The muskrat the only species in genus Ondatra, is a medium-sized semiaquatic rodent native to North America. The muskrat is found in wetlands over a wide range of climates and habitats. It has important effects on the ecology of wetlands. It is the largest species in the subfamily Arvicolinae, which includes 142 other species of rodents, mostly voles and lemmings. Muskrats are referred to as "rats" in a general sense because they are medium-sized rodents with an adaptable lifestyle and an omnivorous diet. They are not, however, so-called "true rats", that is, members of the genus Rattus.