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Sorex eximius

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Western Pygmy Shrew
(Sorex eximius)
Range
Taxonomic classification
Order:Eulipotyphla
Suborder:Erinaceota
Family:Soricidae
Subfamily:Soricinae
Tribe:Soricini
Genus:Sorex
Subgenus:Otisorex
Species group:Sorex hoyi group
Binomial details
Sorex eximius Osgood, 1901

Sorex eximius was split from Sorex hoyi by Hope et al. (2020); Sorex eximius represents the western species of the pygmy shrew and Sorex hoyi represents the eastern species of the pygmy shrew.

Description

From Armstrong et al. (2010) describing Sorex hoyi in Colorado, which is now Sorex eximius: "the pygmy shrew is very small, even by shrew standards. The pelage of Coloradan specimens is dark brown, shading to slightly paler on the venter. In other populations, color varies from reddish brown to gray-brown. The tail is indistinctly bicolored and relatively short."

External measurements

Length measurements are in millimeters (mm) and weight measurements are in grams (g), unless stated otherwise. If available, the sample size (n=) is provided. If a range is not provided and n= is not given, then the listed measurement represents an average.

Part of range Reference Total length Tail length Hindfoot length Ear length Mass
Colorado Armstrong et al. (2010) 70–90 25–31 9–11 2–5
North Dakota Seabloom et al. (2020) 84 30 10 4 2


Skull

Sorex eximius skull characters
units in mm
Skull
Condylobasal length:13.0–15.8 (Hall & Kelson, 1959); 15.0–16.2 (Álvarez-Castañeda, 2024)
Postmandibular canal:Absent
Shape:Skull relatively flat; braincase slightly rounded
Dental
Upper unicuspids:5
Unicuspid notes:U3 and U5 are tiny and vestigial
Tines present:Yes
Tine size:Large and long
Tine position:Well below upper limit of pigment on upper incisors
Shape upper incisors:Slightly curved
Dental characters:U3 and U5 are barely or not at all visible in side view, but may be seen in occlusal view

Sorex eximius have five upper unicuspids. The first, second, and fourth unicuspids are clearly visible. The third and fifth unicuspids are tiny and vestigial. The medial tines on the upper incisors are large and well within the pigmented area.

From Armstrong et al. (2010) describing S.hoyi in Colorado, which is now S. eximius: "Greatest length of skull is under 16 mm for these Rocky Mountain specimens. The skull is narrow and flattened, similar to other small species of Sorex. The third unicuspid tooth has a flattened disclike shape, and the fifth unicuspid is minute; both are barely or not at all visible in the side but may be seen in occlusal view. The distinctive dentition of S. hoyi, its small size, shorter tail, and normally darker color help to distinguish it from the montane shrew [Sorex monticola]."

  • Upper incisors of Sorex eximius, scale is 1 mm.
  • Unicuspids of Sorex eximius, scale is 1 mm.
  • Lateral view of skull of Sorex eximius, scale is 1 mm.

Similar species

Comparison with Sorex hoyi from Hope et al. (2020): "Until more definitive criteria become available, distinguishing between the two distinct North American pygmy shrews (S. eximius and S. hoyi) remains problematic in a field setting. We suggest careful consideration of the known geographical extent of genetic lineages based on the present study and emphasize the need for further field sampling through regions of potential sympatry. These areas include North Dakota, Minnesota, Manitoba and Saskatchewan, in addition to further north around Hudson Bay and the Gaspe Peninsula. Given the small size of North American pygmy shrews and high variance associated with the measurement of tiny mammals (Stephens et al., 2015), it is unlikely that standard field measurements alone will distinguish these species reliably. More detailed skeletal analyses based on samples with known a priori genetic identity may resolve diagnostic differences (Nagorsen et al., 2017), and field-based molecular barcoding is increasingly tractable (Ivanova et al., 2012)."

  • Sorex eximius
  • Eastern Pygmy Shrew (Sorex hoyi)
  • Range of Sorex eximius
  • Range of Sorex hoyi

Differentiating Sorex eximius in the field from other sympatric brown shrews may be difficult. Correct identification will likely require careful examination of the unicuspids. The third and fifth unicuspids in Sorex eximius and Sorex hoyi are tiny and not clearly visible, whereas the third unicuspid in other sympatric brown shrews are clearly visible in lateral view.

  • Unicuspids of Sorex eximius. Notice that U3 and U5 are tiny are not clearly visible.
  • Unicuspids of the Masked Shrew (Sorex cinereus) for comparison. Notice that U3 is clearly visible. The third and fifth unicuspids in Sorex eximius and Sorex hoyi are tiny and not clearly visible, whereas the third unicuspid in other sympatric brown shrews are clearly visible in lateral view.

References

Álvarez-Castañeda ST. 2024. Order Eulipotyphla. In Mammals of North America-Volume 1: Systematics and Taxonomy (pp. 397-513). Cham (Switzerland): Springer Nature Switzerland.

Armstrong DM, Fitzgerald JP, Meaney CA. 2010. Mammals of Colorado, Second Edition. Denver (CO, USA): University Press of Colorado.

Hope AG, Stephens RB, Mueller SD, Tkach VV, Demboski JR. 2020. Speciation of North American pygmy shrews (Eulipotyphla: Soricidae) supports spatial but not temporal congruence of diversification among boreal species. Biological Journal of the Linnean Society 129(1): 41-60.

Ivanova NV, Clare EL, Borisenko AV. 2012. DNA barcoding in mammals. DNA barcodes: Methods and protocols pp.153-182.

Osgood WH. 1901. Natural History of the Cook Inlet Region, Alaska. North American Fauna 21, 71.

Seabloom R, Hoganson JW, Jensen WF. 2020. The mammals of North Dakota. Fargo (ND, USA): North Dakota State University Press.

Stephens RB, Karau KH, Yahnke CJ, Wendt SR, Rowe RJ. 2015. Dead mice can grow–variation of standard external mammal measurements from live and three postmortem body states. Journal of Mammalogy 96(1): 185-193.

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