If fertilization does not occur, she will come into heat about every 5 days. A female averages six litters per year, with nine young per litter. However, under ideal conditions, litters may contain 20 young, and 14 litters have been recorded during one year. The babies from one pair of rats would be more than 3.5 million in 3 years under ideal conditions and ignoring the death rate. In natural conditions, however, many die, but in a year, as many as 60 to 70 offspring from one female may mature.
Breeding is greatest in spring and fall, drops some in summer, and drops substantially in winter.
The common house mouse depends less on humans than rats do. It commonly inhabits grassy fields and cultivated grain crops and adapts well to living away from humans. For example, some have been captured on open tundra in Alaska, miles away from human settlements. On the other hand, they adapt well to living with humans, as indicated by a report of their living 1,800 feet below ground in a coal mine, probably feeding on lunch scraps of the miners. The house mouse has a small range.