Mammalian Species 47(923):57–62
Thomomys clusius (Rodentia: Geomyidae)
Nichole L. Cudworth and Martin B. Grenier
Nongame Program, Wyoming Game and Fish Department, 260 Buena Vista Drive, Lander, WY 82520, USA; [email protected]
(NLC); [email protected] (MBG)
Present address of MBG: Ducks Unlimited, 2926 East Mulberry Street, Fort Collins, CO 80524, USA
Key words: endemic, fossorial, rodent, Species of Greatest Conservation Need, subterranean, United States, Wyoming
pocket gopher
© 2015 by American Society of Mammalogists
Synonymy completed 14 December 2012
DOI: 10.1093/mspecies/sev006
Nomenclatural statement.—A life science identifier (LSID) number was obtained
for this publication: urn:lsid:zoobank.org:pub:047054D7-0E56-4F6B-89A1-33AD750D46C6.
www.mammalogy.org
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 (Patton 2005). 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. talpoides 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
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.
57
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Abstract: Thomomys clusius Coues, 1875 is a geomyid commonly called the Wyoming pocket gopher. Despite a varied taxonomic
history, T. clusius is now recognized as a distinct species and as the smallest of 12 species in the genus Thomomys. The species is
endemic to Wyoming, where it is restricted to about 1.97 million ha in the southern part of the state. T. clusius is often found in flat
areas characterized by Gardner’s saltbush (Atriplex gardneri) and fine-textured soils. Although the United States Fish and Wildlife
Service concluded in 2010 that listing under the Endangered Species Act was not warranted, state and federal agencies continue to
recognize T. clusius as a sensitive species due to its limited distribution and population size.
58
mammalian species47(923)—Thomomys clusius
followed Thaeler and Hinesley (1979) and regarded T. clusius as
a species.
DIAGNOSIS
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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
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 35 mm.
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
47(923)—Thomomys clusius
mammalian species59
(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).
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
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 M3 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 (12.3–22.6 mm), which shows
no relationship between baculum length and head and body
length (Patterson and Thaeler 1982; Verts and Carraway 1999).
ECOLOGY
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).
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
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DISTRIBUTION
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 (HaydenWing Associates 2008; Griscom et al. 2010). No fossils of T.
clusius are known.
60
mammalian species47(923)—Thomomys clusius
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
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.
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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
47(923)—Thomomys clusius
mammalian species61
(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
The authors thank C. Okraska for generously providing a
photograph of Thomomys clusius, R. Timm at the University of
Kansas for providing the skull, and C. Cudworth for assisting in
preparation of the skull plate. We also thank D. Jensen for creating the distribution map and B. Abel for compiling measurements
for General Characters. L. Carraway, S. Patla, L. Yandow, and 1
anonymous reviewer provided helpful comments on earlier drafts.
LITERATURE CITED
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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 DarkSmiley 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).
ACKNOWLEDGMENTS
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