Geographic Variation in the Pocket Gopher, Thomys bottae, in Colorado.
by Phillip M. Youngman.
INTRODUCTION
Two species of pocket gophers of the genus _Th.o.m.omys_ (Family Geomyidae) occur in Colorado, _Th.o.m.omys bottae_ (see fig. 1) in the low valleys in the south-central and southwestern parts of the state and _Th.o.m.omys talpoides_ mainly in the mountains and high valleys.
_Th.o.m.omys bottae_ occurs primarily in the Pinon-juniper, Ponderosa Pine, and Short Gra.s.s zones of Daubenmire (1943) but in some localities is found in the Douglas Fir Zone. _Th.o.m.omys talpoides_ occupies primarily the Douglas Fir Zone and Engelmann Spruce-Subalpine Spruce Zone but is found also in the Pinon-juniper and Short Gra.s.s zones in some localities.
The ranges of the two species do not overlap in the strict sense but interdigitate in a parapatric type of distribution.
Two other pocket gophers, _Geomys bursarius_ and _Cratogeomys castanops_, also occur in Colorado--in the Upper Sonoran Life-Zone.
_Geomys bursarius_ occupies much of the Great Plains, whereas _Cratogeomys castanops_ is found only on the plains in the southeastern part of the state.
The objectives of the study, reported on here, were to learn the geographic distribution of _Th.o.m.omys bottae_ in Colorado, to find means for recognizing the different subspecies, and to describe individual and geographic variation.
I am indebted to Mr. Sydney Anderson and Professor E. Raymond Hall for many helpful suggestions and for their critical reading of the ma.n.u.script, to Dr. Richard S. Miller, who made the collection of many of the specimens possible, and to Dr. Richard M. Hansen for numerous suggestions. I wish to express my appreciation also to the following for the loan of specimens in their care: Alfred M. Bailey and A. A. Rogers, Colorado Museum of Natural History, Denver, Colorado; David H. Johnson, United States National Museum, Washington, D. C; Robert W. Lechleitner, Colorado State University, Fort Collins, Colorado; and Robert Z. Brown, Colorado College, Colorado Springs, Colorado.
METHODS
Adults of approximately equal age were compared in the study of geographic variation. Three criteria of adulthood are: (a) suture obliterated between supraoccipital and exoccipital, (b) suture at least partly obliterated between basisphenoid and basioccipital, (c) supraorbital crests not widely separated and almost parallel. In males the crests encroach on the lateral borders of the interparietal; in females the crests approach the lateral borders of the interparietal but are more widely separated than in males.
[Ill.u.s.tration: Fig. 1. Geographic distribution of _Th.o.m.omys bottae_ in southwestern Colorado.
1. _T. b. howelli_ 2. _T. b. aureus_ 3. _T. b. pervagus_ 4. _T. b. cultellus_ 5. _T. b. internatus_ 6. _T. b. rubidus_]
In studying geographic variation, greater emphasis was placed on females than on males. As noted by Grinnell (1931:4), males vary more than females, especially in length of rostrum and a.s.sociated nasal measurements.
Color terms are those of Munsell (1954). Color measurements were standardized by the use of a single 100 watt General Electric blue daylight bulb in a 12 inch white reflector suspended 24 inches above the specimen. All other light was excluded. The individual hairs of _Th.o.m.omys bottae_ are either bicolored or tricolored. The darkness of a specimen often may be attributed to the presence of dark-tipped hairs.
The color given in the description is the basic reddish or yellowish color of the hairs. The presence of a grizzled effect or a dark dorsal stripe, or any other pattern resulting from dark hairs, is noted in the remarks.
Specimens examined are listed by counties in the following order:
Mesa Montrose San Miguel Dolores Montezuma La Plata Archuleta Conejos Chaffee Fremont El Paso Pueblo Custer Huerfano Alamosa Las Animas
Localities are listed from north to south within a county. If two localities lie on the same line of lat.i.tude, the western precedes the eastern. Localities omitted on the map in order to prevent overlapping of symbols are in Italics. Unless otherwise indicated, specimens are in the University of Kansas, Museum of Natural History. The following initials are used to designate specimens in other collections:
CSU--Colorado State University, Fort Collins, Colorado.
CMNH--Colorado Museum of Natural History, Denver, Colorado.
ERW--E. R. Warren Collection, Colorado College, Colorado Springs, Colorado.
USNM--United States National Museum, Washington, D. C.
The following measurements of the skull are listed in the tables:
_Condylobasal length._--The shortest distance between the anteriormost projections of the premaxillaries and a line touching the posterior surfaces of the exoccipital condyles.
_Length of nasals._--The distance from the most anterior projection of the nasal bones to the most posterior projection of a nasal bone.
_Zygomatic breadth._--The greatest distance across the zygomatic arches, at right angles to the long axis of the skull.
_Squamosal breadth._--The greatest distance between the mastoidal processes of the squamosal.
_Length of rostrum._--The shortest distance from the shallow notch that lies lateral to the hamulus of the lacrymal bone, to the tip of the nasal on the same side of the skull.
_Breadth of rostrum._--The greatest width of the rostrum, anterior to the maxillae, transverse to the long axis of the skull.
_Alveolar length of upper maxillary tooth-row._--Distance between the anterior margin of the alveolus of the first cheek-tooth and the posterior margin of the alveolus of the last upper cheek-tooth, on one side of the skull.
_Least interorbital breadth._--The least distance across the frontal bones at the interorbital constriction as seen in dorsal view.
PHYSIOGRAPHY
_Th.o.m.omys bottae_ occurs in the Colorado Plateau Province (terminology of Fenneman, 1931), the Southern Rocky Mountain Province and a small part of the Great Plains Province.
The Colorado Plateau Province, in the southwestern part of the state, is mostly above 5000 feet and is characterized by the great number of canyons cut by rivers and streams in the nearly horizontal strata.
Prominent features of the landscape are cuestas, such as Mesa Verde, and laccoliths, such as Ute Peak.
The Southern Rocky Mountain Province consists mainly of high granitic mountains running north and south, many of which extend to more than 14,000 feet above sea level. Included in this region are several large basins, such as North Park and South Park and the San Luis Valley. The San Juan Mountains, which separate the Colorado Plateau Province from the San Luis Valley, and the Sangre De Cristo and Wet mountains, which intervene between the San Luis Valley and the Great Plains, importantly influence the distribution of _Th.o.m.omys bottae_.
The Great Plains Province is a broad highland that slopes gradually eastward from the Rocky Mountains. Of importance to the present study are two subdivisions of the Great Plains, the Colorado Piedmont and the Raton Section.
The Colorado Piedmont is a much dissected fluviatile plain, roughly extending from the vicinity of the Arkansas River to the northern boundary of the state. In general the topography of the Colorado Piedmont is broadly rolling with greater relief than the high plains to the east; however, b.u.t.tes and steep bluffs occur locally.
The Raton Section imperceptibly blends into the southern boundary of the Colorado Piedmont and extends south into New Mexico and Texas. A trenched peniplane of greater relief and alt.i.tude than the Colorado Piedmont, it is characterized by high mesas, extensive dissected lava-capped plateaus, deep canyons, and mountains of volcanic origin.
GEOGRAPHIC VARIATION
Six subspecies of _Th.o.m.omys bottae_ occur in Colorado. _T. b. aureus_ and _T. b. howelli_ occupy the Colorado Plateau Province (see fig. 1) and are characterized by a yellowish color; nasals posteriorly truncate or rounded; posterior extensions of premaxillae long; basioccipital wide; and interpterygoid s.p.a.ce U-shaped with a median spicule.
_T. b. internatus_, _T. b. cultellus_, and a new subspecies from the vicinity of Canon City described on page 376, inhabit the Sangre De Cristo and Wet mountains in the Southern Rocky Mountain Province and adjacent parts of the Colorado Piedmont and Raton Section of the Great Plains Province (see fig. 1). This group of closely related subspecies is characterized by reddish color; posterior margins of nasals forming a V; posterior extensions of premaxillae short; basioccipital narrow; and interpterygoid s.p.a.ce V-shaped, lacking a median spicule.
_T. b. pervagus_ occupies part of the San Luis Valley to the west of the Rio Grande (see fig. 1). In Colorado _T. b. pervagus_ is isolated from _T. b. internatus_ and _T. b. cultellus_ by the Sangre De Cristo and Culebra ranges and is separated from _T. b. aureus_ by the San Juan Mountains. _T. b. pervagus_ occupies an area geographically intermediate between _T. b. aureus_ to the west and _T. b. internatus_ and _T. b.