University of Kansas Publications

Museum of Natural History

Volume 7, No. 7, pp. 489-506, 2 figures in text

July 23, 1954

Subspeciation in the Meadow Mouse,Microtus montanus, in Wyomingand Colorado

BY

SYDNEY ANDERSON

University of Kansas
Lawrence
1954

University of Kansas Publications, Museum of Natural History

Editors: E. Raymond Hall, Chairman, A. Byron Leonard,Robert W. Wilson

Volume 7, No. 7, pp. 489-506, 2 figures in text
Published July 23, 1954

University of Kansas
Lawrence, Kansas

PRINTED BY
FERD VOILAND, JR., STATE PRINTER
TOPEKA, KANSAS
1954

25-3560

Subspeciation in the Meadow Mouse,Microtus montanus, in Wyomingand Colorado

BY

SYDNEY ANDERSON

Microtus montanus reaches the eastern limits of its geographicdistribution in Wyoming and Colorado. There the mountains, butin general not the lowlands, are occupied by this species. A certainminimum of moisture may be of direct importance to the mouse andcertainly is indirectly important, because certain hydrophytic ormesophytic grasses used by the mouse for food, for protection fromenemies, and for shelter from the elements are dependent on themoisture. Areas suitable for Microtus montanus are separated bydeserts that are dominated by sagebrush and other xerophytic plantsor by forests or rocky exposures at higher altitudes. A relativelysmall percentage, probably less than ten per cent, of the total areaeven in the more favorable parts of the range of the species is suitablefor occupancy. In these mice, as in other microtines (Elton,1942; Piper, 1909), there are seasonal, and irregularly multiannualfluctuations in population density, which sometimes are extreme.Consequently the mice at some times seem to be absent from suitablehabitats, and at some other times occur there in amazinglylarge numbers.

Because the species is broken up into partly isolated, or at timescompletely isolated, colonies or local populations it may be supposedthat various evolutionary forces such as selection and random geneticdrift operate to foster variation. The degree to which racial distinctionis attained may depend upon these forces and the time available.In Microtus montanus in the eastern Rocky Mountains thedegree of subspecific distinction is not great.

The study here reported upon is based on 1,187 specimens ofMicrotus montanus from Wyoming, Colorado, Idaho and Montana,and on work in the field. I spent approximately four months in thefield in this area, in the summers of 1950, 1951, and 1952. The specimensstudied were arranged according to localities and the largerseries were compared statistically. Each of two series, totaling 136specimens, was studied intensively to ascertain the kind and rangeof variation within single populations. Twenty-seven measurements,various proportions based on these measurements, and differencesin color were analyzed. Fifteen characters, judged to be most significant,were selected for use in comparing all series. In addition,certain characters that can not be expressed easily by measurements,such as inflation of the auditory bullae and the curvature of thezygomatic arch, were observed. The studies by A. B. Howell(1924) of variation in Microtus montanus yosemite Grinnell in Californiaand those by O. B. Goin (1943) of Microtus pennsylvanicuspennsylv