Sorex nanus Merriam 1895

53.

Dwarf Shrew

Sorex nanus

French: Musaraigne naine / German: \Westamerikanische Zwergspitzmaus / Spanish: Musarana enana

Other common names: Rocky Mountain Dwarf Shrew

Taxonomy. Sorex tenellus nanus Merriam, 1895,

“ Estes Park [Larimer Co.], Colorado,” USA.

Sorex nanus is closest to S. tenellus, and both are in the S. oreopolus group and subgenus Otisorex. Although S. tenellus and S. nanus are similar and once considered conspecific, they are morphologically distinguishable and distinct based on allozyme frequencies. They seem to have only recently diverged, but their specific status needs additional assessment with genetic nd morphometric data. Monotypic.

Distribution. WC USA from Montana S through Wyoming, SW South Dakota, extreme NW Nebraska, Utah, Colorado, Utah, Arizona, and New Mexico; probably SE Idaho as well.

Descriptive notes. Head—body 50-60 mm, tail 27-45 mm, hindfoot 10-11 mm; weight 2-4 g. The Dwarf Shrew is very small, similar to the Inyo Shrew (S. tenellus) but smaller in size with proportionally shortertail. Dorsal pelage is pale gray, with brownish wash; venteris silvery gray. Winter pelageis lighter than summer pelage. Feet are pale; ears are rounded and small. Tail is ¢.60-75% of head-body length and bicolored, being pale brown above and paler below, with small blackish tuft at tip. Teeth are pigmented dark red. There are five unicuspids,third is smaller than first, second, and fourth, and fifth is significantly smaller than the rest.

Habitat. Various alpine habitats including rocky areas of alpine tundra, sedge marshes, subalpine meadows, dry brushy slopes,arid shortgrass prairies, dry stubble fields, and pinyon-juniper woodlands. Dwarf Shrews most often occur in high-elevation open areas, although they occasionally occur in subalpine coniferous forests.

Food and Feeding. Dwarf Shrews primarily eat rock-dwelling small invertebrates (worms,snails, centipedes, insects, spiders), especially soft-bodied spiders and insects. They also eat carcasses of small vertebrates (e.g. mice and other shrews), salamanders, and some plant material on rare occasions. They are known to cache extra prey to eat later.

Breeding. Although breeding season of Dwarf Shrews is not completely known, they seem to begin breeding in June because first litter is generally produced in late July or early August. Dwarf Shrews might breed earlier and have more offspring at lower elevations. Two litters, sometimes three, are produced each year, with second appearing around late August or early September. Litters have 4-8 young (average c.6-5).

Activity patterns. Dwarf Shrews seem to be active day and nightlike other shrews.

Movements, Home range and Social organization. Dwarf Shrews are solitary except when breeding and rearing young.

Status and Conservation. Classified as Least Concern on The IUCN Red List. The Dwarf Shrew has a wide distribution and does not seem to have any major threats; however, because of their high-elevation lifestyle, they might be affected by global climate change. They have been considered rare, although this could be due to difficulty in capturing them.

Bibliography. Backlund (1995), Berna (1990), Brown (1967), Cassola (2016p), Clark & Stromberg (1987), George (1988, 1990), Hafner & Stahlecker (2002), Hoffmann & Owen (1980), MacCracken et al. (1985), Thompson (1977).