Spermophilus spilosomaspotted ground squirrel

Geographic Range

Spermophilus spilosoma ranges from south-central South Dakota, through the sand hills of Nebraska, and into south-eastern Wyoming. It also occurs throughout eastern Colorado, northwestern Oklahoma, western Kansas, western Texas, most of New Mexico, the southeastern corner of Utah, and eastern Arizona, south to central Mexico (Streubel 1978).

Habitat

Spermophilus spilosoma is generally found in areas with deep sandy soils and sparse vegetation. In Colorado, S. spilosoma is most abundant in the sand hills of the northeastern part of the state and along the Arkansas River in the southeast. In Kansas, S. spilosoma inhabits areas of sparse vegetation. Drifted sand along rivers in southwestern Kansas is commonly inhabited. In Texas, S. spilosoma inhabits the desert scrub association consisting primarily of creosote Larrea divaricata and blackbrush Flourensia cernua. In New Mexico, S. spilosoma burrows predominately on short-grass mesas or along banks of arroyos. Some inhabit disturbed areas along highways where the soil consists of gravelly sand with enough clay to hold the particles together. In Utah, S. spilosoma prefer lighter soils for their burrows. The entrances to these burrows are usually located under low shrubs. In Nebraska, S. spilosoma is characteristic of the sand hills where it occurs sympatrically with S. tridecemlineatus. In south-eastern Wyoming, S. spilosoma inhabits three distinct community types - sand dunes, yucca grass, and sage-grass. In all three communities, though, the soil is sand to loamy sand, the height of the dominant vegetation exceeds 250 mm, and more than 40% of the soil surface is bare. The evidence indicates that S. spilosoma favors land that is heavily grazed. In northeastern Colorado, S. spilosoma is most abundant on deep sand soils that were grazed heavily by cattle (Streubel 1978).

Physical Description

Spermophilus spilosoma is a small ground squirrel with white non-linear dorsal spots, and white underparts. Measurements (in mm) range as follows: total length 185 to 253, tail length 55 to 92, hind foot length 28 to 30, and greatest length of skull 34 to 43 (Streubel 1978). The dorsal surface of the tail of S. spilosoma resembles the back but is black at the tip. The ventral surface of the tail is a shade of cinnamon (Hall 1959). Color variation exists in S. spilosoma in regions with varied substrates (Streubel 1978).

Spermophilus spilosoma has two distinct pelages and molts each year. Adults emerge from hibernation in winter pelage and undergo molt by late May. The autumn molt is not conspicuous in adults, but is very noticeable in juveniles. Generally, the pelage of adults is paler than the pelage of juveniles (Sumrell 1949).

Spermophilus spilosoma is classified as a high desert species. Observations have indicated that S. spilosoma 1) has a low pulmocutaneous water loss, 2) has a high level of conductance irrespective of body weight, 3) can decrease conductance and increase insulation at high ambient air temperatures, and 4) has a basal metabolism about 60% of that expected from standard metabolic equations (Hudson 1973).

  • Range length
    185 to 253 mm
    7.28 to 9.96 in

Reproduction

The gestation period for S. spilosoma is 24 days (Sumrell 1949). The breeding season begins in mid-April, two to three weeks after emergence from hibernation. Most males are capable of breeding through June, but by mid-July their breeding season ends. The males have descended testes by mid-April and abdominal testes by the first of August. Among the females, most breeding occurs in May and June but some occurs in early July. The extended breeding season results from later breeding on the part of yearling females. S. spilosoma is diestrous in the southern part of its range (Streubel 1978). Litter sizes for S. spilosoma range from 4 to 12. Juveniles weigh 40 to 50 g at the time of emergence from their nest burrows (Sumrell 1949).

  • Breeding interval
    These squirrels breed once annually.
  • Breeding season
    mid-April to mid-July
  • Range number of offspring
    4 to 12
  • Average gestation period
    24 days
  • Average time to independence
    2 to 3 weeks

The dependence of juvenile S. spilosoma on their mothers ceases two to three weeks after their first emergence from the nest burrow (McCarley 1966).

  • Parental Investment
  • pre-fertilization
    • provisioning
  • pre-hatching/birth
    • provisioning
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female
  • pre-independence
    • provisioning
      • female
    • protecting
      • female

Behavior

In S. spilosoma, foraging and feeding make up about 66% of all activity above ground; alert behavior 15%; other maintenance behavior (including sunning, grooming, eliminating, resting in the shade, and sandbathing) 8%; investigative (non-foraging) behavior 6%; and sexual and agonistic behavior less than 2% for males, and less than 0.5% for females.

In the breeding season, the time spent in definite sexual and agonistic behavior is 5 to 7% for males and 2 to 2.5% for females. Sexual behavior is indicated by a noticeable frenzy of activity at a particular burrow system. This activity is characterized by individual males repeatedly approaching females, which generally adopt a threat posture. Occasionally, a male and female enter a burrow system together and mate.

Spermophilus spilosoma exhibits seven different alert behaviors. The most prominent alert behavior is foot stomping (alternately, rapidly stomping the hind feet). This behavior signifies threat or apprehension, and it might occur when a bull snake investigates a female burrow (Streubel 1978).

Daily activity of S. spilosoma varies seasonally and with weather conditions. Emergence from burrows generally occurs when air temperature is between 20 and 33 degrees Celsius. In March and April, S. spilosoma is most active between 0800 and 0900 hours and remains in its burrow after 1600 hours. In July and August, S. spilosoma emerges from its burrow from 0630 to 0700 hours, returns to its burrow by 1100 hours, and re-emerges from 1530 to 1600 hours, remaining active until after 1700 hours. On bright, hot days, S. spilosoma remains in its burrow until the air temperature drops to 33 degrees Celsius or less. During strong winds or rain, S. spilosoma becomes inactive (Sumrell 1949).

The first S. spilosoma to emerge from burrows are young of the previous year, followed shortly by the adults. Males begin entering hibernation in late July, and by mid-August all have entered hibernation. Most young of the year enter hibernation in late September. The seasonal activity cycle is estimated to be 115 to 135 days for males and 95 to 125 days for females (Streubel 1978).

Communication and Perception

The most prominent alert behavior is foot stomping (alternately, rapidly stomping the hind feet). This behavior signifies threat or apprehension, and it might occur when a bull snake investigates a female burrow (Streubel 1978).

Food Habits

Spermophilus spilosoma feeds mostly on seeds and green plant parts. The six main plant species eaten by S. spilosoma are Salsola pestifer, Tribulis terrestris, Oryzopis hymenoides, Curcurbita foetitissima, Thelesperma megapotamicum, and Erigonum sp. Spermophilus spilosoma relies heavily on the seeds of Solanum rostratum, Mentzelia nudastricta, and Croton texensis early in the spring. Green grass shoots are eaten as they appear, and in late May and June the flowers and seeds of Festuca octoflora are also eaten. In late July and August, S. spilosoma feeds heavily on grasshoppers and insect larvae. Spermophilus spilosoma has also been observed feeding on kangaroo rats and lizards (Streubel 1978).

  • Animal Foods
  • mammals
  • reptiles
  • Plant Foods
  • leaves
  • roots and tubers
  • wood, bark, or stems
  • seeds, grains, and nuts

Predation

Bull snakes, Pituophis catenifer represent a major predator on S. spilosoma. Additionally, red-tailed hawks, Buteo jamaicensis have been observed capturing S. spilosoma (Streubel 1978).

Ecosystem Roles

Although the specific ecosystem roles of this species have not been investigated, it is likley that they play some role in local food webs. They are prey to other animals. In addition, they are seed and grass predators, so they probably influence vegetative growth to some degree. Their burrowing behavior helps to aerate the soil.

Conservation Status

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Other Comments

Spermophilus spilosoma is classified as a high desert species. Observations have indicated that S. spilosoma 1) has a low pulmocutaneous water loss, 2) has a high level of conductance irrespective of body weight, 3) can decrease conductance and increase insulation at high ambient air temperatures, and 4) has a basal metabolism about 60% of that expected from standard metabolic equations (Hudson 1973).

Contributors

Nancy Shefferly (editor), Animal Diversity Web.

Michael Misuraca (author), University of Michigan-Ann Arbor.

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

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.

chemical

uses smells or other chemicals to communicate

colonial

used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.

crepuscular

active at dawn and dusk

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.

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

fertilization

union of egg and spermatozoan

folivore

an animal that mainly eats leaves.

fossorial

Referring to a burrowing life-style or behavior, specialized for digging or burrowing.

granivore

an animal that mainly eats seeds

herbivore

An animal that eats mainly plants or parts of plants.

hibernation

the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.

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.

native range

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

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

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

soil aeration

digs and breaks up soil so air and water can get in

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).

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.

vibrations

movements of a hard surface that are produced by animals as signals to others

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.

References

Hall, E. R. 1959. The mammals of North America. Ronald Press Co., New York, 1: 1-546.

Hudson, J. W. 1973. Metabolism, pulmocutaneous water loss and respiration of eight species of ground squirrels from different environments. Comp. Biochem. Physiol. 45A:69-100.

McCarley, H. 1966. Annual cycle, population dynamics, and adaptive behavior of Spermophilus spilosoma. Jour. Mammal. 47:294-316.

Streubel, D. P. 1978. Spermophilus spilosoma. Mammalian Species, No. 101, pp. 1-4. American Society of Mammalogists, New York.

Sumrell, F. 1949. A life history study of the ground squirrel Citellus spilosoma major (Merriam). M.S. thesis, Univ. New Mexico, Albuquerque, 100 pp.