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Sigmodon arizonaeArizona cotton rat

By Jim Porter

Geographic Range

Arizona cotton rats (Sigmodon arizonae) are found in the Southern U.S., through Mexico and Central America to the northern South America. They have been collected on coastal plains and the lower Pacific-facing slopes of the Sierra Nevada Occidental. They have also been found at moderate elevations in eastern Nayarit (Mesa del Nagar, 1300 m; Ocota Airstrip, 1900 m) but occur more often in the coastal plains of the western and central portion of their range. (Andersen, 1994; Cameron and Spencer, 1981; Carelton, et al., 1999)

Habitat

Sigmodon arizonae is found in a wide variety of landscapes, but seem to prefer tall, dense grasslands, usually with ample water availability. They are generally found in areas supporting sedges, rushes, and cane grass-like plants. However, this species has been found within the arid upper tropical sub-zone where the dominant vegetation was savanna-woodland, pine and oak forest, deciduous tropical forest, palm forest, or mangrove swamp. These animals are often found near rivers and their floodplains, lakes and ponds, or drainage sloughs through agricultural fields and pastures. These animals can be found in different types of riparian vegetation, including stands of cattails and water hyacinths. (Andersen, 1994; Brylski, 1999)

  • Range elevation
    0 to 1900 m
    0.00 to 6233.60 ft

Physical Description

These cotton rats greatly resemble their congeners, Sigmodon hispidus, and until recent studies of their chromosomes revealed that they were a distinct species, were lumped with S. hispidus. (Davis and Young, 1999)

Sigmodon arizonae is a large cotton rat, measuring 200 to 349 mm in length, of which 85 to 156 is contributed by the tail, and weighing between 125 and 211 g. This species can be distinguished from other cotton rats in the region, Sigmodon fulviventer and Sigmodon ochrognathus by its larger size and longer hind foot, which usually measures more than 34 mm. There are also differences in coloration of fur. (Davis and Young, 1999)

The pealge of these animals is not remarkable. They appear to be salt and pepper, brownish rats. Although they look like voles, they are much larger. (Cameron and Spencer, 1981; Davis and Young, 1999)

  • Range mass
    125 to 211 g
    4.41 to 7.44 oz
  • Average mass
    172 g
    6.06 oz
  • Range length
    200 to 349 mm
    7.87 to 13.74 in

Reproduction

There is little information to be found on this subject for S. arizonae, or other members of the genus.

Little is known about the reproduction of this species as separate from S. hispidus because until recently they were considered the same species. However, it is likely that what is known about reproduction in S. hispidus, the sister species of S. arizonae will apply reasonably well to the latter species. (Davis and Young, 1999)

Sigmodon hispidus is known to undergo a postpartum estrus. These animals copulate 3 to 6 hours after giving birth, with ovulation following between 6.5 and 12 hours after partuation. (Cameron and Spencer, 1981)

Gestation in cotton rats lasts approximately 27 days. The litter size in S. hispidus ranges from 1 to 15. The neonates are well-developed for rodents, and are able to run at birth, although their eyes are not opened. Neonates weigh about 7.23 g. Growth is rapid, with the young gaining 1 or 2 g per day. Eyes typically open within 60 hours of birth. The young are weaned between 10 and 25 days. (Cameron and Spencer, 1981; Davis and Young, 1999)

In females of S. hispidus, conception took place at 40 days of age and estrus could occur from as early as 10 days of age. Sexual maturity in males, as determined by the presence of sperm in the epididymis, was always achieved by 3 months of age. (Cameron and Spencer, 1981)

  • Breeding interval
    The breeding interval of this species is not known, but related species can breed as often as once per month.
  • Breeding season
    Although there are no reports for S. arizonae, related species have reproductive peaks in spring and autumn.
  • Range number of offspring
    1 to 15
  • Average gestation period
    27 days
  • Range weaning age
    10 to 25 days
  • Range age at sexual or reproductive maturity (female)
    10 to 40 days
  • Range age at sexual or reproductive maturity (male)
    3 (high) months

Although no information is available for this species, it is likely that parental care is not extensive. The young are born well developed, and attain independence very early in life. Females probably provide the bulk of parental care, nursing the young for 10 to 25 days. It is likely that they also groom and protect the young. Male parental care has not been reported for this genus. (Cameron and Spencer, 1981; Davis and Young, 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
      • female

Lifespan/Longevity

Longevity of S. arizonae has not been reported. However, S. hispidus individuals appear not to live much beyond a year, given the length of residence any individual has been known to have in an area. (Cameron and Spencer, 1981)

Behavior

Cotton rats are active at all hours of the day. They are both diurnal and nocturnal. Activity patterns are influenced by biotic and abiotic factors. Although typically terrestrial, moving around on four feet, these animals are known to swim when necessary. When swimming, cotton rats use their hind feet for propulsion while holding their front feet near their body, and never swimming under water. (Cameron and Spencer, 1981)

Sigmodon hispidus builds nests on land. These appear to be mainly surface and burrow nests made of woven grasses. Nests range in shape from cup-shaped to a hollow ball-shaped. They typically have only one entrance. (Cameron and Spencer, 1981)

Sigmodon hispidus is considered a solitary species. There are dominance interactions, which appear to be based mainly on weight of animals, with heavier animals exerting dominance over lighter animals. Although there are no reports for the sociality of S. arizonae, it is likely that they resemble their sister species. (Cameron and Spencer, 1981; Davis and Young, 1999)

Home Range

The home range size for this species has not been reported.

Communication and Perception

There is no information or data found on this subject for S. arizonae. However, it is safe to make some generalizations based on the fact that these animals are mammals. Most mammals are known to communicate with vocalizations. Tactile communication is important in the context of agonistic and reproductive behavior. Scent cues are often used, especially in marking territories, and identification of individuals. Often there are visual signals in communication, such as body posture.

Food Habits

Sigmodon arizonae feeds mainly on grasses, but will also feed on citrus, other crops, some insects, and carrion. (Brylski, 1999)

  • Plant Foods
  • leaves
  • seeds, grains, and nuts
  • fruit

Predation

Although there is a paucity of information on this subject for S. arizonae, this species is likely preyed upon by a variety of raptors and carnivores. Given their apparent ability to reproduce quickly, they could be an important component of the diets of such animals, as are individuals from their sister species, S. hispidus. (Cameron and Spencer, 1981)

Ecosystem Roles

Arizona cotton rats are likely a source of food for a variety of raptors and carnivores.

Economic Importance for Humans: Positive

Cotton rats are used for laboratory experiments for polimyetitis and diptheria they are also used for food by some people. (Cameron and Spencer, 1981)

  • Positive Impacts
  • food
  • research and education

Economic Importance for Humans: Negative

They can eat crops, cause diseases, and alter landscapes. (Cameron and Spencer, 1981)

  • Negative Impacts
  • injures humans
    • carries human disease
  • crop pest

Conservation Status

Sigmodon arizonae is not listed by IUCN or CITES. However, the species may have some conservation concerns. There are reported to be five distinct subspecies of S. arizonae. However, two of these are reported to be probably extinct, and the status of the others is not currently known. (Davis and Young, 1999)

Other Comments

There are five reported subspecies of S. arizonae. These include S. a. arizonae, and S. a. jacksoni, each of which were reported from only one locality in Central Arizona and which are probably extinct. There are also S. a. cienegae, which is found in central and sourth-eastern Arizona into Mexico, S. a. plenus, found along the Colorado River, and S. a. major, found through southern Sonoro and Sinaloa in Mexico. (Davis and Young, 1999)

Contributors

Nancy Shefferly (editor), Animal Diversity Web.

Jim Porter (author), University of Wisconsin-Stevens Point, Chris Yahnke (instructor), University of Wisconsin-Stevens Point.

Glossary

Nearctic

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

acoustic

uses sound to communicate

agricultural

living in landscapes dominated by human agriculture.

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.

carrion

flesh of dead animals.

chemical

uses smells or other chemicals to communicate

crepuscular

active at dawn and dusk

diurnal
  1. active during the day, 2. lasting for one day.
dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates

endothermic

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.

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

fertilization

union of egg and spermatozoan

folivore

an animal that mainly eats leaves.

food

A substance that provides both nutrients and energy to a living thing.

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

herbivore

An animal that eats mainly plants or parts of plants.

iteroparous

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).

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

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

nocturnal

active during the night

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

scrub forest

scrub forests develop in areas that experience dry seasons.

sedentary

remains in the same area

sexual

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

solitary

lives alone

tactile

uses touch to communicate

temperate

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).

terrestrial

Living on the ground.

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

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.

savanna

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.

visual

uses sight to communicate

viviparous

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

References

Andersen, D. 1994. Demographics of small mammals using anthropogenic desert riparian habitat in Arizona.. Journal of Wildlife Management, 58(3): 445-454.

Brylski, P. 1999. "Arizona Cotton Rat" (On-line). California Wildlife Habitat Relations System. Accessed April 28, 2004 at http://www.dfg.ca.gov/whdab/html/M124.html.

Cameron, G., S. Spencer. 1981. Sigmodon hispidus. Mammalian Species, No. 158: 1-9.

Carelton, M., R. Fisher, A. Gardner. 1999. Identification and distribution of cotton rats, genus Sigmodon (Muridae: Sigmodontinae), of Nayarit, Mexico. Proceedings of the Biological Society of Washington, 112(4): 813-856.

Davis, R., P. Young. 1999. Arizona Cotton Rat, Sigmodon arizonae . Pp. 591-592 in D Wilson, S Ruff, eds. The Smithsonian Book of North American Mammals. Washington and London: The Smithsonian Institution Press.

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