Spermophilus spilosoma
spotted ground squirrel

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).

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).

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).

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).

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).

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).

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).

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.

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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).