Microtus montanus pratincolus new subspecies
Type.—Female, adult, skull and skin, No. 34004, Univ. Kansas, Mus. Nat. Hist.; from 6 mi. E Hamilton, 3700 ft., Ravalli County, Montana; obtained on August 14, 1949, by John A. White; original number 477.
Geographic distribution.—Flathead and Bitterroot valleys of western Montana.
Diagnosis.—Size small for the species (see measurements). Color: Essentially as in Microtus montanus nanus. Skull: Small, slender, and comparatively smooth; rostrum moderately depressed distally; nasals moderately inflated distally and extending posteriorly not quite to tips of premaxillary tongues; nasals usually truncate posteriorly, but rounded in some individuals; premaxillary tongues terminating posteriorly in a short medial spine; zygomatic arches lightly constructed and usually more widely spreading posteriorly than anteriorly; interparietal comparatively long and terminating in a small, but distinct, medial spine, otherwise approximately rectangular in shape; exposed parts of upper incisors short and, for the species, only slightly procumbent; molar dentition weak and, in most specimens, especially so posteriorly; tympanic bullae large and well inflated, especially ventrolaterally; basioccipital narrow owing to the encroachment of the tympanic bullae.
Comparison.—Among named forms, Microtus montanus pratincolus most closely resembles Microtus montanus nanus. The geographic range of M. m. nanus adjoins that of M. m. pratincolus on three sides; there is no conspecific subspecies adjoining the range of M. m. pratincolus on the north. From M. m. nanus, M. m. pratincolus differs as follows (measurements are all of males, those of M. m. nanus being of nine topotypes and near topotypes from central Idaho): size smaller (149 mm. as opposed to 165), tail shorter (37 as opposed to 39), hind foot shorter (19 as opposed to 20), upper molar series shorter (expressed as a percentage of basilar length, 25.5 as opposed to 26.3), mastoidal region broader (expressed as a percentage of basilar length, 48.6 as opposed to 46.7), braincase slightly more vaulted (depth of braincase expressed as a percentage of basilar length, 31.3 as opposed to 30.4) and more inflated laterally; tympanic bullae more inflated, this inflation being the most conspicuous difference between the two subspecies. The tympanic bullae of M. m. pratincolus have approximately a fourth more volume than those of M. m. nanus.
Remarks.—Northwardly in the Bitterroot Valley, specimens of M. m. pratincolus morphologically approach M. m. nanus, especially in the reduced degree of inflation of the tympanic portion of the bullae. On geographic grounds we think that the geographic range of M. m. pratincolus extends southward to the southern end of the Bitterroot Valley; we have not seen specimens from that area. Although we have not examined the specimen reported upon by Davis (Murrelet, 18:26, September 4, 1937) from Canyon Creek, "a few miles west of Hamilton", Montana, we think that it will be found to belong to M. m. pratincolus.
Our examination of specimens from localities in Montana east of the range here ascribed to M. m. pratincolus indicates that, among named kinds of Microtus, those specimens are best referred to M. m. nanus. These specimens are listed below under comparative materials. It should be mentioned here that although Bailey (loc. cit.) applied the name Microtus nanus canescens to Montanan specimens from Flathead Lake and Hot Springs Creek, the subspecies M. montanus canescens now is thought to be restricted to Washington and the adjoining part of British Columbia; M. m. canescens does not occur so far east as Montana.
Grateful acknowledgment is made to those persons in charge of the Biological Surveys collection for permission to study the specimens in that collection, and to the Kansas Endowment Association for support of the field work which yielded the specimens from six miles east of Hamilton, Montana. The study here reported upon was aided also by a contract between the Office of Naval Research, Department of the Navy, and the University of Kansas (NR 161–791).
Measurements.—The following measurements in millimeters are those of the holotype and the average, maximum, and minimum, respectively, of eleven adult males from various places in the range of the subspecies. Except as noted below, we are unable to detect significant morphological differences in the populations sampled and believe that pooling of the measurements is justifiable in this case. Measurements are: Total length, 129, 149 (156–141); length of tail-vertebrae, 27, 37 (39–31); length of hind foot, 18, 19 (20–18) (all preceding measurements are those of the collectors); basilar length, 22.2, 24.5 (25.7–23.4); greatest length of nasals, 6.7, 6.9 (7.4–6.4); zygomatic breadth, 14.2, 14.6 (14.9–13.9); mastoidal breadth, 11.3, 11.8 (12.3–10.8); alveolar length of upper molar series, 6.0, 6.2 (6.5–5.9); depth of braincase (shortest distance from ventral surface of basioccipito-basisphenoidal suture to the dorsal surface of the cranium, and not perpendicular to the long axis of the skull), 7.7, 7.7 (7.9–7.5); width of rostrum, 4.7, 4.8 (5.0–4.6); interorbital breadth, 3.2, 3.4 (3.6–3.2). Measurements of females, other than those of the holotype, are not given owing to the lack of sufficient material. Females, however, do not appear to differ appreciably in measurements from males.
Specimens examined (in U.S. Nat. Mus., Biol. Surv. Coll., except as otherwise indicated).—Total, 26, all from Montana, as follows: Sanders Co.: Hot-springs Cr., 4. Lake Co.: end of W arm Flathead Lake, 5; Ravalli, 8. Ravalli Co.: Florence, 2; 8 mi. NE Stevensville, 4000 ft., 1; Corvallis, 4; 6 mi. E Hamilton, 3700 ft., 2(K.U.).