Type.—Adult male, skin and skull, number 37873, University of Kansas, Museum of Natural History, obtained by Rollin H. Baker, original number 1343, 12 miles north and 2 miles east of Sage, 6100 ft., in Lincoln County, Wyoming, on July 19, 1950.
Range.—North-central Utah, eastern Idaho, western Wyoming, and southwestern Montana. See Figures 1 and 2.
Diagnosis.—Size average; color dark, especially in southern part of range; tail relatively long; molar teeth small; nasals relatively broad; maxillary septum relatively small.
Comparisons.—From M. p. uligocola, the subspecies to the southeast, M. p. pullatus differs as follows: relatively darker in southern part of its range (see Figure 1); smaller, tail relatively longer. In 6 of 7 pairs of skulls compared of pullatus (from Lincoln Co., Wyoming) and uligocola (from Larimer Co., Colorado), pullatus had relatively broader nasals (Confidence Limit .85); uligocola had larger maxillary septa (C. L. .97) and larger molar teeth (C. L. .90). From insperatus, the subspecies to the east, pullatus differs as follows: both summer and winter pelage darker; tail longer both actually and relatively; upper molar tooth-row shorter. Ten pairs of skulls of specimens from near Afton, Wyoming, representing pullatus, and from northeastern Wyoming, representing insperatus, revealed no significant differences in the features observed by the "method of pairs". Although not compared in detail with the subspecies to the north, M. p. drummondi (Audubon and Bachman), examination of specimens from western Montana and the accounts of other authors indicate that topotypes of pullatus are darker, longer-tailed, slightly larger-skulled and perhaps longer over all.
Remarks.—In this subspecies there is a cline in color from dark in extreme southwestern Wyoming to pale in north-central Wyoming and Montana as the range of M. p. insperatus is approached. There is thus a broad zone of intergradation in color and the line separating the subspecies must be drawn somewhat arbitrarily. In Wyoming the most distinct break in this cline is in the Big Horn Basin and if a detailed study of the species were made in Montana the break would probably be found where the mountains meet the plains, roughly as shown in Figure 1. There is a similar cline in western Montana in color. The mice are paler farther north as one approaches the Canadian border although they do not become so pale as insperatus. Darkness is a characteristic of several non-adjacent subspecies of Microtus pennsylvanicus, for example M. p. kincaidi Dalquest in central Washington (Dalquest, 1948:347), M. p. finitus, and M. p. nigrans Rhoads in eastern Virginia, but these subspecies presumably differ in other characters.
Some morphological features of the same kind and degree that differentiate subspecies in one place may not vary geographically in another place. Furthermore the geographic variation in one feature may be only partly correlated with the variation in another feature. The variation in M. p. pullatus is an example: Specimens from near Pocatello, Idaho, are darker than topotypes of modestus but specimens from Fremont County, Idaho, are indistinguishable from topotypes of modestus (Davis, 1939:315). I have examined a number of mice from the Bitterroot Valley in western Montana and the color value for 12 adults is 2.7. They are slightly but not significantly paler than topotypes of modestus. This is a result of the cline mentioned above and does not indicate relationship with modestus. Some average measurements of 10 skulls from this series are as follows: condylobasilar length, 24.9; zygomatic breadth, 14.4; interorbital breadth, 3.4; lambdoidal breadth, 11.5; prelambdoidal breadth, 9.1; alveolar length of upper molar teeth, 6.5; and depth of braincase, 7.6. Average external measurements of 9 specimens are as follows: total length, 157; length of tail, 36; length of hind feet, 19.4. Mice from the Bitterroot Valley were compared with topotypes of modestus by the "method of pairs," and modestus had a larger foramen magnum in 6 of 6 pairs (Confidence Limit .97) and larger first upper molar teeth in 5 of 6 pairs (C. L. .75).
A comparison of topotypes of pullatus with modestus shows a similar difference in the teeth, modestus being larger, but in the size of the foramen magnum there is no difference. A comparison of the measurements of pullatus (Bitterroot Valley), pullatus (near topotypes), and modestus (topotypes) shows that the two series of topotypes differ significantly in condylobasilar length (of borderline significance), zygomatic breadth and lambdoidal breadth (both of which vary greatly with age), and length of molar series; the specimens from the Bitterroot Valley agree with pullatus rather than modestus in all of these characters. The specimens from the Bitterroot Valley are smaller than pullatus (topotypes) in total length; they more closely resemble modestus in length of tail, and the hind foot is shorter than in either modestus or pullatus (which do not differ significantly). Specimens from western Montana resemble modestus in certain respects but in most respects resemble topotypes of pullatus and are referred to pullatus. Specimens from Coeur d'Alene, Idaho, and from Blackfoot, Montana (marginal records of modestus, Hall and Cockrum, 1953:410) may be referred to drummondi. Marginal records of pullatus in Montana to my knowledge are: Florence, Ravalli Co.; Highwood Mtns., Chouteau Co.; 7 mi. NE Hilger, Fergus Co.; 10 mi. NW Park City, Stillwater Co. (all represented by specimens in the USBS). The line separating pullatus and drummondi is tentatively drawn as shown in Figure 1.
Specimens examined.—Total 256. Wyoming: Yellowstone National Park: Mammoth Hot Springs, 17 (USNM). Park Co.: 31/5 mi. E, 3/5 mi. S Cody, 5020 ft., 15. Teton Co.: Whetstone Creek, 4 (Mich); 5 mi. N Moran, 13 (Mich); Moran and environs (4 localities within 4 miles of Moran), 6200 ft., 54; Teton Park, Trappers Lake, 3 (Mich); Teton Park, Jenny Lake, 1 (Mich); Teton Park, String Lake, 1 (Mich); Sheep Creek, 1 (Mich); Jackson and environs, 115 (Mich 113, USBS 1). Sublette Co.: 34 mi. N, 4 mi. W Pinedale, 7950 ft., 2; Kendall, 5 (Mich). Lincoln Co.: 9½ mi. N, 2 mi. W Afton, 6100 ft., 1; 9 mi. N, 2 mi. W Afton, 3; 7 mi. N, 1 mi. W Afton, 11; 15 mi. N, 3 mi. E Sage, 6100 ft., 1; 12 mi. N, 2 mi. E Sage, 4; 6 mi. N, 2 mi. E Sage, 2.
Microtus pennsylvanicus insperatus (Allen)
Arvicola insperatus Allen, Bull. Amer. Mus. Nat. Hist., 6:347, December 7, 1894.