Voles are small rodents that grow to 3–9 in (7.6–22.9 cm), depending on the species. Females can have five to ten litters per year. Gestation lasts for three weeks and the young voles reach sexual maturity in a month. As a result of this biological exponential growth, vole populations can grow very large within a short time. A mating pair can produce a hundred more voles in a year. The muskrat (Ondatra zibethicus) is a medium-sized semiaquatic rodent native to North America and an introduced species in parts of Europe, Asia, and South America. The muskrat is found in wetlands over a wide range of climates and habitats. It has important effects on the ecology of wetlands, and is a resource of food and fur for humans.