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

by J. Knox Jones; B. Mursaloglu

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; occipitonasal length shorter owing to shorter rostrum.

"Reithrodontomys dychei nebrascensis," named by Allen (1895:122) from Kennedy, Nebraska, was distinguished in the original description from dychei by "slightly larger size, relatively longer ears, and more strongly fulvous coloration." Allen applied the name nebrascensis to harvest mice from Montana south to central Colorado and western Nebraska. Howell (1914:30-31) placed nebrascensis in synonymy under dychei because he found specimens from Kennedy to be "indistinguishable from specimens of typical dychei in comparable pelage." We concur with Howell. Topotypes of nebrascensis that we have examined average only slightly paler than topotypes of dychei in the same pelage (some specimens from each series can be matched almost exactly), and do not differ significantly in any external or cranial measurements. The "fulvous" upper parts of the series from Kennedy (all taken in late April) that was available to Allen resulted from worn winter pelage. We think that Allen was led astray also by his erroneous assumption that geographic variation in color of R. megalotis on the Great Plains paralleled that found in Peromyscus maniculatus. Actually, R. megalotis varies in color much less geographically in the region concerned than does P. maniculatus.

Specimens from the northwestern part of the range of dychei (Wyoming, Montana and western South Dakota), like those from western Nebraska, average slightly paler dorsally than topotypes and other specimens from eastern Kansas and Nebraska (a few approach aztecus in this regard), but do not otherwise differ. Most specimens from northern Colorado, southwestern Nebraska (Hitchcock and Dundy counties) and western Kansas average slightly paler than typical specimens and have longer rostra, approaching aztecus in these particulars, but have the shorter ears and shorter tail of dychei. In general, these intergrades resemble dychei to a greater degree than aztecus and are accordingly assigned to the former. One exception is a series from Muir Springs, 2 mi. N and 2½ mi. W Ft. Morgan, Colorado. Specimens in this series approach typical dychei in color, but resemble aztecus in having long ears and long rostra (average 15.3 and 7.5, respectively, in 13 adults). The specimens from Muir Springs resemble aztecus to a greater degree than dychei, but are assigned to the latter because specimens from farther west and farther south in Colorado are assignable to dychei. 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. Eastern populations of dychei have, it is true, somewhat darker upper parts than mice from western localities, but the differences are slight; also, judging from the literature, the "pectoral spot" is more common in eastern mice.

It should be noted that R. m. dychei probably has extended its range both eastward and westward in the last century as a result of agricultural practices—clearing of land in the east and irrigation in the west.

Specimens examined.—1145, as follows:

Colorado. Adams County: South Platte River, 5 mi. N Denver, 1; 3 mi. S, 1 mi. W Simpson, 1. El Paso County: 5 mi. E Payton, 1; 4 mi. S maingate of Camp Carson, 1. Larimer County: 3 mi. N Loveland, 1; 9¼ mi. W, ½ mi. N Loveland, 5600 ft., 1; 16 mi. W Loveland, 6840 ft., 1; 3½-4½ mi. W Loveland, 5030 ft., 7; 6 mi. W, ½ mi. S Loveland, 5200 ft., 14; 7 mi. W, 2½ mi. S Loveland, 5370 ft., 1. Morgan County: Muir Springs, 2 mi. N, 2½ mi. W Ft. Morgan, 21. Washington County: Cope, 6. Yuma County: 1 mi. W to 1 mi. E Laird, 6.

Kansas. Atchison County: 1½ mi. S Muscotah, 10; 4½ mi. S Muscotah, 2. Barton County: 3 mi. N, 2 mi. W Hoisington, 3. Brown County: 1 mi. E Reserve, 2; 5 mi. S Hiawatha, 4. Cheyenne County: 23 mi. NW St. Francis, 1; 1 mi. W St. Francis, 12; 8 mi. S, 1½ mi. W St. Francis, 1. Decatur County: 1 mi. N, 2 mi. E Oberlin, 4; 5 mi. S, 8 mi. W Oberlin, 1. Doniphan County: Geary, 2. Douglas County: 5 mi. N, ½ mi. E Lawrence, 1; 1 mi. NW Midland, 1; 4½ mi. N Lawrence, 2; 4 mi. N, 1¾ mi. E Lawrence (sec. 8, T. 12 S, R. 20 E), 10; ½ mi. NW Lecompton, 1; 2½ mi. N, 1 mi. W Lawrence, 2; 2 mi. N Lawrence, 2; U.P. Railroad tracks, N of Lawrence, 1; 91⁄5 mi. W Lawrence, 1; 5 mi. W Lawrence, 1; 2 mi. W Lawrence, 4; 1 mi. W Lawrence, 4; Fort Lake, Lawrence, 1; Lawrence, 24; 1 mi. SW Lawrence, 2; 1 mi. S, 1½ mi. W Lawrence, 2; 1¾. mi. S, 3½ mi. E Lawrence, 1; 2 mi. SW Lawrence, 2; 7-7½ mi. SW Lawrence, 4; Rock Creek, 850 ft., 10 mi. SW Lawrence, 8; N end Lone Star Lake, 9 mi. S, 7 mi. W Lawrence, 1; no specific locality, 6. Ellis County: ½ mi. S, 3½-4 mi. W Hays, 2250 ft., 12. Franklin County: 4 mi. N Ottawa, 2; ½ mi. S, 1¾ mi. E Ottawa, 4. Gove County: Castle Rock, 4; no specific locality, 1. Jackson County: ½ mi. N, 3 mi. W Holton, 4. Leavenworth County: Ft. Leavenworth, 2; no specific locality, 3. Logan County: no specific locality, 2. Marshall County: 2 mi. N, 4 mi. E Oketo, 1; ½ mi. N, 1½ mi. E Waterville, 1; 1 mi. E Waterville, 5; ½ mi. SW Waterville, 4. Mitchell County: ½ mi. S, 3½ mi. W Beloit, 1500 ft., 4. Nemaha County: Nebraska-Kansas line, 7 mi. N Sabetha, 1; 10½ mi. N Seneca, 1; 2½ mi. S Sabetha, 6. Norton County: 1½ mi. N, ¼ mi. E Norton, 1; ½ mi. N, 4 mi. E Norton, 5; 1 mi. SW Norton, 10; 4 mi. W, 1 mi. S Logan, 3. Osage County: 3 mi. N Lyndon, 1. Osborne County: ½ mi. W Downs, 5. Phillips County: 2¼ mi. SE Long Island, 1. Pottawatomie County: 1 mi. NW Fostoria, 1. Rawlins County: 2 mi. NE Ludell, 17; 2 mi. S Ludell, 2; Atwood, 3; Atwood Lake, 2. Republic County: 1½ mi. S, 1 mi. E Belleville, 1; Rydal, 8. Scott County: State Park, 2. Shawnee County: 1 mi. S Silver Lake, 857 ft., 2. Sherman County: ½ mi. S, 1½ mi. E Edson, 1. Smith County: 2 mi. E Smith Center, 9. Stafford County: 16 mi. N, 4 mi. E Stafford, 1. Thomas County: 10 mi. N, 6 mi. E Colby, 5. Trego County: 16 mi. S, 4½ mi. E Wakeeney, 1. Wichita County: 15 mi. W Scott City, 5.

Montana. Big Horn County: Big Horn River, 14 mi. S Custer, 2750 ft., 4. Dawson County: 1 mi. W Glendive, 2070 ft., 3. Phillips County: 1 mi. N, 1 mi. W Malta, 2248 ft., 1. Powder River County: Powderville, 2900 ft., 1.

Nebraska. Antelope County: Neligh, 16 (6 NSM, 9 USNM). Boyd County: 5 mi. WSW Spencer, 1; 5 mi. S, 2 mi. E Spencer, 2; 6 mi. SSE Spencer, 1. Box Butte County: Alliance, 2 (USNM). Buffalo County: Kearney, 2 (USNM). Burt County: 1 mi. E Tekamah, 3. Butler County: 2 mi. N, 2 mi. W Bellwood, 2 (NSM); 4-5 mi. E Rising City, 11; 4 mi. E, 1 mi. S Rising City, 5. Chase County: 2 mi. SE Enders, 1. Cherry County: W of Crookston, 1 (NSM); Valentine, 2 (USNM); Ft. Niobrara Nat'l Wildlife Refuge, 4 mi. E Valentine, 5 (3 NSM); 3 mi. SSE Valentine, 4; 3 mi. S Valentine, 12; 8 mi. S Nenzel, 2; Niobrara River, 10 mi. S Cody, 2 (1 USNM); 11 mi. S, 2 mi. W Nenzel, 1; 18 mi. NW Kennedy, 8 (2 NSM, 6 USNM); Two Mile Lake, 6 (4 NSM, 2 USNM); Watt's Lake, Valentine Nat'l Wildlife Refuge, 3; Hackberry Lake, 12 (UMMZ); 2 mi. W to 4 mi. E Kennedy, 25 (4 UMMZ, 12 USNM); no specific locality, 1 (USNM). Cheyenne County: 15 mi. S Dalton, 4300 ft., 1; 3 mi. N Sidney, 6; 4 mi. E Sidney, 42. Cuming County: Beemer, 1 (USNM). Custer County: 7 mi. NW Anselmo, 1 (UMMZ); within 1 mi. Victoria Spring, 9 (UMMZ); 2 mi. E Lillian, 1 (UMMZ); Comstock, 1 (NSM); Callaway, 3 (USNM); 6 mi. SE Mason City, 1 (UMMZ). Dawes County: Wayside, 1; 3 mi. E Chadron, 2; 6 mi. S Chadron, 1 (NSM); 8 mi. S Chadron, 1 (NSM); 10 mi. S Chadron, 1 (UMMZ); 1 mi. W Crawford, 2 (NSM); Crawford, 2 (UMMZ). Dawson County: ½ mi. S Gothenburg, 5; 3 mi. SSE Gothenburg, 4. Deuel County: 1 mi. N, 2 mi. W Chappell, 3. Dixon County: 3 mi. NE Ponca, 4. Dundy County: Rock Creek Fish Hatchery, 5 mi. N, 2 mi. W Parks, 42; 2 mi. N, 2 mi. W Haigler, 1; Arikaree River, Parks, 2; 2 mi. SW Benkleman, 7; Haigler, 3 (1 NSM, 2 USNM). Franklin County: 1½-2 mi. S Franklin, 10. Gage County: 1 mi. SE DeWitt, 3; ¼ mi. W Homestead Nat'l Mon., 1; 1 mi. S, 1 mi. W Barnston, 1; 1½ mi. S, 2 mi. E Barnston, 18. Garden County: Crescent Lake Nat'l Wildlife Refuge, 1; ½ mi. S Oshkosh, 1. Hall County: 6 mi. S Grand Island, 5. Harlan County: 1 mi. W Alma, 17. Hitchcock County: Republican River, Trenton, 3. Hooker County: Kelso, 3 (UMMZ). Holt County: 6 mi. N Midway, 4; 1 mi. S Atkinson, 4 (2 NSM); Ewing, 1 (USNM). Jefferson County: 7 mi. S, 2 mi. W Fairbury, 6; 3 mi. S, 1 mi. W Endicott, 1. Johnson County: 1 mi. S, 1½ mi. E Burr, 1. Kearney County: 1¾-3¾ mi. S Kearney, 6. Keith County: 4 mi. WNW Keystone, 69. Keya Paha County: 12 mi. N Springview, 8; 12 mi. NNW Springview, 5. Kimball County: 3 mi. E Kimball, 1; Smeed, 40. Knox County: 3 mi. W Niobrara, 2; 1 mi. SE Niobrara, 5; 2 mi. S Niobrara, 2; Verdigre, 2 (USNM). Lancaster County: within 5 mi. Lincoln, 21 (8 NSM). Lincoln County: 2 mi. N North Platte, 1; Conroy Canyon, SW corner sec. 4, T. 11 N, R. 27 W (5 mi. S, 2½ mi. W Brady), 2 (NSM). Logan County: 1-2 mi. NE Stapleton, 11. Madison County: Norfolk, 1 (USNM). Morrill County: 1 mi. N Bridgeport, 4. Nemaha County: 2 mi. SW Peru, 6; 3 mi. S, 1½ mi. E Peru, 2. Nuckolls County: 2 mi. WSW Superior, 5; 1 mi. SSW Hardy, 9. Otoe County: 1 mi. SE Nebraska City, 3; 3 mi. S, 2 mi. E Nebraska City, 3. Pawnee County: Turkey Creek, 4 mi. NW Pawnee City, 2 (NSM); 4 mi. S, 8 mi. W Pawnee City, 7; 1 mi. S Du Bois, 4. Platte County: Columbus, 3 (USNM). Polk County: 15 mi. W Osceola, 2. Red Willow County: 5 mi. S, 2½ mi. E McCook, 2; 8 mi. S, 3 mi. E McCook, 2. Richardson County: 5 mi. N, 2 mi. W Humboldt, 2 (1 NSM); 4 mi. E Barada, 16; 3½ mi. S, 1 mi. W Dawson, 6; 2 mi. N Falls City, 2; 4-6 mi. W Falls City, 4; ½ mi. S, 1½ mi. W Rulo, 1. Saline County: 2 mi. NE Crete, 1; ½ mi. W DeWitt, 1. Sarpy County: 1 mi. W Meadow, 1. Saunders County: 2 mi. NW Ashland, 3. Scotts Bluff County: 8 mi. NNW Scottsbluff, 1; Mitchell, 1 (NSM); ½-1 mi. S Mitchell, 13; 5 mi. S Gering, 10; 7 mi. S Gering, 1; 11-12 mi. S Scottsbluff, 4600-4800 ft., 8; 12 mi. SSW Scottsbluff, 4700 ft., 5. Sioux County: 1 mi. S, 4 mi. W Orella, 1 (NSM); 8 mi. N Harrison, 2 (UMMZ); 6½-7 mi. W Crawford, 3 (1 NSM); 8 mi. N, 1 mi. E Glen, 1 (NSM); 3 mi. NE Glen, 1 (NSM); Glen, 3 (NSM); Agate, 4600 ft., 1. Stanton County: 1½ mi. S Pilger, 3; 6 mi. SE Norfolk, 1. Thomas County: 1 mi. W Halsey, 2; Halsey, 1 (NSM). Thurston County: 1 mi. S Winnebago, 8. Valley County: 2 mi. W Ord, 1; 2 mi. S, 4 mi. E Ord, 6. Washington County: 1 mi. E Blair, 6; 3 mi. SE Blair, 2; 6 mi. SE Blair, 7; 3 mi. S, 2 mi. E Ft. Calhoun, 1 (NSM). Wayne County: ½ mi. W-2½ mi. E Wayne, 3. Webster County: 3 mi. S Red Cloud, 2.