True, or long-tailed, mice belong to the youngest group in the animal kingdom, approximately 15 million years old. Over 200 species of mice exist, but the most common is the house mouse. The house mouse is the second most widely distributed mammal on Earth, behind only humans. Despite their name, house mice often live in fields, but they usually live near human dwellings. House mice eat almost anything, but they prefer grains and grain products.
Mice reach sexual maturity at two to three months of age.
After a gestation period of 20 to 21 days, they deliver a litter averaging six blind, bald, helpless babies. House-dwelling mice can bear young continually, but if overpopulation becomes a problem some female mice will remain infertile. Mice are often considered to be pests, or worse. They can cause serious crop damage, as well as food contamination. In addition, mice can carry viral, bacterial, and parasitic disease.