A cotton rat is any member of the rodent genus Sigmodon. Their name derives from their damaging effects on cotton as well as other plantation crops, such as sugarcane, corn, peanut and rice. 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 United States, Mexico, Central America, and South American countries of: Venezuela, Ecuador, Colombia, Peru, Brazil, Guyana, and Suriname. Many of the species are found in Mexico. They are primarily herbivores. The molars of cotton rats are S-shaped when viewed from above. The genus name literally means S-tooth. Sigmodon hispidus was the first model organism to be used in polio research.

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