Sorex nanus

Dwarf Shrew
(*Sorex nanus*)
Dwarf Shrew (Sorex nanus)
Range
Taxonomic classification
Order:	Eulipotyphla
Suborder:	Erinaceota
Family:	Soricidae
Subfamily:	Soricinae
Tribe:	Soricini
Genus:	Sorex
Subgenus:	Otisorex
Species group:	Sorex oreopolus group
Binomial details
	Sorex nanus Merriam, 1895
Other resources
	Full taxonomic details at MDD

Description

From Hoffman and Owen (1980): "Very small shrew. Summer pelage of S. nanus is hair brown to olive brown dorsally, extending far down the sides where it merges rather sharply with the smoke gray, somewhat buffy, venter (color terms from Ridgway, 1912). The tail is indistinctly bicolored to the tip, dark above and buff below. Winter pelage is lighter and grayer, especially on the back."

From Beauvais and Dark-Smiley (2003): "The dwarf shrew is one of the smallest of all mammals, with adults weighing only 1.8 to 3.2 g and having total body lengths of 82 to 105 mm. The hind foot length of adults varies only slightly from 10 to 11 mm (Hoffman and Owen 1980, Clark and Stromberg 1987). Sexes apparently do not differ in size. The dwarf shrew has an olive brown fur on its back that extends down its sides where it merges sharply with a smoky gray or buffy underside (Ridgway 1912). Winter pelage is lighter and grayer, especially on the back. The dwarf shrew has a long (27-40 mm) tail that is indistinctly bicolored to the tip, dark above and lighter below (Hoffman and Owen 1980)."

From Armstrong et al. (2010) for Colorado: "a delicate animal with a relatively long tail. The color is generally medium brown dorsally merging to a grayer color ventrally. The tail is indistinctly bicolored. Winter pelage is paler and grayer."

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	Mass
Arizona (San Francisco Peaks)	Hoffmeister (1986)	86–97 (n=15)	36–44 (n=15)	9–10.5 (n=15)
Colorado	Armstrong et al. (2010)	82–105	27–45	10–11	1.8–3.2
not reported	Hoffman and Owen (1980)	82–105 (n=10)	27–45 (n=10)		1.8–3.2
rangewide	Wilson & Ruff (editors, 1999)	82–105	27–45		1.8–3.2

Skull

Sorex nanus skull characters

units in mm

Skull

Condylobasal length:	13.8–14.8 (Hoffman & Owen, 1980)
Postmandibular canal:	Absent
Shape:	Braincase narrow and appears flattened when viewed from the side

Dental

Upper unicuspids:	5
Unicuspid notes:	U3<U4
Tines present:	Yes
Tine size:	Medium
Tine position:	Below upper limit of pigment on upper incisors
Shape upper incisors:	Diverging
Dental characters:	A diagnostic dental measurement is the distance from the posterior border of the alveolus of the fifth unicuspid to the anterior margin of the alveolus of the incisor ranges between 1.8 and 2.4 mm.

From Hoffman and Owen (1980): "Third upper unicuspid teeth smaller than fourth; fifth smaller than third. Accessory cusp on first upper incisor well developed and heavily pigmented. Size small, condylobasal length of skull less than 15.2 mm."

From Beauvais and Dark-Smiley (2003): "The dwarf shrew can usually be identified by dentition, although as with all shrews it is difficult to identify old individuals to species because of excessive tooth wear. In dwarf shrews the third unicuspid on the upper tooth row is smaller than the fourth, and both the third and fifth upper unicuspids are easily visible (Clark and Stromberg 1987). The accessory cusp on the first upper incisor is well developed and heavily pigmented. The condylobasal length of the skull of the dwarf shrew is less than 15.2 mm, with a rather flat profile (Hoffman and Owen 1980)."

From Armstrong et al. (2010): "The skull is slim and delicate, with a braincase that appears flattened when viewed from the side. The foramen magnum extends farther into the supraoccipital than into the basiooccipital. The condylobasal length is less than 15 mm. The third unicuspid is smaller than the fourth but visible from the side. A diagnostic dental measurement for the dwarf shrew is that the distance from the posterior border of the alveolus of the fifth unicuspid to the anterior margin of the alveolus of the incisor ranges between 1.8 and 2.4 mm (A. Spencer 1966). This measurement and the size of the third unicuspid allow distinction of the dwarf shrew from similar species."

Skull of Sorex nanus. Lower jaw is not shown. Large scale bar is 1 cm; short scale bar is 1 mm.
Unicuspids of Sorex nanus.
Upper incisors of Sorex nanus, scale is 1 mm.

Similar species

From Hoffman and Owen (1980): "No characters known to us, or to Diersing (in litt.) will reliably separate the two allospecies, although S. tenellus tends to be slightly larger, and with somewhat paler pelage (see also Spencer, 1966)."

From Hoffmeister (1986): "S. nanus differs from S. merriami, S. arizonae, and S. monticolus in much smaller size; length of body less (52 mm or less, rather than 53 mm or more); skull smaller in all measurements, including features of lower jaw. S. nanus differs additionally from S. merriami in the presence of tines on medial side of the first upper incisors, rather than none; tips of first upper incisors not close together; and third upper unicuspid equal to or smaller than fourth."

From Beauvais and Dark-Smiley (2003): "No known morphological characters will reliably separate the two taxa, although S. tenellus tends to be slightly larger and have somewhat paler pelage. Within its known range, the dwarf shrew is especially difficult to distinguish from the pygmy shrew (S. hoyi) and vagrant shrew (S. vagrans) (Clark and Stromberg 1987; WYNDD unpublished data). In contrast with the dwarf shrew, the third and fifth unicuspids of the pygmy shrew are difficult to see, and while the vagrant shrew’s hind foot length is over 11 mm (Clark and Stromberg 1987). Masked (S. cinereus) and Merriam’s (S. merriami) shrews have distinctly bicolored tails (Hoffman and Owen 1980). Habitat information may also be useful in identifying various species of shrews, although the general lack of specimens and knowledge of shrew life history make such extrapolations tenuous at best."

From Armstrong et al. (2010) for Colorado: "A diagnostic dental measurement for the dwarf shrew is that the distance from the posterior border of the alveolus of the fifth unicuspid to the anterior margin of the alveolus of the incisor ranges between 1.8 and 2.4 mm (A. Spencer 1966). This measurement and the size of the third unicuspid allow distinction of the dwarf shrew from similar species."

References

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

Beauvais GP, Dark-Smiley DN. 2003. Species assessment for Dwarf Shrew (Sorex nanus) in Wyoming, p 27. Wyoming Natural Diversity Database and USDI Bureau of Land Management, University of Wyoming, Laramie, WY. Link

Clark TW, Stromberg MR. 1987. Mammals in Wyoming. Lawrence (KS, USA): University Press of Kansas. Kansas.

Hoffmann RS, Owen JG. 1980. Sorex tenellus and Sorex nanus. Mammalian Species (131): 1-4.

Hoffmeister DF. 1986. Mammals of Arizona. Tucson (AZ, USA): University of Arizona Press.

Merriam CH. 1895. Synopsis of the American Shrews of the Genus Sorex. North American Fauna 10, 81.

Ridgway R. 1912. Color standards and color nomenclature. Published by the author, Washington, DC.

Spencer AW. 1966. Identification of the dwarf shrew, Sorex nanus. Journal of the Colorado-Wyoming Academy of Science 5:89.

Wilson DE, Ruff S, editors. 1999. The Smithsonian Book of North American Mammals. Washington D.C. (USA): Smithsonian Institution Scholarly Press.