Thomomys clusius Coues, 1875

Cudworth, Nichole L. & Grenier, Martin B., 2015, Thomomys clusius (Rodentia: Geomyidae), Mammalian Species 47 (923), pp. 57-62 : 57-61

Thomomys clusius Coues, 1875

Wyoming Pocket Gopher

Thomomys clusius Coues, 1875b:138 . Type locality “ Bridger’s Pass , [Carbon County, Wyoming (29 km southwest of Rawlins )] Rocky Mountains.”

Thomomys talpoides clusius: Bailey, 1915:100 . Name combination.

CONTEXT AND CONTENT. Order Rodentia ( Patton2005) . Early work placed Thomomys clusius in suborder Sciuromorpha ( Hall 1981) , but more recently it has been placed in suborder Castorimorpha , family Geomyidae , genus Thomomys , subgenus Thomomys ( Patton 2005) . T. clusius is 1 of 12 species in the genus Thomomys ( Patton 2005; Hafner et al. 2011; Mathis et al. 2013a, 2013b). T. clusius is monotypic ( Patton 2005).

NOMENCLATURAL NOTES. Thomomys clusius was described as a species by Coues (1875b) but subsequently was placed as a subspecies of a widespread and polytypic T. talpoide s by Bailey (1915) in his revision of the genus. Bailey’s action was followed by subsequent authors (Hall and Kelson 1959; Long 1965; Hall 1981) until Thaeler and Hinesley (1979) raised clusius to species status by demonstrating both karyotypic uniqueness and sympatry or near-sympatry between clusius and talpoides . Recent systematics compilations (e.g., Patton, 1993, 1999, 2005) have followed Thaeler and Hinesley (1979) and regarded T. clusius as a species.

DIAGNOSIS

Thomomys clusius ( Fig. 1 ) can be distinguished from its sympatric congener, T. talpoides (northern pocket gopher), by its small size, lack of dark auricular patches, and white fringe on the ears, as opposed to the dark fringe on T. talpoides (Thaeler and Hinesley 1979) . The presence or absence of an auricular patch may be especially useful in field identification ( Keinath et al. 2014). These species can be further distinguished through genetic analyses; T. clusius has a karyotype with 2n = 46 chromosomes, whereas T. talpoides has a karyotype with 2n = 48 or 56 chromosomes where it overlaps with T. clusius (Thaeler and Hinesley 1979) . In contrast, T. idahoensis (Idaho pocket gopher) has a karyotype with 2n = 56–58 chromosomes, dark auricular patches similar to those of T. talpoides , and a range that likely lies slightly to the west of T. clusius ( Thaeler 1972, 1980; Clark and Stromberg 1987; Patton 2005). The Green River in southwestern Wyoming may represent a range boundary between T. idahoensis and T. clusius ( Keinath et al. 2014) . Geomys bursarius (plains pocket gopher) is also expected to occur in Wyoming but reaches its western boundary in the Great Plains grasslands in eastern Wyoming and does not overlap T. clusius (Clark and Stromberg 1987) .

GENERAL CHARACTERS

Thomomys clusius is the smallest member of the genus Thomomys (Thaeler and Hinesley 1979) . The pelage is pale, yellowish-gray with slight brownish tinge and dull gray at the base of the hairs ( Coues 1875b). The dorsal pelage is the same color across the cheeks and dorsal part of the head (Thaeler and Hinesley 1979). The margins of the tiny ears are fringed with white hairs, and auricular patches are absent ( Coues 1875b; Thaeler and Hinesley 1979). The venter, foot, and tail are white, but the snout is blackish ( Coues 1875b).

Thomomys clusius lacks sexual dimorphism (Thaeler and Hinesley 1979). Means ± SE (mm; ranges) of external and cranial ( Fig. 2 ) measurements of 22 individuals of mixed sexes were: total length, 172.7 ± 1.36 (161–184); body length, 127.6 ± 1.24 (112–134); hind foot length, 20.7 ± 0.13 (20–22); occipito-incisoral length (distance from occipital condyles to anterior end of the incisors), 31.64 ± 0.165 (29.8–33.0); rostral breadth, 6.27 ± 0.037 (6.0–6.8); incisor width, 1.69 ± 0.013 (1.6–1.8); zygomatic breadth, 18.73 ± 0.125 (17.5–19.8); cranial breadth, 16.81 ± 0.140 (16.0–17.8); interorbital breadth, 5.69 ± 0.038 (5.4–6.0); length of molar to incisor tip (distance from the anterior base of P4 to the tip of the incisor on the same side), 10.88 ± 0.097 (10.3–11.6); and bullar index, 1.52 ± 0.042 (1.30–2.0—Thaeler and Hinesley 1979). Ranges (mm) of additional external measurements were: tail length, 50–70 and ear length, 5–6 mm (sample size not provided—Clark and Stromberg 1987). Both relative zygomatic breadth (zygomatic breadth/least interorbital breadth) and relative length of the maxillary toothrow (alveolar length of maxillary toothrow/ greatest length of skull) vary by age ( Wilkins 1988). Ranges (mm) of external measurements for an additional 21 T. clusius of mixed sexes were: hind foot length, 15–23 and body length, 86–128 ( Griscom et al. 2010). Upon initial description of the species, Coues (1875b) provided the following measurements (mm) for the adult male type specimen: hind foot length, 19.05; fore foot length, 16.51; fore claw length, 7.62; and tail length, 38.1. Mean ± SE of body mass (g; range) of 22 T. clusius was 57.86 ± 1.731 (44.0–71.5—Thaeler and Hinesley 1979), and range (g) of an additional 21 T. clusius was 43–66 ( Griscom et al. 2010). The auditory bullae are greatly inflated ventrally, the mastoid bullae are inflated posteriorly, protruding to the level of the occipital condyles or beyond (Thaeler and Hinesley 1979), and the sphenoidal fissure may be incompletely closed ( Thaeler 1980).

DISTRIBUTION

Thomomys clusius may be sympatric with 2 other species of Thomomys — T. talpoides and T. idahoensis ( Patton 2005) . T. clusius is endemic to Wyoming, United States and is found only in southeastern Sweetwater and southwestern Carbon counties in the southern part of the state, covering an area of about 1.97 million ha ( Fig. 3 ; Thaeler and Hinesley 1979; Clark and Stromberg 1987; Keinath et al. 2014). Distribution extends from roughly 52 km southeast of Rock Springs, Sweetwater County to 25 km southeast of Rawlins, Carbon County (Thaeler and Hinesley 1979). The distribution was extended northwest of Rawlins with the capture of 4 individuals in 2009 ( Griscom et al. 2010). However, results of recent survey efforts have resulted in few discernible modifications to distributional boundaries (Hayden- Wing Associates 2008; Griscom et al. 2010). No fossils of T. clusius are known.

FORM AND FUNCTION

The dental formula of Thomomys clusius is the same as for other geomyids: i 1/1, c 0/0, p 1/1, m 3/3, total 20 ( Baker et al. 2003). The molars frequently have a 3rd enamel plate on the protomere ( Thaeler 1980). Although T. clusius shows no differences in enamel bands between sexes, significant age variation for the right M 3 in males and right anterior and posterior columns of P4 for females was reported by Wilkins (1988) in his evaluation of chewing direction in 9 species of Thomomys . However, explanation for why individuals display this asymmetry is not discussed. Deviation from 90° to the midsagittal plane of the skull for enamel ridges on upper check teeth of T. clusius is significant for 9 of 10 enamel bands, with angles ranging from 81.9° for the left anterior column of P4 to 98.0° for the right M2 ( Wilkins 1988). Only right upper cheek teeth have alignment of tooth scars significantly different from 0, ranging from −1.3° for M1 to −3.2° for P4. Although alignment of enamel bands and tooth scars for T. clusius and congenerics is significant, overall differences are biologically negligible, indicating that chewing in extant Thomomys is propalinal ( Wilkins 1988).

Thomomys clusius has large cheek pouches, measuring 44.45 mm from the beginning of the fold of skin at the snout ( Coues 1875a). Baculum length (mm; range) for 8 T. clusius was 12.05 (10.4–13.8), which is short in relation to head and body length when compared to the baculum of another small-bodied pocket gopher, T. idahoensis pygmaeus (19.8 mm —Thaeler and Hinesley 1979; Patterson and Thaeler 1982) and generally shorter than the larger T. talpoides (12.3–22.6mm), which shows no relationship between baculum length and head and body length (Patterson and Thaeler 1982; Verts and Carraway 1999).

ECOLOGY

Little work has been done to evaluate the ecology and life-history traits of Thomomys clusius due to its limited distribution and taxonomic history. T. clusius is closely related to T. talpoides and is believed to have similar reproductive, ecological, and behavioral traits (Clark and Stromberg 1987; Verts and Carraway 1999; Beauvais and Dark-Smiley 2005; Keinath and Beauvais 2006), but these remain to be determined. Most research on T. clusius has evaluated habitat requirements and capture techniques, primarily in response to a petition to list the species under the United States Endangered Species Act.

Thomomys clusius has been collected from areas characterized by well-drained, gravelly soils along ridgelines and stream-cut river banks, often in association with greasewood ( Sarcobatus — Thaeler and Hinesley 1979). T. clusius is found in areas characterized by more bare soil (ranging from 50% to 80% cover) and less litter and rock cover than unoccupied random sites ( Griscom et al. 2010). The species is also associated with areas that produce less big sagebrush ( Artemisia tridentata ) and more Gardner’s saltbush ( Atriplex gardneri ; 0.01–15.00% cover) and winterfat ( Krascheninnikovia lanata ) than unoccupied sites ( Griscom et al. 2010). When compared to sites occupied by T. talpoides , T. clusius is found in areas with flatter slopes, more bare ground, fewer surface rocks, less litter cover, less big sagebrush cover, less yellow rabbitbrush ( Chrysothamnus viscidiflorus ) cover, greater Gardner’s saltbush cover, and greater winterfat cover ( Keinath et al. 2014). The presence of Gardner’s saltbush may be the most useful factor in identifying T. clusius habitat, although it is unclear if T. clusius is selecting for this habitat or is being relegated to it by competition with T. talpoides ( Keinath et al. 2014) . Soil analyses suggest T. clusius occupies areas with soils with higher clay content and finer mean particle size, whereas T. talpoides tends to occupy soils with more sand ( Keinath et al. 2014).

Tunnel diameter may be used to distinguish between burrows of T. clusius and T. talpoides in the field; overall, tunnels of T. clusius are significantly narrower than those of T. talpoides ( Griscom et al. 2010; Keinath et al. 2014). From a survey of 21 T. clusius capture sites and 42 T. talpoides capture sites, tunnels averaging <55 mm in diameter had a high probability of being occupied by T. clusius , tunnels averaging> 80 mm in diameter had a high probability of being occupied by T. talpoides , and tunnels with intermediate diameters could have either species ( Griscom et al. 2010). Incorporating habitat metrics with burrow diameters may provide a means to allow researchers to distinguish between species without requiring capture.

The distribution of T. clusius in Wyoming may be further defined by a number of habitat and climate variables, the most important of which include a positive association with Gardner’s saltbush and intermediate daily and annual temperature ranges and a negative association with sagebrush and variable topography ( Keinath et al. 2014).

Although T. clusius and T. talpoides differ in habitat, individuals of both species are often found in close proximity (<100–269 m apart—Thaeler and Hinesley 1979; Hayden-Wing Associates 2008; Griscom et al. 2010). In one case, T. talpoides was captured <20 m from a T. clusius and within the same burrow system ( Griscom et al. 2010). However, no hybridization has been detected (Thaeler and Hinesley 1979; McDonald and Parchman 2010).

Many trap types have been used to capture T. clusius , with mixed results. Sherman traps (H.B. Sherman Traps, Inc., Tallahassee, Florida) had higher capture rates than gopher-specific traps (Baker and Williams 1972), with 14.9% and 6.5% capture success, respectively ( Hayden-Wing Associates 2008). However, Griscom et al. (2010) evaluated 5 types of live traps and found little influence on capture success of pocket gophers in southwestern Wyoming ( T. clusius , T. idahoensis , and T. talpoides ) but substantial differences on mortality rates. Overall trap success averaged 4.8 captures per 100 trap nights (range 1.6–6.8 captures per 100 trap nights— Griscom et al. 2010). The somewhat low capture success is likely the result of a low population density overall and trap saturation in an effort to capture individual gophers (D. A. Keinath, in litt.) Only Harmony (Harmony Metalworks, Laramie, Wyoming) and Sherman traps were used extensively, with Harmony traps resulting in lower mortality rates ( Griscom et al. 2010). Mortality rates can be alleviated by providing food, covering the traps to insulate from temperature fluctuations, checking traps frequently, and decreasing handling time (D. A. Keinath, in litt.).

Thomomys clusius received its name from a behavior common to all pocket gophers whereby individuals plug burrows and other excavations with dirt ( Coues 1875a; from the Latin root “clus,” meaning “to close or shut”). T. clusius is suspected to be solitary, with only one individual occupying a burrow complex outside the breeding season (Clark and Stromberg 1987).

GENETICS

Thomomys clusius has a karyotype of 2n = 46 chromosomes ( Fig. 4 ), which differs from nearly all other Thomomys in the slender-rostrum group, including T. idahoensis , T. monticola —the mountain pocket gopher, and T. talpoides , with the exception of T. mazama —the western pocket gopher (Thaeler and Hinesley 1979; Thaeler 1980; McDonald and Parchman 2010). Unlike many other species of Thomomys , T. clusius is invariant in number of chromosomes ( Thaeler 1980). T. clusius is considered to be genetically distinct from other congenerics and differs from T. talpoides in central Wyoming (2n = 48 chromosomes) by ≥ 4 paracentric inversions or other chromosomal rearrangements combined with Robertsonian fusion (Thaeler and Hinesley 1979). Based upon 456 Amplified Fragment Length Polymorphism markers, T. clusius forms a monophyletic clade that, along with T. idahoensis , forms a well-resolved clade distinct from T. talpoides ( McDonald and Parchman 2010). This suggests a common ancestor for both T. clusius and T. idahoensis that diverged before the radiation of T. talpoides subspecies ( McDonald and Parchman 2010). No intergradation between T. clusius and sympatric T. talpoides has been observed in either karyotypes or morphology, suggesting these species do not hybridize (Thaeler and Hinesley 1979; McDonald and Parchman 2010).

CONSERVATION

Thomomys clusius was petitioned for listing under the Endangered Species Act, but the United States Fish and Wildlife Service determined listing was not warranted because current information did not support that T. clusius faces significant threats that place the species in danger of extinction ( United States Fish and Wildlife Service 2010). However, because oil, gas, and wind energy development pose potential threats to the persistence of the species, additional investigation is needed ( United States Fish and Wildlife Service 2010). T. clusius is listed as a species of “Least Concern” on the International Union for Conservation of Nature and Natural Resources Red List of Threatened Species; population trends are unknown (Lindzey and NatureServe 2008).

Both Region 2 of the United States Forest Service (2009) and the Wyoming Bureau of Land Management (2010) list T. clusius as a sensitive species. The Wyoming Natural Diversity Database lists T. clusius as a species of concern that is imperiled with extirpation at both state and global scales ( Keinath et al. 2003). The Wyoming Game and Fish Department (2010) recognizes T. clusius as rare and classifies it as a Species of Greatest Conservation Need with a Native Species Status of 3, because limiting factors due to human activity are severe, and populations are vulnerable due to limited population size or distribution. The Wyoming Game and Fish Commission (1998) provides further protection for T. clusius by requiring permits for take and possession as well as for educational and scientific purposes.

The dearth of information available on T. clusius makes designing conservation actions difficult (Beauvais and Dark- Smiley 2005; Keinath and Beauvais 2006; Wyoming Game and Fish Department 2010). Many conservation issues have been suggested to negatively impact T. clusius , although few have been examined. Potential impacts include soil compaction due to oil and gas exploration and extraction, stochastic weather events, specific habitat requirements, habitat fragmentation, and a limited distribution (Beauvais and Dark-Smiley 2005; Keinath and Beauvais 2006; Keinath et al. 2014).

Figures

Fig. 1.—An adult female Thomomys clusius from about 22.5 km south of Wamsutter, Sweetwater County, Wyoming, 3 September 2009. Used with permission of the photographer, C. Okraska, Hayden-Wing Associates, LLC.

Fig. 2.—Dorsal, ventral, and lateral views of skull and lateral view of the mandible of an adult female Thomomys clusius (KU [University of Kansas Museum of Natural History] 43909) from 29 km NNE Sinclair, 6,500 feet, Carbon County, Wyoming. Occipitonasal length is 35mm.

Fig. 3.—Geographic distribution of Thomomys clusius in Wyoming, United States. Drainages (as defined by 10-digit Hydrologic Unit Codes) with known T. clusius captures are shown within the predicted distribution range (dashed line); triangles represent capture locations. County names are provided for reference. Locations from Wyoming Natural Diversity Database download from 12 February 2015. Map redrawn from Clark and Stromberg (1987) with modifications from Keinath et al. (2014).

Fig. 4.—Representative karyotype of a male Thomomys clusius from 23.2 km S, 9.7 km E Rawlins, Carbon County, Wyoming. Figure reprinted from Thaeler and Hinesley (1979), with permission from the author and the publisher,Journal of Mammalogy,Allen Press Publishing Services.