Rats and mice are polyoestrous and have no specific breeding season. Once bred, mammary development normally occures within 15 to 16 days. A few days before the young are born, a flury of nest building will occur resulting in large nests. The nest should be left undisturbed for 2-3 days after birth. The babies, or pups, are blind, hairless, and immature at birth. They develop rapidly, however, and may be weaned at three weeks.
Sexual Maturity
Safe breeding
Oestrus cycle
litter size
Ave. weaning age
Rats8-10 weeks10 weeks4-5 days20-22 days6-1621-28 days
Mice6-7 weeks7 weeks4-5 days19-21 days8-1221-28 days