A cotton rat is any member of the rodent genus Sigmodon. They are called cotton rats because they build their nests out of cotton, and can damage cotton crops. Cotton rats have small ears and dark coats, and are found in North and South America. Members of this genus are distributed in the Southwestern USA, Mexico, Central America and South American countries of: Venezuela, Ecuador, Colombia, Peru, Brazil, Guyana and Suriname. Many of the species are found in Mexico. Sigmodon hispidus was the first model organism to be used in polio research.