Meadow Voles have a remarkable reproductive output: they are the world's most prolific mammals. Females can breed when they are a month old and produce litters of 3-10 pups every three weeks for the rest of their lives. Learn more about the Microtus pennsylvanicus, more commonly known as a Meadow Vole, in this easy-to-read species overview by the Smithsonian's National Museum of Natural History.