This entrance is closed when the mouse is inside. Both the males and females help build the nest. In winter, the nests are usually larger and are sometimes shared with other mice. By sleeping together, the mice produce extra warmth, which helps keep them warm during the cold weather. The male mice look for female mice to mate with in late winter and early spring.
Breeding continues into the summer and fall.
The female may have up to four litters each year. The young babies are born after a gestation period of about 25 days. The usual size of the litter is about four but can be as large as 10 or 11. The mother has been known to help the babies in the birthing process by pulling them out with her teeth. The young are very small and weigh only 2.8 grams.