Subspeciation in the Meadow Mouse, Microtus pennsylvanicus, in Wyoming, Colorado, and Adjacent Areas
BY
SYDNEY ANDERSON
INTRODUCTION
In the region including Wyoming and Colorado,Microtus pennsylvanicushas been divided into two subspecies: the paleM. p. insperatus(J. A. Allen) inhabits the Black Hills of the northeasternmost part of Wyoming; the darkM. p. modestus(Baird) inhabits extensive areas in both Wyoming and Colorado. Initial examination ofMicrotus pennsylvanicusrevealed that specimens from the Big Horn Mountains of north-central Wyoming (within the range ofmodestusas mapped by Hall and Cockrum 1952:407), in color at least, resemble the subspeciesinsperatusmore than they domodestus, 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 frommodestusnamed from southern Colorado. Also, there seemed to be a hiatus of at least 180 miles between the ranges ofmodestusin northern Colorado andmodestusin eastern Wyoming, and an even greater distance separating populations ofmodestusin northern Colorado from those in western Wyoming.Microtus pennsylvanicushas not been taken in central or southeastern Wyoming despite extensive collecting there, which yielded numerous records of other kinds ofMicrotus(M. longicaudus,M. montanus, andM. ochrogaster). Subsequent study revealed a pattern of geographic variation within the range now ascribed tomodestuswhich, 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 1½ 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 v