Measurements.—Five males and 6 females, showing moderate wear on the third upper molars, from Walla Walla County, average, respectively: total length 141, 137.5; length of tail 71.8, 68.1; hind foot 17.2, 16.7; weight (males only) 10.5 grams. A female, 136 mm. long, from Grant County, weighed 9.5 grams. Over the range of megalotis in the United States, males average consistently larger than females in length of the hind foot and consistently less in length of head and body.
Distribution.—The Columbian Plateau, the Upper Sonoran Life-zone area west of the Columbia River and south of the Wenatchee Mountains in Kittitas, Yakima, and Benton counties, the north side of the Columbia River in Klickitat County, and Okanogan County east of the Okanogan River. Marginal localities are: Timentwa (W. W. D.) on the north, Maryhill (M. V. Z.) on the west, Wallula (M. V. Z.) on the south, and Colfax ([Taylor] and [Shaw], 1929: 21) on the east.
Fig. 105. Distribution of the western harvest mouse, Reithrodontomys megalotis megalotis, in Washington.
Remarks.—Harvest mice from the states of Washington, Oregon, Idaho, Utah, Nevada, California, Arizona, and New Mexico were examined to ascertain the validity of the name Reithrodontomys megalotis nigrescens [Howell] (type from Payette County, Idaho). No cranial characters or measurements could be found to separate nigrescens from megalotis. Specimens from Washington, Oregon, and west-central Idaho, within the range ascribed to nigrescens, averaged slightly darker in color with a more distinct, blackish dorsal line than specimens from New Mexico. This difference is slight, and specimens from any one locality vary greatly in color. Some Washington specimens are as pale as the palest material from New Mexico, and some of the darkest nigrescens can be matched by specimens of megalotis from Arizona and New Mexico. This slight color variant is not worthy of recognition as a distinct subspecies, and I agree with [Howell] (1939: 390) that nigrescens is a synonym of megalotis.
Description.—The western harvest mouse is about the size and shape of the common house mouse. The length of the tail is approximately equal to the length of the head and body. The tail is slim and lightly haired. The upper parts are brownish buff lightly washed with blackish. A faint but distinct dark dorsal stripe is usually present. The underparts are white. From Mus and Peromyscus, Reithrodontomys can be distinguished by the grooves on the anterior face of the upper incisors and the long brown hairs at the bases of the ears.
This genus reaches the extreme northern limit of its distribution in the state of Washington, where it ranges to the Canadian border. A single subspecies occurs in Washington. The harvest mouse is a resident of the Upper Sonoran Life-zone and it is most abundant in dense growths of grass, weeds, and other vegetation. Along the Yakima River, near Ellensburg, it was abundant in thick, river-side jungle. Near Yakima many specimens were taken in a marsh of cattails and tules. In the Grand Coulee they live in thick grass. Near Moses Lake several were found in their nests in a thick growth of tumbleweed. Harvest mice occur also, although never in great numbers, in relatively open areas. At the Potholes near Moses Lake, Grant County, a few were caught on sand dunes among scattered bushes of sage. They were taken on sagebrush flats in several localities. [Gray] (1943: 191) estimates their numbers in the sagebrush area of the Yakima Valley at 5 per acre.