Thirteen-lined Ground Squirrel
Order Rodentia : Family Sciuridae : Spermophilus tridecemlineatus (Mitchill)

Description. A small ground squirrel with usually 13 alternating dark and light stripes, the dark ones containing a series of squarish buffy spots, the light stripes occasionally broken into spots; dark dorsal stripes dark brown or black in color, the light stripes continuous and buffy white; underside of tail russet at base, shading to orange buff toward tip; lower sides cinnamon buff; belly pinkish buff; chin white; ear small. External measurements average: total length, 285 mm; tail, 105 mm; hind foot, 40 mm. Weight of males averages 154 g (up to 212 g); females, 160 g (to 220 g).

Distribution in Texas. Known from northern Texas and in a corridor extending from Tarrant and Dallas counties in north-central Texas south to Atascosa, Bee, and Calhoun counties along the Gulf Coast.

Habits. These squirrels are typically inhabitants of short-grass prairies, but they have invaded the tall-grass areas in Texas where they live principally in pastures and along fencerows. They live in burrows in the ground from which radiate well-marked paths to the feeding grounds. In tall grass the paths may become tunnels. In cultivated areas they seem to prefer fence-rows and excavate their burrows near fence posts. Occasionally, they usurp abandoned burrows of pocket gophers or even those of prairie dogs. Their own burrows are about 5 cm in diameter, have two or three openings, descend to a depth of 10-115 cm, and may be 7 m or more in length.

These squirrels are strictly diurnal but their annual cycle of activity includes a very long period of hibernation. In Texas, studies conducted by Howard McCarley revealed that the period of hibernation lasts about 240 days. Adults enter hibernation in July and young-of-the-year in August or September. They emerge from the middle of February to the first of March in the Texas Panhandle. In southern Texas they have been observed above ground as late as October 27 and as early as January.

Their food is chiefly green grasses and herbs in early spring but seeds, flower heads, and insects contribute importantly to their diet as the season advances. Grasshoppers are often conspicuous items in their stomach contents, and often more than half of the stomach contents consists of insects, including grasshoppers, crickets, caterpillars, beetles, ants, and insect eggs. They also eat mice and have been reported capturing and eating small chickens. Quantities of dry seeds stored in underground caches probably serve to carry the squirrels through the period of scarcity shortly after they emerge in the spring.

Mating activities begin about 2 weeks after squirrels emerge from hibernation. The males are sexually active for only 2-3 months which of necessity restricts the length of the breeding season. Normally only one litter is produced annually, but one study found about 25% of the females observed in a marked population produced two litters. The gestation period is 27-28 days. The young vary in number from two to 13; the yearling females produce the smallest litters. The young are blind, hairless, and toothless at birth and weigh from 3 to 4 g each. By the eighth day they are dark dorsally; on the 12th the stripes begin to appear and hair sparsely covers the back; on the 26th their eyes begin to open. The female then begins to wean them and at the age of 6 weeks they are entirely dependent upon their own resources. They mature sexually at about 9 or 10 months of age.

Where concentrated in pastures and farming areas these squirrels may cause serious loss of forage and crops, but their fondness for insects partly offsets any damage they may do. On rangelands they usually do no serious damage.

Photo courtesy of U.S. Fish and Wildlife Service.