University of Kansas Publications
Museum of Natural History

Volume 14, No. 2, pp. 9-27, 1 fig. in text
July 24, 1961

Geographic Variation in the Harvest Mouse,
Reithrodontomys megalotis,
On the Central Great Plains
And in Adjacent Regions

By

J. KNOX JONES, JR. AND B. MURSALOĞLU

University of Kansas
Lawrence
1961

University of Kansas Publications, Museum of Natural History

Editors: E. Raymond Hall, Chairman, Henry S. Fitch,
Robert W. Wilson

Volume 14, No. 2, pp. 9-27, 1 fig. in text
Published July 24, 1961

University of Kansas
Lawrence, Kansas

PRINTED IN
THE STATE PRINTING PLANT
TOPEKA, KANSAS
1961

Geographic Variation in the Harvest Mouse,
Reithrodontomys megalotis,
On the Central Great Plains
And in Adjacent Regions

By

J. KNOX JONES, JR. AND B. MURSALOĞLU

The western harvest mouse, Reithrodontomys megalotis, inhabits most parts of the central Great Plains and adjacent regions of tall grass prairie to the eastward, shows a marked predilection for grassy habitats, is common in many areas, and is notably less variable geographically than most other cricetids found in the same region. R. megalotis occurs (see Hall and Kelson, 1959:586, map 342) from Minnesota, southwestern Wisconsin, northwestern Illinois, Iowa and Missouri westward to, but apparently not across, the Rocky Mountains from southeastern Alberta to Colorado; it is known in Oklahoma only from the Panhandle, thence southward through the Panhandle and Trans-Pecos areas of Texas to southern México, westward across the mountains in New Mexico to the Pacific Coast, and northward to the west of the Rockies to southern British Columbia.

Hoffmeister and Warnock (1955) studied western harvest mice from Illinois, Iowa, northeastern Kansas, Minnesota and Wisconsin, concluded that one subspecific name (Reithrodontomys megalotis dychei J. A. Allen, 1895, with type locality at Lawrence, Douglas Co., Kansas) applied to all, and relegated Reithrodontomys megalotis pectoralis Hanson, 1944 (type locality at Westpoint, Columbia Co., Wisconsin) to synonymy under dychei. Our study, based upon an examination of 1350 specimens, concerns the area west of the Missouri River from Kansas and Nebraska westward to Montana, Wyoming, Colorado and northern New Mexico. Our objectives were to study variation in R. megalotis in the region indicated and to decide what subspecific names properly apply to populations of the species that occur there.

Aside from the name R. m. dychei, currently applied to western harvest mice from a large part of the region here under study, three other subspecific names need consideration:

"Reithrodontomys aztecus" J. A. Allen, 1893 (type locality, La Plata, San Juan Co., New Mexico), currently applied to specimens from northern New Mexico and southern Colorado (and adjacent parts of Arizona and Utah) east to southwestern Kansas and the Oklahoma Panhandle;

"Reithrodontomys megalotis caryi" A. H. Howell, 1935 (type locality, Medano Ranch, 15 mi. NE Mosca, Alamosa Co., Colorado), proposed for, and currently applied to, harvest mice from the San Luis Valley, Colorado, but possibly a synonym of aztecus according to Hooper (1952:218); and

"Reithrodontomys dychei nebrascensis" J. A. Allen, 1895 (type locality, Kennedy, Cherry Co., Nebraska), proposed for harvest mice from western Nebraska and adjacent areas, but regarded as a synonym of dychei by A. H. Howell (1914:30-31).

Our comments concerning the taxonomic status of these several names appear beyond.

We are grateful to Dr. W. Frank Blair, University of Texas, for the loan of a specimen from the Texas Panhandle (TU), and to Dr. Richard H. Manville, U.S. Fish and Wildlife Service, for the loan of specimens of R. m. caryi from the Biological Surveys Collection (USNM). We are grateful also to persons in charge of the following collections for allowing one of us (Jones) to examine Nebraskan specimens of R. megalotis in their care: University of Michigan Museum of Zoology (UMMZ); University of Nebraska State Museum (NSM); and U.S. National Museum (USNM). A research grant from the Society of the Sigma Xi facilitated travel to the institutions mentioned. Specimens not identified as to collection are in the Museum of Natural History of The University of Kansas. All measurements are in millimeters, and are of adults (as defined by Hooper, 1952:12) unless otherwise noted.

Secondary Sexual Variation

Hooper (1952) did not accord separate treatment to males and females in taxonomic accounts of Latin American harvest mice because (p. 11): "In no species ... does sexual dimorphism in the measurements, if present at all, appear to be sufficient to warrant separating the sexes in the analysis." Hooper did not statistically test the validity of treating the sexes together in R. megalotis. He did test a series of R. sumichrasti from El Salvador, in which he found no basis for separate treatment of males and females.

Some authors (Verts, 1960:6, for instance) have recorded females of R. megalotis as larger than males in external measurements, whereas others (Dalquest, 1948:325, for instance) have recorded males as the larger. In order to learn something of secondary sexual variation, and to decide whether or not to separate the sexes in our study, we compared adult males and females from the southern part of the Panhandle of Nebraska (Cheyenne, Keith, Kimball, Morrill and Scotts Bluff counties) in four external and twelve cranial measurements (see Table 1). The external measurements are those customarily taken by collectors and were read from the labels of the specimens; cranial measurements were taken to the nearest tenth of a millimeter by means of dial calipers, and are those described by Hooper (1952:9-11). Females from our sample averaged larger than males in all external and several cranial measurements, but individual variation greatly exceeded secondary sexual variation in each of these measurements and in no case was the greater size of females statistically significant. Therefore, and because we found no qualitative external or cranial differences between the sexes, males and females have been considered together in each population studied.

Table 1. Analysis of Secondary Sexual Variation in Adult Reithrodontomys megalotis From the Southern Part of the Nebraska Panhandle. For Each Measurement, the Number of Specimens Used, the Average, the Extremes, and One Standard Deviation Are Given.