Northern short-tailed shrew
|Northern short-tailed shrew|
|Northern short-tailed shrew range|
The northern short-tailed shrew (Blarina brevicauda) is the largest shrew in the genus Blarina, and occurs in the northeastern region of North America. It is a semifossorial, highly active and voracious insectivore present in a variety of habitats. It is one of the few venomous mammals. The specific epithet, brevicauda, is a combination of the Latin brevis and cauda, meaning "short tail".
B. brevicauda is a red-toothed shrew, one of three or four species (depending on the authority) in the genus Blarina. It was formerly considered to be a sister subspecies of the southern short-tailed shrew (Blarina carolinensis). The species has been divided into 11 subspecies based on morphological characteristics, which are grouped into two semispecies: brevicauda and talpoides. These groupings were mirrored by a molecular systematics study of the mitochondrial cytochrome b sequence. The two groups of subspecies are thought to have been kept isolated from each other by Pleistocene glaciers.
This shrew has a total length of 108 to 140 mm (4.3 to 5.5 inches), of which 18 to 32 mm (0.7 to 1.3 inches) is tail; it weighs 15 to 30 grams (0.5 to 1.1 ounces). The northern short-tailed shrew 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 total body length (as measured from tip of snout to base of tail) length. The species has one upper incisor, five unicuspids (simple teeth that cannot be easily identified), a premolar (the fourth), and three molars in the upper jaw. The lower dentition is identical, except only one is unicuspid. Three well-developed scent glands are present, one on each side of the animal and one ventral. The scent may be used for marking territories, though the shrew's sense of smell is thought to be poor.
Most records of B. brevicauda are from the Pleistocene, though one record from the late Pliocene (Blancan land mammal age) is tentatively attributed to this species. Another source indicates the earliest record of the genus Blarina is a specimen of the talpoides subspecies of B. brevicauda, from the Blancan (early Pleistocene) in Kansas. The species is thought to have arisen in the middle or late Pliocene. The B. b. brevicauda subspecies appeared later.
This shrew is found throughout central and eastern North America, from southern Saskatchewan to Atlantic Canada and south to Nebraska, Georgia, and northern Arkansas. It is probably the most common shrew in the Great Lakes region. Population densities usually range from 5–30 shrews per hectare (2–12 per acre), but rarely exceed 200 per hectare (80 per acre). The typical home range of a shrew is 2.5 hectares, and may overlap slightly with the ranges of other shrews.
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 moist with a decent amount of leaf litter or thick plant cover. Burned-over forests are not quickly recolonized by B. brevicauda, and shrews quickly depart clear-cuts.
The northern short-tailed 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 amphibians, especially 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.
The saliva of the northern short-tailed shrew contains a kallikrein-like protease, used to paralyze and subdue its prey. The toxin is strong enough to kill small animals, up to sizes somewhat larger than the shrew itself, and results in painful bites to humans who attempt to handle the shrew. The poisonous saliva is secreted from submaxillary glands, through a duct at the base of the lower incisors, where the saliva flows along the groove formed by the two incisors, and into the prey. The toxin is very similar in structure to the one used by the Mexican beaded lizard (Heloderma horridum), but evolved independently, however, from the same precursor protein.
One of the venom components, a peptide called soricidin, has been patented and is being investigated in Canada for pain control and as an anticancer drug. Another component is being studied in Japan as an antihypertensive agent.
Their sense of smell is thought to be poor, and their eyes are degenerate and vision is thought to be limited to the detection of light, but these shrews compensate by using echolocation and a fine sense of touch.
Its ability to consume almost anything it can catch allows the northern short-tailed shrew to survive the cold winters of temperate regions. The thermoneutral zone of this species (no extra energy must be expended by the animal to maintain its body temperature, which averages 38–38.5 °C) is from 25 to 33 °C; when the ambient temperature is within this range. Food consumption is 43% higher in winter than in summer, as the shrew must increase its metabolic rate to maintain its body temperature under the cold conditions. Temperatures at or above 35 °C are lethal for this shrew. A study of captive shrews found, though they were primarily nocturnal, the degree of nocturnality changed with the season. That is, during the colder winter months, the shrews exhibited more out-of-burrow activity earlier in the evening, but were active later in the night during the summer. The study indicated this seasonal pattern was due to solar radiation and changing daily temperatures, and it allows the shrews to minimize the energy needed for thermoregulation. Other winter adaptations include the creation of a lined nest, which helps the shrew conserve heat, caching of food in case of prey shortages, foraging below the leaf litter or snow where the temperature is milder, and decreasing activity levels during cold periods. Along with these behavioral adaptations, this shrew increases its ability to generate body heat during the winter by nonshivering thermogenesis in brown adipose tissue.
Blarina brevicauda is a semi-fossorial animal meaning that it spends the majority of its time underground. The shrews use their front feet to dig tunnels underground for shelter that often will resemble a honeycomb complex. Despite living underground, they have been known to climb trees and other structures in the short bouts of time spent on the surface. 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 about 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 shrews live in underground nests that are spherical in shape. These nests are often lined with the fur of meadow voles. The shrew constructs a nest up to 20 cm (8 in) in diameter underground or underneath a log. 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.
B. brevicauda has a high resting metabolic rate with their activity level is tied to the weather. The animals are most active on cloudy days as opposed to rainy or sunny days. The northern short-tailed shrew is widely considered solitary and unsocial which is likely due to its territorial nature. Pairs of shrews 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 is killed within a few hours.
Another interesting aspect of B. brevicauda behaviour is their use of echolocation. The eyesight of the northern short-tailed shrew is particularly poor and they must accordingly rely on other sensory methods to succeed. The animals have strong olfactory senses and are able to use echolocation. They use echolocation for exploration of their environment and tunnel building.
Reproduction in Blarina brevicauda occurs during the breeding season, which lasts from early February until September. There are two peaks of breeding, the first in spring and the second in late summer to early autumn.
During the reproductive period, female shrews tend to increase in size as well as appetite and will diversify their food sources and venture further to gain nutrients. Because of their high resting metabolic rate, the shrews reproduce frequently with upwards of 20 or more matings occurring in a single day. The repetitive copulation is needed in order to induce ovulation, as a minimum of six matings is required. Copulation in the short-tailed shrew can last as long as 25 minutes with the average length being 5 minutes. The female directs the copulation, positioning herself accordingly while the male is asleep. 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, weighing less than a gram, may become sexually mature in as soon as two to three 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.B. brevicauda is promiscuous. Male shrews in captivity were observed to make clicking sounds while courting a female.
Once pregnancy has occurred, the gestation period is 21 to 22 days on average, with litters approaching a mean number of 6 or 7 young. The weaning of the young occurs by 25 days old. Most B. brevicauda do not live beyond a year and in the case of the death of the litter, estrous in the mother will reoccur. The number of offspring in a population per year varies on a random basis leading to fluctuations in numbers but as the species is a generalist as are its predators, the impact of these random changes is not significant.
The northern short-tailed shrew has a high mortality rate, though it attempts to escape predation by remaining hidden under vegetation, soil, leaf litter, or snow; only 6% of a marked group of shrews survived to the next year, and winter mortality of 90% has been recorded, probably due to cold stress. This shrew is consumed by many predators: snakes, raptors, canids, cats, mustelids, skunks, raccoons, and opossums, though mammalian carnivores appear to be deterred by the musky odor produced by the shrew's scent glands.
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