Geographic Variation in the Pocket Gopher, Cratogeomys Castanops, in Coahuila, México

Part 1

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~University of Kansas Publications~ ~Museum of Natural History~

Volume 7, No. 12, pp. 591-608

March 15, 1955

Geographic Variation in the Pocket Gopher, Cratogeomys castanops, in Coahuila, México

BY ROBERT J. RUSSELL AND ROLLIN H. BAKER

~University of Kansas~ ~Lawrence~ 1955

~University of Kansas Publications, Museum of Natural History~

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

Volume 7, No. 12, pp. 591-608 Published March 15, 1955

~University of Kansas~ ~Lawrence, Kansas~

PRINTED BY FERD VOILAND, JR., STATE PRINTER TOPEKA, KANSAS 1955

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Geographic Variation in the Pocket Gopher, Cratogeomys castanops, in Coahuila, México

Robert J. Russell and Rollin H. Baker

The plateau pocket gopher, Cratogeomys castanops, inhabits open lands from southeastern Colorado southward onto the Mexican Plateau as far south as southern San Luis Potosí and southeastern Zacatecas and southeastward to the Coastal Plain of northern Tamaulipas. This species occurs at elevations from as low as 26 feet at Matamoras in Tamaulipas to as high as 8700 feet in valleys of south-eastern Coahuila. In 1934, Nelson and Goldman (Proc. Biol. Soc. Washington, 47:135-154, June 13, 1934) revised the genus Cratogeomys and decided that six subspecies of C. castanops occurred in Coahuila. In the present account, we describe four previously unknown subspecies from Coahuila, exclude from the state two others recorded from there by Nelson and Goldman, and show that three others named previously from adjacent Mexican states do occur in Coahuila. This makes eleven subspecies now known from that state. From Coahuila Nelson and Goldman had 35 study specimens of C. castanops from seven localities and we have had 234 specimens from 63 localities. Consequently we have been able to define with greater certainty, than formerly was possible, the geographic distribution of C. castanops in this Mexican state and similarly analyze more completely the geographic variation.

Coahuila is near the center of the geographic range of C. castanops. The occurrence of 11 subspecies within the state seems to be the result of partial or perhaps, in some cases, total isolation of populations of C. castanops because of the highly dissected topography and the variability of the soil. Cratogeomys castanops is a sedentary animal preferring open plains mantled by suitable soils, preferably sandy in texture, in which the animals can dig their elaborate underground systems of runways. Thin soils of hard texture and rocky soils do not offer optimum habitat for C. castanops, and the animals usually are absent or uncommon in such situations. Desert mountains with their thin rocky soils, elevated passes, perpendicular rocky cliffs, and stands of oaks and conifers at higher elevations present impassable barriers for pocket gophers of this species. The Río Grande, bordering Coahuila to the north, in many places flowing through steep-walled cañons, also seems to be a barrier that this fossorial rodent does not cross; distinct subspecies occur on the two sides of the river directly opposite each other (also see Nelson and Goldman, op. cit.: 143). Smaller streams, such as the Río Salado, Río Nazas and Río Salinas, seem to be unimportant barriers to the passage of these pocket gophers. The food supply of C. castanops seems adequate in most situations and consequently food is unimportant in governing the distribution of this species. Principal foods of C. castanops are fleshy tuberous roots of well-distributed desert shrubs, but in the valleys of the high mountains of southeastern Coahuila, where desert shrubs are absent, roots and leaves of low-growing forbs are eaten.

Three distinct habitats for C. castanops occur in Coahuila. The state is crossed by a series of mostly impassable, mountainous ridges beginning at the northwestern boundary at the Cañon de Boquillas on the Río Grande and extending southeastward to the east-central border. This divides Coahuila into a more humid and less elevated northeastern area which is an inland extension westward of the Coastal Plain and a more arid and higher western and southern area which is a part of the "Mesa del Norte" of the Mexican Plateau. In the extreme southeast the still higher elevated plains and intermontane valleys within the Sierra Madre Oriental afford a third habitat for populations of this species. The subspecies of these pocket gophers found in any one of these three habitats show greater affinity to each other than they do to any subspecies found in the other habitats.

Generally speaking, populations of C. castanops from northeastern Coahuila are related, as a group, in color and cranial features. Partial isolation of subspecies in this area results chiefly from discontinuity of suitable soils rather than from topography. These pocket gophers occur most commonly in the deep, sandy soils which are found along streams, especially where farm lands are irrigated. In western and southern Coahuila, mountains extending in both north-south and east-west directions act as partial barriers to the passage of C. castanops. Within this large area, pocket gophers occur in desert basins many of which are enclosed on two or more sides by mountains. Even so, with the exception of the smaller C. c. consitus of northwestern Coahuila, all known subspecies occurring at lower elevations in the western and southern part of the state show close relationships in color and cranial features. Those subspecies in the higher parts of southeastern Coahuila by their small size and dark color reflect to a high degree their isolation in an elevated habitat.

Males of C. castanops differ greatly from females of equal age; consequently animals of the same sex, as well as of the same age, are used herein for taxonomic comparisons. Since, of any given age-group, females show less individual variation than do males, we have relied more on the characteristics of the females in this taxonomic study. Only specimens taken at approximately the same times of the year have been compared for color of pelage. Capitalized color terms are those of Ridgway, Color Standards and Color Nomenclature, Washington, D. C., 1912. Specimens made available through the courtesy of the authorities of the Biological Surveys Collection of the United States National Museum are indicated in the accounts of subspecies as BSC; other specimens listed are in the collection of the University of Kansas Museum of Natural History. Assistance with field work is acknowledged from the Kansas University Endowment Association and the National Science Foundation.

In any one of the lists of "Specimens examined" beyond, the order of arrangement of the localities is from north to south. Those localities listed in Roman type are represented on the distribution map (Figure 1) by blacked-in circles. Each of several circles covers two or more localities because the localities are close together. In any such instance the northernmost place is listed in Roman type and the names of the other places follow in Italic type. Measurements in millimeters are given in table 1 for females and in table 2 for males.

+Cratogeomys castanops convexus+ Nelson and Goldman

1934. Cratogeomys castanops convexus Nelson and Goldman, Proc. Biol. Soc. Washington, 47:142, June 13, type from 7 mi. E Las Vacas [= Villa Acuña], Río Grande Valley, Coahuila (opposite Del Río, Texas).

Distribution.--Extreme northern Coahuila, east and north of the Serranías del Burro (see fig. 1).

Diagnosis.--Previously known from only one specimen, a subadult female, this subspecies has not been well diagnosed. At hand we have five near topotypes of convexus (including two adult females and one adult male) and specimens assignable to this subspecies from several other localities. This subspecies may be characterized as follows: Size medium (see tables 1 and 2); dorsal profile of skull convex in females and flat, especially posteriorly, in males; zygomata weakly constructed and not widely flaring; mastoid and tympanic bullae inflated; nasals short; rostrum broad and short; maxillary teeth large.

Guide to subspecies 4. C. c. bullatus 8. C. c. subsimus 1. C. c. convexus 5. C. c. ustulatus 9. C. c. goldmani 2. C. c. consitus 6. C. c. jucundus 10. C. c. subnubilus 3. C. c. sordidulus 7. C. c. excelsus 11. C. c. planifrons]

Comparisons.--From topotypes of Cratogeomys castanops angusticeps Nelson and Goldman, found to the north and east across the Río Grande in Texas, convexus differs in: Body larger; upper parts more reddish, especially on sides; skull with zygomata less heavy, nasals broader, pterygoids smaller, maxillary teeth larger. For comparisons of convexus with the subspecies of C. castanops found to the west, south and southeast, see accounts of the subspecies to follow.

Remarks.--The geographic range of convexus is restricted, being bounded on the west and southwest by mountains, especially the Serranías del Burro, and on the north and east by the Río Grande. The range of the subspecies found to the southeast may not be continuous with that of convexus. At least, in the area between Villa Acuña and Piedras Negras, along the Río Grande, no specimens were obtained and no sign was observed. We suspect that in this area the species occurs only locally if at all.

Part 2

A specimen taken near the Río Grande in Coahuila, opposite Samuels, Texas, and assigned to Cratogeomys castanops clarkii by Nelson and Goldman (op. cit.:140), has been examined by us and is referable to convexus. This specimen is typical of convexus except for the lesser inflation of the mastoid bullae and tympanic bullae. Conspicuous differences between convexus and angusticeps indicate that the Río Grande is an effective barrier to passage by these rodents.

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