Separate dens are used for aestivation. While the squirrel is in aestivation the entrances to the summer burrow are solidly plugged with tamped earth. Aestivation dens are short and may or may not communicate with the summer burrow. Shaw (1925B: 58) measured 50 such dens and found the aestivation cell to average 2 feet 6 inches beneath the surface with extreme depths of 6 inches and 4 feet 11 inches. Ordinarily a drainage burrow is dug beneath the level of the nest. [Illustration: FIG. 85. Distribution of the Columbian and Beechey ground squirrels in Washington. A. _Citellus columbianus columbianus._ B. _Citellus columbianus ruficaudus._ C. _Citellus beecheyi douglasii._] The time and length of aestivation is variable. Old males enter aestivation before the females and young, and emerge earlier in the spring. In the lowlands, as about Pullman, Whitman County, the squirrels begin to disappear in mid July. In the mountains of northeastern Washington they are active until late August. They emerge again in late February or early March. Shaw (1925B) mentions that food is occasionally stored in aestivation dens of male squirrels. This food is not eaten in the winter but in the spring, when the squirrels awaken. The gestation period was determined by Shaw (1925C: 108) as 24 days. Two to five, rarely seven, young are born. The average litter is about 3.5. Mating occurs in late March and the young are born about the middle of April. Most green vegetation occurring in its habitat is food for the Columbian ground squirrel. Bulbs, seeds, fruit, berries, grain, clover, alfalfa, and garden truck are eagerly eaten. The squirrels are especially fond of wheat and great damage results from their depredations. In northeastern Washington some wheat fields are almost entirely destroyed. Shaw (1925G) showed that one squirrel destroyed an average of more than 50 pounds of wheat in a season. When populations range as high as ten squirrels to the acre, little wheat is left. In Montana the Columbian ground squirrel is a known reservoir of Rocky Mountain spotted fever (Birdseye, 1912: 1-46) while sylvatic plague has been reported in this species in Oregon (Meyer, 1936: 965). Control of the Columbian ground squirrel by traps, poison, and shooting is only locally successful. =Citellus columbianus columbianus= (Ord) _Arctomys columbianus_ Ord, Guthrie's Geography, 2d American Edition, 2:292 (description, p. 303), 1815. _Spermophilus columbianus_ Merriam, N. Amer. Fauna, 5:39, July 30, 1891. _Citellus columbianus_ Allen, Bull. Amer. Mus. Nat. Hist., 19:536, October 10, 1903. _Type._--None designated. Description based on Lewis and Clark's account of animals taken by them between the forks of the Clearwater and Kooskooskie rivers, Idaho. _Racial characters._--Smaller size and paler color than in _ruficaudus_.