Spermophilus tridecemlineatus - Thirteen-Lined Ground Squirrel

Description:

Spermophilus tridecemlineatus is a rat-sized rodent reaching lengths from 225 – 300 mm (8.9 – 11.8 in) with a tail length of 75 – 109 mm (3.0 – 4.3 in).  Weight ranges from 100 – 220 g, depending on the season.  The back is the most distinctive part of the animal with seven dark brown stripes alternating with six thinner tan stripes beginning at the top of the head and extending to the rump.  Squarish light spots break up five of the darker stripes.  The stripes locating the sides of the body are less distinctive than those on the back.  The chest and belly are of a light color.  The ears are very small and the tail slender with a black stripe on the top.  Their call is a high-pitched trilling ch-r-r-r (Kurta 1998).

The skull is small with a slightly shorter toothrow than other ground squirrels.  The bullae are not inflated and a postorbital process is present.  The lower jaw is v-shaped and the infraorbital foramen is small and shifted forward.  The dental formula is 1/1, 0/0, 2/1, 3/3= 22.  Like most rodents, incisors are very large and canines are lacking (Struebel and Fitzgerald 1978).

Distribution in Wisconsin:

S. tridecemlineatus is found north from central Alberta, Manitoba, and Saskatchewan south to Texas and New Mexico, and east from central Ohio west to Colorado.  Originally, the animal was limited to the prairies of the North American Great Plains but clear cutting and agriculture has increased its range (Kurta 1998).  S. tridecemlineatus is found throughout Wisconsin and inhabits grassland ecosystems as well as parks, yards, fields, golf courses, and cemeteries.  In Wisconsin, the species greatly extended its range after forest areas were cleared for farming.

The fossil record includes lower jaws and maxillaries found in a Pleistocene deposit in Kansas that appear to be identical to modern populations.  Twenty cheekteeth were collected from a mid-Pleistocene deposit in Texas (Struebel and Fitzgerald). 

Ontogeny and Reproduction:

Five days after emerging from hibernation female squirrels are in full heat. They stay sexually active for 2 weeks.  Ovulation occurs within 48 hours of copulation.  Gestation lasts for 28 days and litter size varies from 3 to 14, with an average of 9 young.  In Wisconsin, copulation dates can range from mid April to early May and litters emerge in late June (Struebel and Fitzgerald 1978).  Females produce one litter per year.

At birth, S. tridecemlineatus is naked, blind and toothless, and weighs about 3 – 4 g.  Newborns grow at a rate of 1.3g/day and up to 90% of the litter may die from predation.  Newborns acquire a light coat of hair at day 4; stripes appear at day 12, lower incisors show at day 17, upper incisors at 19, and molars at day 24, and eyes open at 21 - 31 days.  Adult body length is reached at 11 weeks.  After 6 to 12 weeks the young are weaned and on their own.  Shortly after being weaned they will construct their own burrow, usually in close proximity (100m) from their birth site.  Young squirrels are sexually mature by 9 or 10 months (Kurta 1998). 

Ecology and Behavior:

S. tridecemlineatus typically inhabits grasslands, commonly avoiding wet regions.  Black or clay soil is preferred to sand and areas inhabiting short vegetation chosen over those with vegetation receiving irrigation or fertilizer.

This species is strictly diurnal and rarely ventures out of the burrow on dark or damp days.  They survey the area around their burrow by standing in an upright position with arms held closely to the chest (Cleary and Craven 1994).  If the threat of danger is suspected, they quickly retreat to the nearest burrow.  Burrows are 2 inches in diameter, often covered with vegetation, and may have 2 or 3 openings.  The entrance descends for 6 inches until it angles and sharply turns an “L” shape to confuse predators.  The nesting chamber is 9 inches in diameter and lined with dry grass.  S. tridecemlineatus also may use burrows of other animals such as pocket gophers or prairie dogs.        

Hibernation begins in late September or early October and emergence occurs between late March and early May.  Males tend to begin hibernation and emerge earlier than females.  Body temperatures decrease to within 3ºC of ambient air temperature.  Their heartbeat decreases from 350 to 5 beats per minute.  Prior to hibernation, individuals become more aggressive and intolerant of others, their home range is reduced, and they become relatively inactive (Streubel and Fitzgerald 1978).

S. tridecemlineatus is omnivorous with 50% of their diet consisting of animal matter.  Prey items consist of grasshoppers, caterpillars, Lepidoptera larvae, beetles, ants, insect eggs, mice, small passerines, small chickens, and even each other.  Vegetation consumed is seeds, green shoots, roots, vegetables, fruits and cereal grains (Kurta 1998).  Water is rarely consumed and hydration is achieved from their diet.  Large quantities of seeds are cached to be eaten during food shortages.  Predators include hawks (especially Circus cyaneus), badgers, weasels, foxes, coyotes, domestic cats, bull snakes, and black snakes.  Many parasites live internally and externally on this squirrel including fleas, lice, mites, and ticks.

Males tend to have a higher mortality rate than females because of their longer distances traveled away from the burrow after weaning, leaving them more vulnerable to predation (Streubel and Fitzgerald 1978).  The home range of males is about 5 hectares and for females 1.5 hectares.  Males extend to the greatest lengths of their home range during the breeding season while females extend farthest during pregnancy and lactation.  Females tend to use the same home range area year after year.  Both sexes are generally non-territorial.

S. tridecemlineatus emits a distress call when threatened.  Females give an alarm call to young after their emergence from the burrow until three or four days later. Young respond by diving into a burrow entrance, rather than returning to their mother (Arenz and Leger 1999).

Remarks:

S. tridecemlineatus exhibits cannibalism and infanticide, specifically in males (Vestal 1991).

Thirteen-lined ground squirrels may also be referred to as Federation Squirrels, striped gophers, or thirteen-liners.

Spermophilus means seed lover.

Literature Cited:

Arenz, C.L. and D.W. Leger.  1999.  Thirteen-lined ground squirrels (Sciuridae: Spermophilus tridecemlineatus) antipredator vigilance decreases as vigilance cost increases.  Animal Behaviour 57:97-103.

Cleary, E.C. and S.R. Craven.  1994.  Prevention and Control of Wildlife Damage: Thirteen-Lined Ground Squirrels.  Wildlife Damage Handbook B: 165-169

Kurta, A.  1998.   Mammals of the Great Lakes.  The University of Michigan Press, Ann Arbor, Michigan, USA.

Streubel, D.P. and J.P. Fitzgerald.  1978.  Mammalian Species: Spermophilus tridecemlineatus. American Society of Mammalogists.  103:1-5.

Vestal, B.M.  1991.  Infanticide and cannibalism by male thirteen-lined ground squirrels.  Animal Behaviour 41:1103-1104.

Reference written by Reba Riedner, Biol 378: Edited by Chris Yahnke. Page last updated 4-29-04.