The species is very prolific and will breed throughout the year. Several litters may be produced annually, aver-aging 2 to 15 young per litter. The gestation period is 27 days, and the young are weaned in 10 to 15 days.
Most young breed for the first time at 2 to 3 months of age. Therefore, several generations may live in the same nest at one time. The average life span is 6 months.
Cotton rats are basically nocturnal but will venture out in the daytime and are active year-round. The home range is small — from 1/4 to 3/4 acre (0.1 to 0.3 ha) for females and 1 to 1 1/4 acres (0.4 to 0.5 ha) for males. Cotton rats do not store food or hibernate. They can swim and do not hesitate to do so. This species is excitable, pugnacious, and aggressive toward mice living in the same fields. Their nests are a crude mass of dry grass fibers stripped from larger plant stems, placed in shallow surface depressions, among clumps of coarse grasses, underground in shallow tunnels, or under rocks or logs.
Cotton rats prefer dense cover such as grassy fields, overgrown roadsides, or fencerow vegetation adjacent to cultivated fields. They also occupy meadows, marshy areas, cactus patches, and weedy ditch banks. Under the protective cover, the cotton rat will have well-defined runways radiating in all directions from the nest site.
Cotton rats are normally herbivores, eating the roots, stems, leaves, and seeds of a wide variety of plants. They also feed on sugarcane, fruits, berries, and nuts. Cotton rats will cut tall plants off at the base and continue to cut them into shorter sections. They also eat insects, the eggs and young of ground-nesting birds (particularly quail), and the carcasses of dead animals.