Mice fertility questions?

Question

I was wondering if there was a point that anyone found that as their mice aged they stopped having litters. I know mice average gestation period is 21 days. I have recently aquired some mice from a friend of mine that was going to breed mice for feeders. The females were suposedly pregnant (for at least a week already) when he got them. None of them have had any litters yet and from what i can tell not even showing the usual signs of being pregnant. I have no idea how old the mice were already when he got them. The females have been separated from the males and i have already gotten some younger mice to replace them as breeders so these ones are going to be retired and just become pets. i was just checking to see if anyone else has come across any that stopped having litters after geting older.

Answer 1

I've found that the males, once they're over 16 or so months old, can take a looong time before they get the females pregnant.  Females should only be bred between 3 months and 9 months of age, it's too hard on their little bodies to raise litters outside that range.  I have had accidental pregnancies, once in a female that was 17 months old at the time who I expected was so old it was safe to house her with a male--wrong!  She had one baby, raised him (barely), and never really recovered.  

Generally I've seen the litter size go down when they get a bit older (in the 7 month old range versus the 3 or 4 month old range).

If you're not sure how old they are, I think you're making a good call on retiring them.