Geographic Variation in the Harvest Mouse, Reithrodontomys megalotis, On the Central Great Plains And in Adjacent Regions Part 1

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

by J. Knox Jones and B. Mursaloglu.

The western harvest mouse, _Reithrodontomys megalotis_, inhabits most parts of the central Great Plains and adjacent regions of tall gra.s.s prairie to the eastward, shows a marked predilection for gra.s.sy 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 Mexico, 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 inst.i.tutions 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 s.e.xual 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 s.e.xual dimorphism in the measurements, if present at all, appear to be sufficient to warrant separating the s.e.xes in the a.n.a.lysis." Hooper did not statistically test the validity of treating the s.e.xes 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 s.e.xual variation, and to decide whether or not to separate the s.e.xes 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 s.e.xual 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 s.e.xes, males and females have been considered together in each population studied.

TABLE 1. a.n.a.lYSIS OF SECONDARY s.e.xUAL 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.

CHARACTER Males Females -------------+--+-----+------------+------+--+-----+----------+----- Total length 27 135.0 (121-149) 6.14 32 141.0 (127-149) 5.36 -------------+--+-----+------------+------+--+-----+----------+----- Length of tail- vertebrae 27 63.9 ( 56-74) 4.63 32 65.2 (58-73) 4.06 -------------+--+-----+------------+------+--+-----+-----------+----- Length of hind foot 27 17.0 ( 16-18) 0.60 32 17.3 (15-19) 0.81 -------------+--+-----+------------+------+--+-----+-----------+----- Length of ear from notch 27 12.9 ( 12-14) 0.55 32 13.0 (12-14) 0.61 -------------+--+-----+------------+------+--+-----+-----------+----- Greatest length of skull 27 21.0 ( 20.2-21.8) 0.43 28 21.3 (20.4-22.2) 0.48 -------------+--+-----+------------+------+--+-----+-----------+----- Zygomatic breadth 25 10.7 ( 10.3-11.0) 0.21 28 10.9 (10.4-11.3) 0.25 -------------+--+-----+------------+------+--+-----+-----------+----- Breadth of braincase 27 10.0 ( 9.6-10.5) 0.22 28 10.1 (9.8-10.7) 0.18 -------------+--+-----+------------+------+--+-----+-----------+----- Depth of cranium 26 7.9 ( 7.4-8.4) 0.20 28 7.9 ( 7.7-8.3) 0.15 -------------+--+-----+------------+------+--+-----+-----------+----- Length of rostrum 27 7.3 ( 6.8-7.6) 0.21 28 7.4 ( 6.9-8.0) 0.27 -------------+--+-----+------------+------+--+-----+-----------+----- Breadth of rostrum 27 3.8 ( 3.6-4.1) 0.11 28 3.8 ( 3.5-4.0) 0.12 -------------+--+-----+------------+------+--+-----+-----------+----- Length of incisive foramen 27 4.4 ( 4.1-4.6) 0.10 28 4.5 ( 4.1-4.9) 0.19 -------------+--+-----+------------+------+--+-----+-----------+----- Length of palate 26 3.5 ( 3.1-3.8) 0.18 28 3.5 ( 3.2-4.0) 0.15 -------------+--+-----+------------+------+--+-----+-----------+----- Alveolar length of maxillary tooth-row 27 3.4 ( 3.2-3.7) 0.14 28 3.4 ( 3.2-3.7) 0.13 -------------+--+-----+------------+------+--+-----+-----------+----- Interorbital breadth 27 3.1 ( 2.9-3.3) 0.12 28 3.1 ( 2.8-3.3) 0.11 -------------+--+-----+------------+------+--+-----+-----------+----- Breadth of zygomatic plate 27 1.9 ( 1.8-2.1) 0.10 28 2.0 ( 1.9-2.3) 0.12 -------------+--+-----+------------+------+--+-----+-----------+----- Breadth of mesopterygoid fossa 26 0.9 ( 0.6-1.1) 0.12 28 0.9 ( 0.8-1.2) 0.12 -------------+--+-----+------------+------+--+-----+-----------+-----

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 spa.r.s.e, and characteristically grayish brown. The molt (post-juvenal molt) from juvenal pelage to subadult pelage seemingly occurs at an early age, perhaps frequently before the young leave the nest, as individuals in juvenal pelage are few among specimens studied by us. Judging from study skins alone, the progress of post-juvenal molt in _R. megalotis_ is similar to that described for _R. humulis_ by Layne (1959:69-71). The subadult pelage is thicker, longer and brighter than juvenal pelage and closely resembles the pelage of adults; it differs from adult pelage dorsally in being somewhat duller and in having less contrast between back and sides.

The pelage of adults varies depending on season. In summer the individual hairs are relatively short (5-6 mm. at the middle of the back) and spa.r.s.e. The over-all color of the dorsum, sides and flanks is brownish to dark brownish, and the venter is grayish. In winter the pelage is dense, long (8-9 mm. at the middle of the back) and lax. The over-all color dorsally in fresh winter pelage in most specimens is paler (more buffy) than summer pelage, the sides are markedly buffy, and the venter is whitish; even the tail is more pilose and more sharply bicolored than in summer. Adults molt, usually completely but occasionally only partially, at least twice a year--once in spring (in May and June in Nebraskan specimens) from winter to summer pelage, and once in autumn (in October and November in Nebraskan specimens) from summer to winter pelage. Of the two molts, the one in spring is most easily discernible because the contrast in color between worn winter pelage and fresh summer pelage is considerably greater than that between worn summer pelage and fresh winter pelage, and because the progress of spring molt is seemingly more regular than that of autumn molt. In spring, molt proceeds posteriorly in a more or less regular line on both dorsum and venter; in most specimens it is completed first on the venter. In autumn, molt is irregular, or at best is coincident over large parts of the body, and frequently is seen only by searching through the pelage with a fine probe or dissecting needle. In both spring and autumn, molt seemingly is delayed in females that are pregnant or lactating.

In both winter pelage and summer pelage, the upper parts have blackish or grayish guard hairs and shorter, more numerous cover hairs. All the cover hairs are gray basally; some have a buffy band terminally and others have a buffy subterminal band with a terminal black tip. The generally darker over-all color of upper parts in summer pelage results (as seen in Nebraskan specimens) from a narrower band of buff on the cover hairs (only approximately one half the width of the band on hairs in winter pelage), a darker buffy band (ochraceous buff rather than pale ochraceous or straw color), and a relative spa.r.s.eness of the pelage, which allows the gray basal portion of some hairs to show on the surface. The more grayish venter of summer-taken specimens results from much more of the grayish basal portion of the white-tipped hairs showing through than in the longer, denser pelage of winter.

Wear on the pelage seems in general to produce a paler over-all color of upper parts, evidently due mostly to abrasion of the terminal black tip of the cover hairs, but possibly actual fading of the pelage is involved also. Worn winter pelage is especially notable for its paleness; the buffy tones are accentuated and the upper parts, especially posteriorly, may even appear fulvous. The difference in color of upper parts between specimens in worn winter pelage and fresh summer pelage (or for that matter specimens in fresh _versus_ worn winter pelage) from the same locality is greater in our material than the difference between some specimens in comparable pelages from localities more than 500 miles apart.

We have seen no specimens taken in winter in which we could discern that the autumn molt had been incomplete, but three old adult males in summer pelage indicate that spring molt is not always completed. KU 50154, obtained on August 14, 1952, 5 mi. N and 2 mi. W Parks, Dundy Co., Nebraska, has the entire posterior back and sides still in old winter pelage and does not appear to have been actively molting; the entire venter is in summer pelage. KU 50146, obtained on August 22, 1952, 3 mi.

E Chadron, Dawes Co., Nebraska, has small patches or tufts of winter pelage remaining on the rump and likewise does not appear to have been actively molting. KU 72085, obtained on October 13, 1956, 4 mi. E Barada, Richardson Co., Nebraska, is in the process of molting from summer to winter pelage, but has tufts of old winter pelage on the rump.

Geographic Variation

Geographic variation, both in color of pelage and in external and cranial dimensions, is less in _R. megalotis_ in the region studied than in most other cricetine species that occur there. Nevertheless, meaningful variation is present. The a.s.sumption that variation in _R.

megalotis_ paralleled in degree that of other species, _Peromyscus maniculatus_ for example, led to untenable taxonomic conclusions by some previous workers.

_Color of Pelage_

Color of pelage is remarkably uniform, considering the geographic extent of the area involved, over most of the northern part of the central gra.s.slands. Perhaps this uniformity results partly from the predilection of the western harvest mouse for gra.s.sy habitats, for in most areas on the Great Plains the species is restricted to riparian communities, princ.i.p.ally along river systems, where soils, cover, and other conditions approximate those of corresponding habitats farther to the east to a much greater degree than do conditions in upland habitats.

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.

To the southwest, on the other hand, a trend toward paler (pale brownish, less blackish) upper parts is apparent. Specimens from southwestern Kansas and adjacent parts of Colorado and Oklahoma average slightly paler in comparable pelages than specimens from northeastern Kansas and eastern Nebraska, but most specimens from farther southwest, in northern New Mexico and southwestern Colorado, are discernibly, although not markedly, paler than mice from northern and eastern populations.

A "pectoral spot," fairly common in some populations of _R. megalotis_ east of the Missouri River (see Hoffmeister and Warnock, 1955:162-163), is present in only a small percentage of the specimens we have studied, and when present is usually only faintly developed.

_External and Cranial Size_

[Ill.u.s.tration: FIG. 1. Geographic variation in five measurements of _Reithrodontomys megalotis_ on the central Great Plains. The size of each sample is given, along with total length, length of tail expressed as a percentage of the head and body, length of ear, greatest length of skull, and length of rostrum. The approximate distribution of the species in the region shown and the approximate boundary between the subspecies _R. m. aztecus_ and _R. m. dychei_ also are indicated.]

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. 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 p.r.o.nounced southwestwardly. Variation in length of tail and length of ear, therefore, appear to be in accord with Allen's Rule. Length of body and length of hind foot seem not to vary significantly in specimens we have studied.

The skulls of specimens examined differed only slightly, except that the rostrum is significantly longer and relatively, if not actually, narrower in specimens from the south and southwest than in mice from the rest of the region under study. 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. aztecus_ is applicable, occurs in the southwest and is characterized by the culmination of trends in the region studied to paler upper parts, longer tail, longer ear, and longer, relatively narrower rostrum--characters that appear at least partly independent of each other as concerns gradation toward the smaller, darker-colored populations to the northward. 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 a.s.signment of some intergrades is admittedly difficult.

_Reithrodontomys megalotis aztecus_ J. A. Allen

_Reithrodontomys aztecus_ J. A. Allen, Bull. Amer. Mus. Nat. Hist., 5:79, April 28, 1893 (type locality, La Plata, San Juan Co., New Mexico).

_Reithrodontomys megalotis aztecus_, A. H. Howell, N. Amer. Fauna, 36:30, June 5, 1914.

_Reithrodontomys megalotis caryi_ A. H. Howell, Jour. Mamm., 16:143, May 15, 1935 (type locality, Medano Ranch, 15 mi. NE Mosca, Alamosa Co., Colorado).

_Distribution._--Western and southern Colorado, southeastern Utah, northeastern Arizona and northern New Mexico, east to the panhandles of Texas and Oklahoma and to southwestern Kansas.

_External measurements._--Average and extremes of 10 adults (5 males, 5 females) from San Juan County, New Mexico, and adjacent Montezuma County, Colorado, are: total length, 140.1 (126-150); length of tail-vertebrae, 67.4 (56-71); length of hind foot, 17.3 (16-18); length of ear from notch, 15.1 (13-17); tail averaging 92.7 per cent of length of body. Corresponding measurements of 13 adults (7 males, 6 females) from Bernalillo and Guadalupe counties, New Mexico, are: 142.1 (129-156); 69.4 (60-75); 17.9 (17-19); 16.3 (15-18); tail averaging 95.4 per cent of length of body.

Corresponding measurements of 22 adults (17 males, 5 females) from Meade County, southwestern Kansas, are: 147.1 (139-162); 71.3 (65-77); 17.6 (17-19); 13.8 (13-15); tail averaging 94.1 per cent of length of body. For cranial measurements see Table 2.

_Remarks._--For comparisons with _Reithrodontomys megalotis dychei_, geographically adjacent to the northeast, see account of that subspecies.