Microtus montanus fusus Hall
Microtus nanus, Bailey, N. Amer. Fauna, 17:30, June 6, 1900 (part); Cary, N. Amer. Fauna 33:123, August 17, 1911.
Microtus montanus fusus Hall, Proc. Biol. Soc. Washington, 51:131-134, August 23, 1938; Warren, The Mammals of Colorado, Univ. of Okla. Press, p. 229, 1942.
Type.—Male, adult, skin and skull; No. 61281, Museum of Vertebrate Zoology; 2½ miles east of summit of Cochetopa Pass, Saguache County, Colorado; Sept. 21, 1933; collected by Annie M. Alexander; original number 2568. Type not seen by me.
Range.—Southern Colorado and northern New Mexico.
Comparisons.—For comparison with M. m. nanus, the subspecies to the northward, see the preceding account of that subspecies. For comparison with M. m. amosus the subspecies to the west see Hall (1938) and Durrant (1952). I have not examined specimens of amosus.
Measurements.—Average and extreme measurements for 17 adults including both males and females from several localities in southern Colorado are as follows: total length, 160 (136-179); length of tail, 42 (35-55); hind foot, 19.2 (17-23); condylobasilar length of the skull, 25.2 (24.0-26.0); zygomatic breadth, 15.0 (14.1-15.5); alveolar length of upper molar tooth-row, 6.4 (6.0-6.7); prelambdoidal breadth, 8.7 (8.3-9.2); lambdoidal breadth, 11.7 (11.1-12.6).
Average and extreme measurements of 4 adults (2 males and 2 females) from the type locality and 11 adults (4 males and 7 females) from other localities in southern Colorado are as follows: total length, 162 (157-168), 157 (137-169); length of tail (means only), 44.5, 40.5; hind foot, 18.8 (18-19), 18.6 (18-23); condylobasilar length of skull, 24.5 (24.0-24.7), 25.2 (24.3-26.1); alveolobasilar length, 14.2 (13.9-14.5), 14.6 (14.1-15.1); palatilar length, 13.2 (13.0-13.4), 13.5 (13.1-14.2); alveolar length of upper molar tooth-row, 6.3 (6.0-6.6), 6.4 (6.3-6.7); zygomatic breadth, 15.0 (14.3-15.5), 14.9 (14.1-15.5); interorbital breadth, 3.5 (3.3-3.6), 3.5 (3.3-3.7); lambdoidal breadth, 11.8 (11.1-12.6), 11.7 (11.2-12.3); prelambdoidal breadth, 8.6 (8.3-9.2), 8.8 (8.3-9.0); depth of braincase, 7.5 (7.2-7.8), 7.6 (7.1-7.9).
Discussion.—There is no sharp boundary between M. m. fusus of southern Colorado and the subspecies to the north, M. m. nanus. Although the line separating these two subspecies is drawn somewhat arbitrarily, on the whole the samples from north of this line more closely resemble nanus. All of the means for total length given above are larger than the maximum given in Hall's description of fusus. 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.
Specimens of M. montanus from northern New Mexico have been referred previously to M. m. arizonensis. When he named M. m. fusus, Hall mentioned its resemblance to arizonensis in reddish coloration, but pointed out that fusus is less reddish. Of six specimens from Valle Santa Rosa, Jemez Mountains (USBS), 8500 ft., Rio Arriba County, New Mexico, three are immature, and the skulls of the remaining specimens are damaged. In reddish color and relatively large size these few specimens resemble arizonensis more than fusus although the locality of occurrence is closer to the geographic range of the northern fusus than to that of arizonensis. The identification of these specimens as arizonensis is provisional; additional specimens are needed from the area, 200 miles wide, which separates the ranges as now known of arizonensis in Arizona from the occurrence in New Mexico. There is a single specimen from this area, the damaged skull of which prevents conclusive identification. The specimen is either M. montanus or M. mexicanus, and is from Nutria, on the southern edge of the Zuni Mountains (USBS). Detailed comparison of fusus and arizonensis is not attempted here although it may be stated that in several characters fusus is intermediate between arizonensis to the south and nanus to the north.
Specimens examined.—Total, 89, distributed as follows (abbreviations for collections are given in the account of M. m. nanus; localities that are not represented in [Fig. 1] because overlapping or crowding of the symbols would result are Italicized):
Colorado: Pitkin Co.: 5 mi. W Independence Pass, 11,000 ft., 1 (Chi). Lake Co.: Independence Pass, 12,095 ft., 2 (Chi). Gunnison Co.: Gothic, 2 (USBS); Decker's Ranch, Crested Butte, 2 (AMNH); Almont, 3 (USBS). Montrose Co.: Coventry, 5 (USBS 4, AMNH 1). Saguache Co.: Cochetopa Pass and environs, 44 (USBS 22). Hinsdale Co.: Ruby Lake, 1 (USBS). Mineral Co.: 3 mi. E Creede, 1; 23 mi. S, 11 mi. E Creede, 9300 ft., 7. La Plata Co.: Florida, 6800 ft., 1. Conejos Co.: 1 mi. S, 19 mi. W Antonito, 10,200 ft., 3; 4 mi. S, 23 mi. W Antonito, 1; 5 mi. S, 24 mi. W Antonito, 9600 ft., 9.
New Mexico: Rio Arriba Co.: 6 mi. W Hopewell, 9900 ft., 6 (USBS); Tusas River, 8700 ft., 1 (USBS).
Some measurements not given above are included in [Table 1], together with the number of specimens and the sex if restricted to one sex. 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. Suitable montane habitat is present and both Microtus pennsylvanicus insperatus and Microtus longicaudus longicaudus occur there. The arid basin of the Powder River presumably is a barrier that has prevented M. montanus from reaching this area.
Table 1. Average Measurements, in Millimeters, of Adults of Microtus montanus.