327.

Hispid Cotton Rat

Sigmodon hispidus

French: Sigmodon hispide / German: Baumwollratte / Spanish: Rata de algodén hirsuta

Taxonomy. Sigmodon hispidus Say & Ord, 1825, St. John’s River, Florida USA.

Traditional concept of S. hispidus as a highly polytypic and broadly distributed species was initially uncovered as incomplete by application of chromosomal data and subsequently refined by molecular phylogenetic analyses. Current understanding restricts S. hispidus to the USA and two areas in northern Mexico, to the exclusion of taxa now recognized as S. arizonae, S. harsutus, S. iopinatus, S. mascotensis, S. peruanua, S. toltecus, and S. zanjonensis . Given radical reconstitution of S. hispidus, any recognition of infraspecific taxa is inappropriate without fresh documentation. Allocation of specimens referred to S. hispidus from the Mexican states of Chihuahua and Coahuila require molecular or chromosomal confirmation. Monotypic.

Distribution. USA and Mexico, from S Nebraska, SE Colorado, and SE Arizona E to Virginia, the Carolinas, Georgia, and Florida, and S through Texas to Nuevo Leon and Tamaulipas, with an isolated population in the lower Colorado River of SW Arizona, SE California, and extreme NW Sonora.

Descriptive notes. Head-body 125-227 mm, tail 81-166 mm, ear 16-24 mm, hindfoot 28-41 mm; weight 100-225 g. There is slight sexual dimorphism in body length and weight, with males being larger. The Hispid Cotton Ratis large-bodied, with its pelage grizzled by blackish or dark brownish hairs, interspersed with buffy or grayish hairs; sides are only slightly paler; and underparts are usually pale to dark grayish, often faintly washed with buff. Tail is bicolored, dark above and paler below, but sparsely haired so that large annual scales (more than 0-5 mm in width) are visible. It differs from members of the fulviventer species group by long and narrow skull, long and broad basioccipital, and shallow palatal pits and from other members of the hispidus species group by generally shorter hindfoot (averaging 32 mm), shorter distance between temporal and occipital crests, smaller diameter of foramen ovale, and well-developed crest on posterior palate. While molecularly distinct from Burmeister’s Cotton Rat (S. harsutus) and the Toltec Cotton Rat (S. toltecus), lack of adequate comparisons preclude useful delineation of diagnosable morphological attributes among these taxa, as has been done with respect to the Arizona Cotton Rat (S. arizonae) and the West Mexican Cotton Rat (S. mascotensis). Basic chromosomal complement is 2n = 52, FNa = 52, but polymorphism exists with 2n varying from 51 to 56 due to supernumerary elements and sex chromosomes varying in morphology geographically.

Habitat. Grassy habitats with dense vegetation, including tall-grass prairie, meadows, agricultural areas, and fallow fields, and mixed grass and brush and mesquite ( Prosopis, Fabaceae) grasslands with little ground cover. Local density of the Hispid Cotton Rat is positively associated with grass height and density. Surface and burrow nests are made of woven grass and range from cup-shaped to hollow ball-shaped structures with single entrance. Nests found in northern areas (Kansas, USA) are more tightly woven and significantly larger that those in southern areas (Florida).

Food and Feeding. The Hispid Cotton Rat is omnivorous. Diet is primarily grasses but varies widely relative to availability of food items; it can include insects. Individuals ingest ¢.0-4% of annual net primary production, with 98% of this energy being used for maintenance costs.

Breeding. Gestation is ¢.27 days. Litters have 1-15 young, with individuals in northern areas having significantly largerlitters (range 4-15, mean 7-3 young) than those in more southerly localities (range 2-5, mean 5 young). Seasonal variation in litter size has been reported, with spring litters significantly smaller than those born in summer or autumn. Larger littersizes are explained as a response to shortened breeding season and lack of winter reproduction. Postpartum estrus occurs, with ovulation occurring within 6-12 hours after parturition; estrus varied from 21 to 123 hours. Young are highly precocious, growth is rapid, and weaning occurs within 10-15 days of birth. Sexual maturation occurs at 2-3 months of age for males and as early as 30-40 days for females.

Activity patterns. Hispid Cotton Rats can be active at all hours of the day and night. Some studies noted primarily nocturnal activity, but others showed peaks in early morning and late evening. This variable activity pattern has been related to biotic and abiotic factors.

Movements, Home range and Social organization. The Hispid Cotton Rat exhibits multiannual variation in numbers; local densities up to 112-5 ind/ha are reported, with average densities of 10-12 ind/ha. Males have larger home ranges than females (0-35-0-39 ha vs. 0-22 ha); female home ranges are exclusive. Average individual daily movements are c¢.13 m, with males exploring over larger distances (17 m) than females (6-6 m). Reproductive males moved farther than do non-reproductive males, but female movement was unrelated to reproductive state. Dispersal was positively correlated with density, with sex ratio and age structure of dispersers similar to those of the resident population. Intraspecific interactions affected density and movement patterns, with aliens introduced into an experimental population being restricted to less preferred habitats, which decreased their survival. The Hispid Cotton Rat has been characterized as solitary, with prolonged social contact only between males and females, depending upon a female’s reproductive state. In captivity, however, male-female pair bonds are formed and are characterized by frequent physical contact. Males exhibited dominance over females in these pairs.

Status and Conservation. Classified as Least Concern on The IUCN Red List. The Hispid Cotton Rat has a wide distribution and presumably large overall population. It has expanded its distribution in some areas, occurs in many protected areas, and lacks conservation threats.

Bibliography. Allen (1920), Audubon & Bachman (1853), Baird (1855a), Bangs (1898b), Bradley, Black et al. (2012), Bradley, Henson & Durish (2008), Cameron & Spencer (1981), Carroll et al. (2005), Cassola (2016d), Chapman (1889), Gardner(1946, 1948b), Goldman (1918b, 1928), Goldman & Gardner (1947), Hall (1951), Howell (1943), Layne (1974), Mearns (1897b), Musser & Carleton (2005), Phillips et al. (2007), Say & Ord (1825), Zimmerman (1970).