Sydney Anderson

“ The caudal index (38 and 35% in two series) is slightly larger than that cited by Hall (33.3%) and is not significantly different from that in nanus (35.2%) . The color in both young and old mice is variable, but in general is more yellowish, and less grayish, than in any other series studied.
There is a large area in western Colorado and eastern Utah, between the known ranges of M. m. fusus and M. m. amosus from which there are no specimens. Probably the species occurs only at certain places in this arid region which seems to be a partial barrier to the species. ”

“ In these mice, as in other microtines (Elton, 1942; Piper, 1909) , there are seasonal, and irregularly multiannual fluctuations in population density, which sometimes are extreme. Consequently the mice at some times seem to be absent from suitable habitats, and at some other times occur there in amazingly large numbers.
Because the species is broken up into partly isolated, or at times completely isolated, colonies or local populations it may be supposed that various evolutionary forces such as selection and random genetic drift operate to foster variation. ”

“ The color in codiensis is lighter than in nanus. As compared to the new subspecies named below from the Big Horn Mountains to the east, codiensis is of similar size in head-body length, but has a relatively as well as actually longer tail; the hind foot averages longer; the upper molar tooth-row is relatively longer ”

“ Discussion.—This subspecies is separated from M. m. codiensis to the west by the Big Horn Basin. A series from along Buffalo Creek, 27 mi. N, 1 mi. E Powder River, 6075 ft., in Natrona County, Wyoming, is intermediate between the topotypes of zygomaticus and nanus in the characters cited above as distinguishing the two, but shows greater resemblance to zygomaticus in the shape of the zygomatic arch, in color which is paler than in topotypes of zygomaticus, and in the short hind foot. On these and on geographic grounds this population is referred to zygomaticus. ”

“ So that the variability can be evaluated more adequately, the coefficient of variability and 2 times the standard error of the mean are included for the measurements in two series. The series consist of all the adult specimens (with a condylobasilar length of 24.0 mm. or more) of both sexes from the areas specified. Various barriers are shown in Fig. 2 for comparison with the distributions of the subspecies and the localities of known occurrence shown in Fig. 1. Microtus montanus has not been taken in the Black Hills area of extreme northeastern Wyoming. ”

“ Subspeciation in the Meadow Mouse, Microtus montanus, in Wyoming and Colorado BY SYDNEY ANDERSON
Microtus montanus reaches the eastern limits of its geographic distribution in Wyoming and Colorado. There the mountains, but in general not the lowlands, are occupied by this species. A certain minimum of moisture may be of direct importance to the mouse and certainly is indirectly important, because certain hydrophytic or mesophytic grasses used by the mouse for food, for protection from enemies, and for shelter from the elements are dependent on the moisture. ”

“ Subsequent sampling from Idaho shows that the series of specimens available to Bailey was made up mostly of young and subadult animals. Finally, caryi does not occupy as Bailey stated "the meadows along streams in the arid sagebrush country of the Bear River, Green River, and Wind River valleys" exclusively, or characteristically. When the localities from which the species actually is known are plotted, it seems that the arid basin serves as a barrier and that the species is more commonly and abundantly found in montane meadows in the Transition and Canadian life-zones. ”

“ Measurements.—Average and extreme measurements of 24 adult males and females from several localities here referred to M. m. zygomaticus are as follows: total length, 159 (150-175) ; length of tail, 37.6 (31-46) ; hind foot, 18.6 (17-20) ; condylobasilar length of the skull, 25.3 (24.2-26.7) ; zygomatic breadth, 15.3 (14.1-16.7) ; alveolar length of upper molar tooth-row, 6.2 (5.7-6.8) ; prelambdoidal breadth, 8.7 (8.3-9.4) ; lambdoidal breadth, 11.9 (11.0-12.5) . Average and extreme measurements of a series of 12 adult male topotypes are as follows: total length, 159 (144-174) ”

“ Range.—Idaho; southwestern Montana; most of the southwestern half of Wyoming; southward to central Colorado. See figure 1.
Comparisons.—Comparisons with subspecies named as new in this paper will be found in the accounts of those subspecies beyond. From Microtus montanus fusus Hall, the subspecies to the south, M. m. nanus from Idaho differs as follows: averages smaller; slightly darker and less reddish and less yellowish in color; slightly wider braincase (see measurement of prelambdoidal breadth) ; larger bullae. ”

“ Minidoka Co.: Heyburn, 2 (USBS) . Cassia Co.: 2 mi. S, 2 mi. W Burley, 5. Bannock Co.: Pocatello, 23 (USBS 4) ; Swan Lake, 1 (USBS) . Bear Lake Co.: Montpelier Creek, 6700 ft., 3 (USBS) . Montana: Gallatin Co.: W. Fork of W. Fork, Gallatin River, 1 (USBS) . Park Co.: Lamar River, 7000 ft., 1 (USBS) ; Gardiner, 1 (USBS) . Sweet Grass Co.: 14 mi. S Big Timber, 1 (USBS) ; McLeod, 1 (USBS) ; West Boulder Creek, 18 mi. SE Livingston, 2 (USBS) .
Microtus montanus codiensis, new subspecies
Type.—Female, adult, skin and skull ”

“ S Lonetree, 1 (USBS) . Sweetwater Co.: Farson, 3; Bitter Creek, 3 (AMNH) ; Kinney Ranch, 21 mi. S Bitter Creek, 6800 ft., 9 (USNM 1, AMNH 2) ; 32 mi. S, 22 mi. E Rock Springs, 7025 ft., on Vermillion Creek, 15. Carbon Co.: 18 mi. ”