A cotton rat is any member of the rodent

Rodent

Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....

 genus Sigmodon. They are called cotton rats because they build their nest

Nest

A nest is a place of refuge to hold an animal's eggs or provide a place to live or raise offspring. They are usually made of some organic material such as twigs, grass, and leaves; or may simply be a depression in the ground, or a hole in a tree, rock or building...

s out of cotton

Cotton

Cotton is a soft, fluffy staple fiber that grows in a boll, or protective capsule, around the seeds of cotton plants of the genus Gossypium. The fiber is almost pure cellulose. The botanical purpose of cotton fiber is to aid in seed dispersal....

, and can damage cotton crops. Cotton rats have small ears and dark coats, and are found in North

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 and South America

South America

South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...

.
They are primarily herbivores.
The molar

Molar (tooth)

Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....

s of cotton rats are S

S

S is the nineteenth letter in the ISO basic Latin alphabet.-History: Semitic Šîn represented a voiceless postalveolar fricative . Greek did not have this sound, so the Greek sigma came to represent...

-shaped when viewed from above. The genus name literally means S-tooth.

Sigmodon hispidus was the first model organism

Model organism

A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the organism model will provide insight into the workings of other organisms. Model organisms are in vivo models and are widely used to...

 to be used in polio research.

Classification

• Genus Sigmodon
  • Subgenus Sigmodon
    • Sigmodon hispidus species group
      • Sigmodon alleni - Allen's Cotton Rat
      • Sigmodon arizonae - Arizona Cotton Rat
      • Sigmodon hirsutus - Southern Cotton Rat
      • Sigmodon hispidus - Hispid Cotton Rat
      • Sigmodon mascotensis - West Mexican Cotton Rat
      • Sigmodon ochrognathus - Yellow-nosed Cotton Rat
      • Sigmodon planifrons - Miahuatlán Cotton Rat
      • Sigmodon toltecus - Toltec Cotton Rat
      • Sigmodon zanjonensis - Montane Cotton Rat
    • Sigmodon fulviventer species group
      • Sigmodon fulviventer - Tawny-bellied Cotton Rat
      • Sigmodon inopinatus - Ecuadorian Cotton Rat
      • Sigmodon leucotis - White-eared Cotton Rat
      • Sigmodon peruanus - Peruvian Cotton Rat
  • Subgenus Sigmomys
    • Sigmodon alstoni - Alston's Cotton Rat