Northern short-tailed shrew

Appearance

This shrew has a total length of 108 to 140 mm, of which 18 to 32 mm is tail; and weighs 15 to 30 g . The species exhibits slight sexual dimorphism in size, with the male being slightly larger than the female. The dorsal fur is thick and velvety, and can be black, brownish black, or silvery gray, with the ventral fur being a bit lighter and grayer. The shrew molts from a summer coat which is shorter and paler than the winter pelage in October and November, and back again sometime in February through July. The tail is quite short, amounting to less than 25% of the total length. The dental formula is I 3-3/1-1, C 1-1/1-1, P 3-3/1-1, M 3-3/3-3 = 16/16 = 32. Three well-developed scent glands are present, one on each side of the animal and one placed ventrally; the scent may be used for marking territories, though the shrew's sense of smell is thought to be poor.

Status

The northern short-tailed shrew is considered a species of least concern in the IUCN Red List, as it is widespread, abundant, and its population is not declining.

Behavior

Other shrews spend more time above ground than does the northern short-tailed shrew, which prefers to tunnel along below ground, through the leaf litter, or at the snow/ground interface. Bouts of frenzied activity, lasting around five minutes, are followed by longer periods of resting, with the total active time amounting to only 16% of a 24-hour day. This animal is capable of digging at a rate of 2.5 cm/min, in between resting.

The shrew constructs a nest up to 20 cm in diameter underground or underneath a log, and lines it with leaves or the fur of the meadow vole . This nest is kept clean, with wastes deposited outside the nest in a latrine area. Other parts of the burrow system are used for food storage.

Typically solitary, ''B. brevicauda'' exhibits several aggressive displays and vocalizations to ward off other members of the species when encounters occur. Pairs of shrews which were introduced to a cage simultaneously coexisted for less than four months before one killed the other, and a new shrew placed in a cage containing an already established shrew will be killed within a few hours.

Habitat

Both disturbed and undisturbed habitats are used by the northern short-tailed shrew, including grasslands, old fields, fencerows, marshy areas, deciduous and coniferous forests, and household gardens, though the preferred habitats are those which are moist with leaf litter or thick plant cover. Burned-over forests are not quickly recolonized by ''B. brevicauda'', and shrews quickly depart clear-cuts.

Reproduction

Mating occurs from March through September, though most births occur early or late in that period. Male shrews in captivity were observed to make clicking sounds while courting a female. During copulation, the male and female are locked together, and the female drags the male along with her. Gestation lasts 21–24 days, and after birth, the six to eight young are suckled for up to 25 days before the babies are weaned. Two litters per season are typical, though three are possible. The female strengthens the nest when the young are nursing, and is more active to support her increased nutritional needs. The young, which were born hairless and blind and weighing less than a gram, may become sexually mature in as soon as 2–3 months; those born in the spring mature more quickly than those born late in the season, and may themselves reproduce in the same year they were born. The juvenile pelage is pale and quite similar to the adults' summer fur, and is molted when the young reaches adult size.

Food

This shrew consumes up to three times its weight in food each day. It eats small quantities of subterranean fungi and seeds, though it is mostly carnivorous. It prefers insects, earthworms, voles, snails, and other shrews for the bulk of its diet, though salamanders and mice are also eaten. This shrew consumes vertebrates more often than other shrews do. The shrew mostly forages within a few hours after sunset, though it is also active during cloudy days. High evaporative water loss requires the shrew to have access to a source of water, though it does obtain water from its food, as well. The shrew often hoards food, especially in the fall and winter, or during a time of prey abundance; one study found it caches 87% of the prey it catches, while 9% is eaten immediately and 4% is left where it was killed.

Evolution

Most records of ''B. brevicauda'' are from the Pleistocene, though one record from the late Pliocene is tentatively attributed to this species. Another source indicates the earliest record of the genus ''Blarina'' is a specimen of ''B. b. talpoides'', from the Blancan in Kansas. The species is thought to have arisen in the middle or late Pliocene. The ''B. b. brevicauda'' subspecies appeared later.