Sigmodon fulviventertawny-bellied cotton rat

Geographic Range

Sigmodon fulviventer (tawny-bellied cotton rat) is a native mammal of the nearctic range. It is found from New Mexico and the southeastern corner of Arizona southward into central Mexico along the Sierra Madre Mountain Range. ("Wild Animals of North America", 1979; Wilson and Ruff, 1999)


Sigmodon fulviventer inhabits grassy areas dotted with shrubby growth. The shrubs afford cover and allow dense growth of grasses. Currently, only scattered habitat patches that have been protected from heavy grazing exist. (Baker and Shump Jr, 1978; Davis and Schmidly, 1994)

Physical Description

Sigmodon fulviventer is the largest of the cotton rats and is distinguished from other cotton rats by its large size and coloration. It is also known as the tawny-bellied cotton rat, due to the buff-brown color of its underside. A salt and pepper pattern is found on the dorsal portion of the pelage. The tail is consistently black, with small tail scales and a heavy coating of hair. (Baker and Shump Jr, 1978; Davis and Schmidly, 1994; "Sevilleta Long Term Ecological Research", 2002; Wilson and Ruff, 1999)

These animals weigh between 200 and 220 g. They measure 223 to 270 mm in length, with a tail length between 94 and 109 mm. The skull is arched, short and broad, and contains 16 teeth. The upper incisors are well developed, and the large molars have high crowns.

  • Sexual Dimorphism
  • sexes alike
  • Range mass
    200 to 220 g
    7.05 to 7.75 oz
  • Range length
    223 to 270 mm
    8.78 to 10.63 in


No information was found on mating systems for S. fulvivener. Little is known about the mating systems of the genus. ("Sevilleta Long Term Ecological Research", 2002; Wilson and Ruff, 1999)

Breeding is seasonal and peaks in late summer or fall. Gestation is between 27 and 33 days in length, and results in a litter size averaging between 7 and 9. Within 18 to 36 hours of birth, the babies of S. fulviventer are fully furred, able to walk, and have opened eyes. They are weaned in 10 to 15 days. The young leave the nest when they are about two weeks old and begin to breed at about six weeks of age. ("Sevilleta Long Term Ecological Research", 2002; Wilson and Ruff, 1999)

  • Breeding interval
    These animals can breed approximately monthly during the breeding season.
  • Breeding season
    Breeding peaks in late summer or fall.
  • Average number of offspring
  • Range gestation period
    27 to 33 days
  • Range weaning age
    10 to 15 days
  • Average time to independence
    2 weeks
  • Average age at sexual or reproductive maturity (female)
    6 weeks
  • Average age at sexual or reproductive maturity (male)
    6 weeks

Sigmodon fulviventer constructs a nest woven out of grasses in which it resides and cares for their young. It is not clear whether the male helps to raise the offspring. The female nurses the precocious youngsters until they are between 10 and 15 days old. Shortly after, the young disperse. (Baker and Shump Jr, 1978; Wilson and Ruff, 1999)

  • Parental Investment
  • no parental involvement
  • precocial
  • pre-fertilization
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female
  • pre-independence
    • protecting


Maximum lifespan/longevity for S. fulviventer is not known. However, in the wild, they are not expected to live beyond 2 months of age. ("Sevilleta Long Term Ecological Research", 2002)

  • Average lifespan
    Status: wild
    2 months


Sigmodon fulviventer prefers tall grassy areas where nests and runways can be hidden. It is reported to be quite excitable and pugnacious. Individuals of this species quarrel frequently. (Baker and Shump Jr, 1978; Cahalane, 1954; "Sevilleta Long Term Ecological Research", 2002)

  • Key Behaviors
  • terricolous
  • motile

Communication and Perception

No information was found on communication for S. fulviventer. However, as mammals, it is likely that they have the ability to perceive visual information, accoustic information, and scent cues. It is also likely that they use these in intraspecific communication. Tactile communication is likely to occur during fighting as well as between mothers and their offspring.

Food Habits

The diet of S. fulviventer consists largely of grasses and sedges, as well as cultivated grains and vegetables. It will also feed on insects, grasshoppers, and quail eggs. (Cahalane, 1954)

  • Animal Foods
  • birds
  • eggs
  • insects
  • Plant Foods
  • leaves
  • roots and tubers
  • seeds, grains, and nuts


Sigmodon fulviventer serves as a principle food for many predators such as coyotes. (Cahalane, 1954)

Ecosystem Roles

Sigmodon fulviventer is part of the small mammal food base for a number of carnivores and raptors. (Cahalane, 1954)

Economic Importance for Humans: Positive

Sigmodon fulviventer is the principal food of numerous predators, serving as a "buffer species" between predators and game birds. (Cahalane, 1954)

In captivity, cotton rats have been influential in developing therapeutic clinical intervention strategies for many viral infections of humans. Examples include influenza virus, respitory wyncytial, adenovirus, poliovirus, and parainfluenza virus. Current research studies are being conducted on cotton rats to see if they are succeptible to HIV and to try to relate them to human HIV research. (Langly, et al., 1998)

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

The reproductive capacity of S. fulviventer is impressive, and when coupled with a plentiful food supply the populations of these rodents can explode. When this happens, farmers may suffer financial losses due to crop damage. These rats will eat all kinds of cultivated grains and vegetables. (Cahalane, 1954)

  • Negative Impacts
  • crop pest

Conservation Status

Sigmodon fulviventer is not listed by CITES or IUCN.


Nancy Shefferly (editor), Animal Diversity Web.

Jenny LaRoche (author), University of Wisconsin-Stevens Point, Chris Yahnke (editor, instructor), University of Wisconsin-Stevens Point.



living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map


uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.


uses smells or other chemicals to communicate

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.


animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.


union of egg and spermatozoan


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.


an animal that mainly eats all kinds of things, including plants and animals

seasonal breeding

breeding is confined to a particular season


reproduction that includes combining the genetic contribution of two individuals, a male and a female


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.


A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.


uses sight to communicate


reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

young precocial

young are relatively well-developed when born


National Geographic Society. 1979. Wild Animals of North America. Washington D.C.: National Geographic Society.

University of New Mexico. 2002. "Sevilleta Long Term Ecological Research" (On-line ). Accessed 10/02/02 at

Baker, R., K. Shump Jr. 1978. Sigmodon fulviventer. Mammalian Species, 94: 1-4.

Cahalane, V. 1954. Mammals of North America. New York: Macmillan Company.

Davis, W., D. Schmidly. 1994. "The Mammals of Texas" (On-line ). Accessed 10/02/02 at

Langly, R., G. Prince, H. Ginsberg. 1998. HIV type-1 Infection Of The Cotton Rat. Proc Natl Acad Sci USA, 95(24): 14355-60.

Wilson, D., S. Ruff. 1999. The Smithsonian Book of North American Mammals. Washington and London: Smithsonian Institution Press.