A female house mouse gives birth to around 6 young about 19 days after mating. She is ready to mate again in two days. She can produce 6 to 10 litters a year. Each of her young is ready to mate in two month. Remarkably, all her children, grandchildren, great grandchildren, and great, great grandchildren can have offspring in the same year. Two mice, starting to breed on New Year’s Day, could theoretically have as many as 31,000 descendants by December 31.