J. Knox Jones

“ As seen in Figure 1, the tail and especially the ear are longer in mice from New Mexico and adjacent areas than in specimens from northern localities. The ear, only slightly variable in size in the northern part of the region, is markedly longer in the southwest, averaging more than 2 mm. longer in specimens from New Mexico and adjacent southwestern Colorado than in specimens from Nebraska and eastern Kansas; specimens in a zone from central Colorado through southwestern Kansas and adjacent Oklahoma generally have ears of a size between the two extremes. ”

“ 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.
Pelage and Molt
Western harvest mice that attain adulthood acquire at least three distinct types of pelage in sequence in the course of their development. The first of these, the juvenal pelage, is short, relatively sparse, and characteristically grayish brown. ”

“ Howell (1914:31) earlier noted that specimens from northern and central Colorado were intergrades between the two subspecies.
The geographic range occupied by R. m. dychei (from east of the Mississippi River in Illinois and Wisconsin to the foothills of the Rockies) is large (although not so large as that currently ascribed to R. m. megalotis, which ranges from southern British Columbia to central México) . Most other small rodents that occur in the same geographic area occupied by dychei are represented there by at least two subspecies, a dark one in the east and a pale one in the west. ”

“ 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. ”

“ Differential selective pressure, therefore, theoretically would be less between eastern and western populations of R. megalotis than in an upland-inhabiting species. In any event, specimens from western Nebraska, Wyoming, northern Colorado, and adjacent areas average only slightly paler dorsally than specimens in corresponding pelages from the eastern parts of Nebraska and Kansas, and many individuals from the two areas can be matched almost exactly. ”

“ The rostrum is longest (average 7.7 mm.) in specimens from the vicinity of the type locality of R. m. aztecus, but is relatively long (7.5-7.6 mm.) in populations from as far north as northeastern Colorado and southwestern Nebraska. An average greater occipitonasal length (greatest length of skull) in specimens from the south and southwest results mostly from the longer rostrum.
Recognition of two subspecies of R. megalotis on the central Great Plains seems justified on the basis of the geographic variation discussed above. One subspecies, for which the name R. m. ”

“ 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) ”

“ As concerns the tail we note a slight trend toward increasing length (best expressed as percentage of length of body) from north to south throughout the central plains, but in general the trend is more pronounced southwestwardly. ”

“ 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. ”

“ Corresponding measurements of 20 adults (14 males, 6 females) from Cherry County, Nebraska, are: 135.3 (122-155) ; 62.9 (56-72) ; 17.5 (17-18) ; 13.0 (12-14) ; tail averaging 86.9 per cent of length of body. For cranial measurements see Tables 1 and 2.
Remarks.—From Reithrodontomys megalotis aztecus, geographically adjacent to the southwest, R. m. dychei differs as follows: upper parts averaging darker (especially in summer pelage) , owing principally to more suffusion of blackish middorsally; tail slightly shorter; ears markedly shorter, rostrum shorter and relatively broader ”

“ 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 ... ”

“ The latter, while exhibiting some differences in color (slightly paler westwardly) and length of tail (shorter northwardly) , stand more or less as a unit in contrast to the mice from the southwest, and represent, in our judgment, a single subspecies, R. m. dychei. The area of intergradation between the two subspecies is relatively broad, considering all the characters mentioned, and assignment of some intergrades is admittedly difficult.
Reithrodontomys megalotis aztecus J. A. Allen
Reithrodontomys aztecus J. A. Allen, Bull. Amer. Mus. Nat. ”