Sigmodon hispidus
hispid cotton rat

Hispid cotton rats have an extensive range. The southern range reaches northern South America in Peru, Ecuador, Columbia, Venezuela, Guyana, French Guiana, and Brazil. The range extends northward through central America and Mexico. In the United States, they are found as far north as Nebraska in the west and coastal and central Virginia to the east. There is also an isolated population in southeastern California in the Imperial Valley along the Colorado River. However, since Sigmodon hispidus was separated genetically from Sigmodon arizonae in recent years, it's unclear whether this California population is indeed Sigmodon hispidus.

In the past 50 to 100 years hispid cotton rats have been extending their range northward and to higher elevations. Genoways and Schlitter (1966) observed that hispid cotton rats moved northward in Nebraska. In 2002, Dunnum et al. observed that they were extending their range into higher elevations in New Mexico. As recently as 2008 (Francl and Meikle, 2009), they were captured in central Virginia - the first record of this species in the Ridge and Valley province. They may also be moving westward in Virginia. (Cameron and Spencer, 1981; Cameron, 1999; Choate, et al., 1994; Dunnum, et al., 2002; Francl and Meikle, 2009; Genoways and Schlitter, 1966)

Hispid cotton rats prefer dense, grassy areas. They are most commonly captured in grassy fields, brushy pastures, canal banks, roadsides, and edges of cultivated fields overgrown with weeds and other brushy vegetation such as broomsedge and honeysuckle. They occasionally have been observed in areas of dense cacti, salt marshes, and in grasslands bordering brackish waters. In Kansas, grasslands are the most common habitat in which they are trapped, but they also have been found in open wetland habitats, in agricultural fields, and in "planted plum-cedar woodlands" (Rehmeier et al. 2005). In Mexico, hispid cotton rats range from sea level to 1130 m in elevation. In radiotelemetry studies, Cameron and Spencer (2008) found that reproductively active females spent more time in habitats that were a mix of monocots and dicots, and actually avoided all-dicot patches. Males show no preference. (Cameron and Spencer, 2008; Espinoza and Rowe, 1979; Linzey, 1998; Rehmeier, et al., 2005; Schwartz and Schwartz, 1981; Webster, et al., 1985)

Hispid cotton rats are small to medium sized rodents, with adults weighing 100 to 225 g (average 159 g). Total length ranges from 80 to 320 mm, with males slightly longer than females. Regional size variation exists; hispid cotton rats in Virginia are smaller than those found in North and South Carolina. The color of both sexes consists of a mixture of tan, brown, and black fur on their dorsal parts, giving them a coarse, or "hispid," appearance. The underparts are white to greyish, the tail is sparsely haired and considerably shorter than the combined length of the head and body. Regional variation in color is common, hispid cotton rats from the Coastal Plain of North and South Carolina are darker than those from Maryland and Virginia. An additional characteristic feature of Sigmodon hispidus is the s-shaped crown pattern on the second and third molars. The dental formula is I 1/1, C 0/0, O 0/0, M 3/3 = 16. (Choate, et al., 1994; Linzey, 1998; Mengak and Laerm, 2007; Schwartz and Schwartz, 1981; Webster, et al., 1985)

Little has been published about the mating systems of hispid cotton rats. Most assumptions about their habits are indirect. For example, hispid cotton rats appear to polygamous because of gender differences in home range sizes. In species where males have a much larger home range than females, polygyny is common. (Sulok, et al., 2004)

Hispid cotton rats have the potential to breed year-round and do so in tropical and semi-tropical portions of their range. In temperate regions breeding seems to be determined by temperature. In Virginia no pregnant females have been reported from November through February. In non-pregnant females heat occurs about every 7 to 9 days. The gestation period is about 27 days. A single adult female typically produces 3 to 4 litters per year, averaging 5 to 7 young per litter. Newborns average 76 mm in total length and 6.5 g. They are usually weaned at about 3 weeks and can be reproductively active in 35 to 40 days. However, most do not reproduce until 2 months and they are fully grown at 5 months. (Espinoza and Rowe, 1979; Linzey, 1998; Schwartz and Schwartz, 1981; Webster, et al., 1985)

Female hispid cotton rats produce more than one litter per year. The young stay in the nest until they are about 3 weeks old when they are weaned and begin to care for themselves. (Linzey, 1998)

Hispid cotton rats, like many rodents, are not long lived in the wild. Few cotton rats (13.2%) live beyond six months. The oldest recorded Sigmodon species individual lived 12 months in the wild. In captivity, the oldest hispid cotton rat recorded is five years, two months old. (Linzey, 1998)

Hispid cotton rats are active year-round and during all hours of the day. Maximum activity is concentrated from late afternoon to about midnight. Although heavy rains and extreme cold decreases activity in this species, light or moderate rains have little or no effect. Interactions are limited to the breeding season, when older males have some dominance over the younger males. They also show cooperative behavior in cooler months, huddling together for warmth. They tend to be aggressive towards other rodent species occupying the same habitat. Hispid cotton rats construct nests using dry grass, fibers stripped from larger plants, and other materials. Nests are built under logs and rocks for protection. Sometimes they use abandoned dens of larger mammals such as skunks or squirrels. They also construct a maze of runways, about 7.5 to 10 cm wide, and tunnels from 2.5 to 5 cm wide and from 2.5 to 10 cm below the surface. New growth of grasses and sedges is trimmed from main runways and trimmings are piled up in an irregular manner.

Homing studies found that cotton rats were more successful in returning to their territories if they were released less than 300 m away. This suggests familiarity with the habitat at this distance. In radiotelemetry studies, Cameron and Spencer (2008) found that reproductively active females spent more time in habitats that were a mix of monocots and dicots, and avoided all-dicot patches. Males showed no preference. The authors speculate that this difference in preference is related to nutritional needs. Females, with larger caloric demands, consume a mix of monocots and dicots. (Cameron and Spencer, 1981; Cameron and Spencer, 2008; Linzey, 1998; Schwartz and Schwartz, 1981)

Hispid cotton rats have a home ranges from 0.10 to 0.50 ha. Males usually have larger home ranges (0.40 to 0.50 ha) than females (0.10 to 0.30 ha). Adult home ranges are larger than those of young adults or juveniles. Movements are typically less than 100 m per day, but can be upwards of 300 m in rare cases. They typically do not defend territories, but there have been reports that females may occasionally do so. (Cameron and Spencer, 1981; Espinoza and Rowe, 1979; Linzey, 1998; Schwartz and Schwartz, 1981)

Little information regarding communication of hispid cotton rats has been reported. One article mentions communication when young were being handled by researchers. The young made a squeaking sound alerting an adult to come close to researchers. However, like most mammals, hispid cotton rats have a keen sense of smell and hearing. (Cameron and Spencer, 1981)

Hispid cotton rats are folivorous, granivorous, and lignivorous. In addition they feed on the eggs and young of ground-nesting birds, insects, crayfish, fiddler crabs, and carrion where possible. They do not cache food. When feeding on tall plants they cut down the plant near its base then cut the whole plant into smaller sections. They drink water but do not require a permanent water source in their habitat. (Linzey, 1998; Schwartz and Schwartz, 1981)

Hispid cotton rats avoid predation by staying in their runways, being alert, and taking advantage of their cryptic coloration. Their rate of reproduction is high, suggesting that this effort may quickly replace those individuals lost to predation. Hispid cotton rats are preyed on by a wide variety of predators including owls, hawks, red foxes, bobcats, raccoons, coyotes, domestic cats, weasels, mink, and snakes.

Hispid cotton rats presumably fall victim to invasive red fire ants, Solenopsis invicta. Pederson et al., 2003 demonstrated that cotton rat densities in habitats containing fire ants in the summer may be half of that in plots where fire ant suppression techniques have been implemented. This may be a result of habitat switching by cotton rats or direct predation on cotton rat young. In a laboratory study by Ferris (1994) high densities of fire ants caused females to abandon their nests, carrying young in tow one by one for as long as they were able. (Cameron and Spencer, 1981; Ferris, 1994; Pedersen, et al., 2003)

Hispid cotton rats are prey for many larger animals such as foxes, dogs, coyotes, weasels, raccoons, minks, domestic cats, bobcats,, hawks, and snakes. This makes them an important food source. Some researchers suggest that their abundance and availability as prey means they are a buffer between predators and game animals. In the piedmont and coastal plains of North Carolina cotton rats appear to be an important food source to bobcats, ranking first or second in frequency of occurrence in their stomachs. Hispid cotton rats have an important relationship with bobwhite quail (Colinus virginianus) because they compete with quail for food resources and feed on the quail eggs. Hispid cotton rats do not feed on all quail nests at one time, suggesting an adaptive reason for staggered quail breeding. This staggering may actually benefit quail for reasons other than predation by Sigmodon hispidus; all nests may not be not subjected to the same adverse weather and environmental conditions as others. In addition, hispid cotton rats are host to many internal and external parasites. External parasites are hosts to mites, ticks, lice, and fleas. They are also hosts to cestode species (Choanotaenia nebraskensis, Hymenolepis diminuta, Raillietina bakeri, Taenia taeniaeformis), bacteria (Rickettsia rickettsii), nematodes (Longistriata adunca, Physaloptera hispida, Mastophorus muris), and ascarid worms. (Linzey, 1998; Mengak and Laerm, 2007; Schwartz and Schwartz, 1981)

There is no evidence hispid cotton rats are economically beneficial.

Hispid cotton rats can greatly reduce crop production. They cause damage to a variety of crops, including cotton, rice, alfalfa, grains, vegetables, fruits, squash, sugarcane, corn, sweet potatoes, and melons. Crop damage is directly related to population densities, which vary by year and by season. Dense populations of hispid cotton rats also cause damage to canal banks. (Espinoza and Rowe, 1979)

Over much of their range hispid cottons rats are common and populations are stable. In Kentucky hispid cotton rats are listed as vulnerable because they occur in only a portion of the state, in portions of just 8 counties in the southwestern and southeastern corners. (Mengak and Laerm, 2007)