Physical Characteristics

"Identification:Head and body 5 - 6 in. (127 - 152 mm): tail 3 4/5 - 4 1/5 in. (97 - 107 mm). This rat may be distinguished by the mixture of black and pale buff hairs of upperparts, and the buff belly....Skull has 16 teet h."(1)

"Diagonsis. A Sigmodon of small to medium size, underparts brownish, buffy, or fulvous, extending from t hroat to anus; tail blackish all around, sometimes slightly lighter below than above; upper parts with a "salt-a nd-pepper" or speckled appearance; antiplantar surface (top) of feet buffy; hind foot and ear short; skull of me dium-size; preoptic foramen (in front and slightly above optic foramen) small but usually present (39 to 43 spec imens); nasals usually tapered and v-shaped posteriorly; skull sharply arched in nasofrontal region. Chromosome s, 2N of 28, 29, or 30; FN of 34." (2)

Habitat

"Life History. Fulvous cotton rats in Arizona have a limited distribution in the southeastern corner of the sta te and are nowhere abundant. In this area we could find no ecological separation between S. fulviventer and S. arizonae. At the mouth of Miller Canyon S. fulviventer used runways in a cocklebur patch, but we did not know whether arizonae or fulviventer constructed them. In Mexico, Baker (5) found S. fulviventer inhabiting mesquite-grassland and in association with bunch grasses. Runways are apparen tly well hidden in thick grassy cover. Baker (5) found one nest, about 75 mm in circumference, under a rock. I n New Mexico, Mohlhenrich (6) found these cotton rats in the pinyon-juniper-live oak association. In Arizona, t hey were in weedy places in the encinal and Mexican oak-pine in the Huachuca Mountains. These cotton rats were in a lower association in the Graham Mountains for they were in grassland with mesquite, yuccas, and sacaton. T hey seemed to prefer the thick patches of sacaton." (2)

"Cotton rats are inhabitants of grasslands where the species of grasses involved are tall and provide good cover . Generally, the rats are found below the woodlands,...During times of population highs, the animals may disper se into a variety of less suitable habitats. Typically, Sigmodon makes runways through the grass, where it spends much of its traveling time. Where open space occurs between the bunches of grass, the animals may tra nsit such clearings rapidly, preferring the shelter of the clumps. Burrows are dug, and nests may be constructe d on the surface of the ground or in burrows." (3)

Range

"Range (In Arizona). Southeastern Arizona; from near San Carlos Lake at the north and Patagonia Mountains to th e west....

Fulvous cotton rats do not seem to be extending their range northward in Arizona. The most northern record, fro m "near Old Camp Goodwin," near San Carlos Lake in graham County was taken in 1976. (August 18)....

The species as now defined ranges from the southern Rio Grande Valley in New Mexico and near San Carlos Lake in Arizona to central Michoacan." (2)

This species if found in the Rio Grande Valley and in the grasslands of southwestern New Mexico."(3)

"Sigmodon fulviventer, which differs from the other two cotton rats in its rich buffy ventral coloration, is an animal of well-developed grassland, presently limited to southwestern New Mexico and the middle Rio Grand e Valley from Socorro to Sandoval counties.

A hiatus exists between the northern and southern segments of the range of this apecies, possibly caused by the recent advent of the hispid cotton rat into favorable habitat in much of the Rio Grande Valley (6). Although in the early part of the century S. fulviventer was recorded from the southern Rio Grande Valley, as at Las Palomas, Sierra County, only S. hispidus is now present there. Similarly, at Bernardo, near the souther n end of the range of the northern segment, Wood obtained S. fulviventer in 1940 (7), but only S. hisp idus now occurs there (8). In the northern segment of the range the animals are mostly found in tall grass and cattails, as at Isleta Marshes, and in similar places in swales along the Santa Fe Railroad roadbed."(4)

Reproductive Characteristics

"Breeding may take place throughout the year in the laboratory, but it is generally seasonal in our area, with a nnual peaks in the late summer or fall. One to fifteen young are born after a gestation period of twenty-seven to thirty-five days. The average number of young in several studies varied between seven and nine. Unlike the young of other New Mexican cricetines, baby Sigmodon are fully furred, able to walk, and open their eyes within eighteen to thirty-six hours. They are weaned in ten to fifteen days. In captivity females conceive as early as thirty-eight days of age, while males may not produce sperm until they are from sixty-nine to ninety d ays old. Young animals are essentially adult in skeletal growth after one hundred days. Once mature, females h ave an estrous cycle of eight to nine days. In one study, the average life expectancy for animals caught in the wild averaged two months." (3)

Etc.

"Those from New Mexico known as S. fulviventer by some authors....

Similar species: (1)Hispid Cotton Rat has a whitish belly; in valleys. (2)Yellownose Cotton Rat has a gray bell y." (1)

"Comparisons. S. fulviventer differs from S. ochrognathus as follows: underparts brownish, buffy, or fulvous rather than whitish; dorsum speckled or "salt and peppery"; tops of feet buffy; tail blackish or nea rly so all around; preoptic foramen usually present rather than usually absent; posterior border of nasals v-sha ped rather than squared or notched; chromosomes 2N of 28, 29, or 30 rather than 52, FN of 34 rather than 66....

S. fulviventer differs from S. arizonae in much the same way that S. hispidus does..., but in addition the tail is shorter; skull averages smaller in all measurements except length of palatine foramina a nd width of rostrum; chromosomes 2N or 28, 29, or 30 rather than 22 or 24, FN of 34 rather than 38....

Remarks. This species has been referred to in earlier literature as Sigmodon minimus, but in 1962 Baker a nd Greer (9) regarded minimus as synonymous with S. fulviventer....The buffy-colored underparts ar e unique to this species in Arizona as well as the salf-and-pepper upper parts, blackish and underside of the ta il, buffy tops of feet, and chromosomal complement" (2)

"Populations may grow as large as thirty to thirty-five per acre, but the animals may essentially vanish from la rge areas during population lows, when they have been recorded at densities of less than one animal per four acr es of habitat. Each individual has a home range of 0.1 to 0.2 acres in good habitat, but this figure must chang e with changes in density. Little is known of social life. Pairing has been said to be transitory in the wild, although pairs form in the laboratory.

Three species occur in New Mexico. The tawny-bellied cotton rat, S. fulviventer, differs from the other two in its ventral surface, which is a rich buffy color contrasting with the blackish or dark brown dorsal surfa ce of the animal....The yellow-nosed cotton rat, S. ochrognathus, has a light-colored belly, and a buffy or yellowish wash on the nose and facial region. It is limited to southern Hidalgo County. Most common is the hispid cotton rat, S. hispidus, which is light bellied and lacks the yellowish nose and face. S. hisp idus is found in the grasslands of southern and eastern New Mexico. All three species may inhabit the same kinds of habitats; but where fulviventer and hispidus coexits, the former prefers more dense and u ndisturbed grasslands, while the latter is more common in disturbed places. Where the three species occur in Hi dlago County, S. ochrognathus is most likely to be found in hillside stands of bunch grass, agaves prickl y pears, oaks, and other shrubs, while the other two species occupy the grasslands at lower elevations."(3)

Literature Cited

1. Burt, W.H., R.P. Grossenheider. 1976. The Peterson Field Guide Series: A Field Guide to the Mammals. Peterson, R.A. (Ed.). Houghton Mifflin Company, Boston, p174.
2. Hoffmeister, D.F. 1986. Mammals of Arizona. University of Arizona Press, Tucson, p398-99.
3. Findley, J.S. 1987. The Natural History of New Mexican Mammals. University of New Mexico Press, Albuquerque, p96-7.
4. Findley, J. S., A. H. Harris, D. E. Wilson, and C. Jones. 1975. Mammals of New Mexico. University of New Mexico Press, Albuquerque, p235-37.
5. Baker, R.H. 1969. Cotton rats of the Sigmodon fulviventer group (Rodentia: Muridae). In J.K. Jones, Ed., Contributions in mammalogy. Misc. Publ. Mus. Nat. Hist. Univ. Kansaas, 51:177-232.
6. Mohlhenrich, J. 1961. Distribution and ecology of the hispid and least cotton rats in New Mexico. J. Ma mm., 42:13-24.
7. Gardner, M. 1947. An undescribed cotton rat from New Mexico. J. Mamm. 29:65.
8. Findley, J and C. Jones. 1963. Geographic variation in the least cotton rat in New Mexico. J. Mamm. 44:307-15.
9. Baker, R.H., and J.K. Greer. 1962. Mammals of the Mexican State of Durango. Mich. State Univ. Publ. Mus., Biol. Ser., 2:25-154.