About Neurotrichus gibbsii (Baird, 1858)
Neurotrichus gibbsii, commonly known as the shrew-mole, shares some morphological traits with shrews: like shrews, it has a pelage made up of guard hairs and underfur. Its fur is dense, soft, and ranges in color from dark gray to sooty bluish-black. This species is the smallest American mole, measuring approximately 10 cm (3.9 in) in total length including a 3 cm (1.2 in) tail, and weighs around 10 g (0.35 oz). It has a long, flattened snout, and a short, thick, bristled tail that is roughly half the combined length of its head and body; the tail is also covered in scales and scattered coarse hairs. Unlike shrews, it has a zygoma (zygomatic arch) and auditory bullae, which are characteristic features of moles. The tooth enamel of Neurotrichus gibbsii is white, unlike the mahogany or reddish-brown enamel seen in shrews, and this species lacks a penis bone. Its front paws are smaller than those of more fossorial moles and do not face outward from the body, making them more similar to the front paws of shrews. Even so, the front paws are broad with bifurcate phalanges that add support to the claws for digging; the three middle claws of the front paws are also elongated, and the eyes are completely covered by skin. Beyond its front paw structure, all of this species' morphological traits support its highly fossorial, subterranean lifestyle. It has a streamlined body that lets it move smoothly through tunnels, and short appendages that are held close to the body. It also has no external ear pinnae. Both of these traits reduce drag while the shrew-mole digs and moves through tunnels. Neurotrichus gibbsii inhabits damp, forested or bushy areas with deep, loose soil in the northwestern United States and southwestern British Columbia. In the northernmost part of its range, it lives along streams or in moist dense woods; in the southernmost part of its range, it occurs in swampy areas overgrown with vegetation such as sedges or shrubs. This shrew-mole can produce several litters each year, though no litters have been recorded between December and January. Females give birth to litters of one to four young. Newborns measure about 30 millimeters long and weigh less than 1 gram. In females, the vagina remains sealed until follicles develop in the ovaries. Males do not have a scrotum, though their testes and associated glands enlarge during breeding season, which increases their overall weight. These reproductive traits make it difficult to externally determine the sex of shrew-moles.
