genetic, so it may be that they just aren't good breeders.  I think you can eliminate the buck as the problem, since he seems to have bred your first two well.

One possibility is that they are getting pregnant, but are not carrying to term.  A pregnancy terminated early-on will result in the babies being reabsorbed into mom's body.  This could result from the mom just not being cut out to handle having pups, but to determine how likely that is you would have to have some idea of how good of breeders her parents and ancestors were.  Supplementing their diets with a bit of extra protein, such as scrambled eggs made without milk or dry cat food, might help them strengthen up a bit for pregnancy, too.  I would also keep a close eye on them for weight loss that might indicate a hidden illness that could be preventing pregnancy, or weight gain that might indicate success that just hasn't started showing yet.  A gram scale, available at most general and food stores, is a good way to take the guesswork out of it.  :)

Much more likely however, is that they are just not tolerating the male's sexual advances.  I would try separating them for a while and letting the buck live alone for a couple of weeks.  This way, he'll mark the whole cage and feel like he's definitely got his own territory.  Then, when you put the females in, he'll be much more forward in his breeding attempts.  Continue to expose the females to his hormones when in their separate cage, though, by putting a little dirty bedding from his cage into theirs with each change.  This will help them continue to cycle despite the absence of a male.  When you reintroduce them, try keeping them in for 2 weeks or so.  This way, they'll have a couple of estrus cycles to attempt pregnancy and can have a break afterward in case he is hassling them and the stress is causing them problems.