Subspeciation in the Meadow Mouse, Microtus Pennsylvanicus, in Wyoming, Colorado, and Adjacent Areas

Volume 9, No. 4, Pp. 85-104, 2 Figures in Text

Published May 10, 1956

UNIVERSITY OF KANSAS Lawrence, Kansas

PRINTED BY FRED VOILAND, JR., STATE PRINTER TOPEKA, KANSAS 1956

Subspeciation in the Meadow Mouse, Microtus pennsylvanicus, in Wyoming, Colorado, and Adjacent Areas

SYDNEY ANDERSON

INTRODUCTION

In the region including Wyoming and Colorado, Microtus pennsylvanicus has been divided into two subspecies: the pale M. p. insperatus (J. A. Allen) inhabits the Black Hills of the northeasternmost part of Wyoming; the dark M. p. modestus (Baird) inhabits extensive areas in both Wyoming and Colorado. Initial examination of Microtus pennsylvanicus revealed that specimens from the Big Horn Mountains of north-central Wyoming (within the range of modestus as mapped by Hall and Cockrum 1952:407), in color at least, resemble the subspecies insperatus more than they do modestus, and that specimens from southwestern Wyoming are notably dark. Durrant (1952:363) noted that specimens from Utah are dark, and Davis (1939:315) did the same for specimens from near Pocatello, Idaho. It seemed, therefore, that dark color might characterize populations of a wide geographic region and distinguish them from modestus named from southern Colorado. Also, there seemed to be a hiatus of at least 180 miles between the ranges of modestus in northern Colorado and modestus in eastern Wyoming, and an even greater distance separating populations of modestus in northern Colorado from those in western Wyoming. Microtus pennsylvanicus has not been taken in central or southeastern Wyoming despite extensive collecting there, which yielded numerous records of other kinds of Microtus (M. longicaudus, M. montanus, and M. ochrogaster). Subsequent study revealed a pattern of geographic variation within the range now ascribed to modestus which, in my opinion, can be described best by the recognition of three new subspecies.

MATERIALS, METHODS, AND ACKNOWLEDGMENTS

To study geographic variation in color a method was devised as follows: A single skin (KU 42407, from 1-1/2 miles east of Buckhorn in Weston County, Wyoming) was selected as a representative of the paler mice and arbitrarily given the number 2. A single skin (KU 17491, from 3 miles east of Moran in Teton County, Wyoming) was selected as a representative of the darker mice from the western part of Wyoming and arbitrarily given the number 4. These mice were selected so that they were respectively paler and darker than the estimated average of the total variation within the populations to be studied, but the two mice were not at the extremes of paleness and darkness. Comparisons were based on visual inspection of the dorsal pelage as a whole. Skins were compared with these two mice and given whole numbers from one to five. If paler than the standard for 2, the skin was numbered one; if not distinguishably paler or darker, it was given the number two; if intermediate in color to the standards for 2 and 4 and not definitely more nearly referable to one than the other, it was given the number three; if it resembled the standard for 4, it was numbered four; and if darker, it was given the number five. In this manner skins from a given locality could be evaluated one by one and the results plotted, averaged, and treated statistically. On Figure 1 the average values for color of 32 series are mapped to show the geographic variation of color. The following series of adults are the basis for Figure 1 (abbreviations for collections other than at the University of Kansas are included in parentheses): Each locality is followed by the month (or months) of capture, the number of specimens, and the average value for color.

Montana: Glacier County, August, 6, 1.8; Hill and Chouteau counties combined (Mich), July, 24, 1.5; Malta, Philips County, August, 14, 1.5; Sheridan County, August, 6, 1.5; Fergus County (USBS), August, 5, 2.4; Ravalli County (KU and USBS), August, 12, 2.8; Silver Bow County, August, 7, 3.0; Sweet Grass County (Mich), June and July, 7, 2.7; Park County, August, 10, 2.6. Idaho: Pocatello and vicinity, November and December, 5, 3.4. Wyoming: Park County, August, 6, 2.8; Sheridan County, September, 9, 1.2; Johnson County, August, 12, 1.5; Campbell and Crook counties, July, 11, 1.4; Weston County, July, 7, 1.6; Teton County, September, 8, 3.4; Teton County (Mich), June, 17, 3.1; Afton and vicinity, Lincoln County, July, 10, 4.2; Sage, Lincoln County, July, 5, 5.0. South Dakota: Pennington County (Chi), June, 14, 2.1; Pennington County (Mich), December and January, 8, 1.1; Walworth County, July, 4, 3.7; Buffalo County, July, 6, 3.2. Colorado: Loveland and vicinity, Larimer County (KU and USBS), July, 13, 2.8; Boulder County (Chi), September, 34, 2.6; Park County (Denv), March, 8, 1.9; Colorado Springs (ERW), March, April, and May, 5, 2.8; Saguache County (USBS), August, 46, 3.0; Conejos County, June, 4, 3.0; Wray, Yuma County (USBS), 3, 4.7. Nebraska: Dundy County, August and November, 14, 4.6. New Mexico: Colfax County, June, 8, 3.2. Variation in color is discussed in the accounts of the subspecies concerned.

a. M. p. pullatus b. M. p. insperatus c. M. p. uligocola d. M. p. finitus e. M. p. modestus f. M. p. aztecus g. M. p. drummondi h. M. p. pennsylvanicus]

For each of the series listed in Table 1 all adult mice having skulls that measured more than 24.0 mm. in condylobasilar length were studied. Total length, length of tail, and length of hind foot were taken from the collector's field labels. The measurements of the skulls listed below were taken by means of dial calipers reading to one-tenth of a millimeter, and in the same fashion as described previously (Anderson, 1954:492). Measurements of specimens in each series were averaged (the arithmetic means were computed). If the averages differed noticeably the significance of the difference was tested statistically. Averages referred to in the text as significantly different differ by as much as, or more than, the sum of two times the standard error of each of the two averages. Linear measurements are in millimeters; color values are in the arbitrary units described in a preceding paragraph. Measurements taken of the skulls are: condylobasilar length, zygomatic breadth, interorbital breadth, lambdoidal breadth, prelambdoidal breadth, depth of braincase, and alveolar length of upper molar tooth-row.

Secondary sexual variation was not detected in the material studied. Variation with age is important to the taxonomist even among specimens designated as "adults", because growth and changes in various proportions continue throughout the life of the mice. The possibility that differences detected in the statistical treatment or observed directly could be the result of differences in average age within the samples of "adults" was considered in each case.

In order to study certain variations, the following "method of pairs" was used. Skulls of two series to be compared were matched in pairs so that they corresponded in size and ontogenetic stage of development. Then the two skulls of each pair were examined for differences in each of the following features: size of circle inscribed by the upper incisor teeth, width of nasal bones relative to their length, curvature of the zygomatic arch, elongation of the braincase relative to its width when viewed from the dorsal aspect, degree of indentation in the anterior edge of the zygomatic arch near the rostrum, degree of depression of the nasal bones when viewed from the side, width in the vertical plane of the zygomatic arch at the suture between the maxillary and jugal bones, length relative to width of the prominent fenestra in the posterodorsal part of the squamosal bone, size of the meatus of the auditory canal, distance between the internal margin of an occipital condyle at its posteriormost point and the tip of the paraoccipital process of the same side of the skull, size of the foramen magnum, vertical height of the supraoccipital bone from the dorsalmost point on the margin of the foramen magnum to the midpoint of the lambdoidal crest, constriction posteriorly or narrowness of the incisive foramen relative to its length, distance across the premaxillary bone from the anteriormost point of the incisive foramen to the posteriormost point of the margin of the alveolus of the upper incisor, area of the maxillary septum (Howell 1926:112, or "zygomatic plate" of Ellerman 1941:1), acuminateness of the anterior border of the palatine opening (internal nares), size of auditory bullae, size of foramen ovale, acuteness of the angle between the basioccipital and basisphenoidal bones at the suture between them (degree to which the area of the suture is raised between the bullae when viewed from the ventral aspect), width of first upper molar tooth, least distance between alveoli of first upper molars. Any differential feature present in more than 75 per cent of the pairs of animals is reported in the discussion of the subspecies concerned. The significance of each difference reported was calculated by the Chi-square test and the confidence limit is given in each case. The probability used in the Chi-square formula is one-half of the percentage of all pairs compared in which the skulls were different in regard to the character being considered. For example, in 68 per cent of the total number of pairs of skulls compared in this study a difference in the size of the auditory bullae was noted. Therefore the probability that a specified skull of a pair will have larger bullae than the other skull was taken as 34 per cent. A different probability for each feature compared was derived in like manner.

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