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Shrews
Page Index:
Introduction
Portrayed in the idyllic summer season of warm and dewy days, the tiny masked shrew would seem to lead a blissful life. But bliss is likely to be an unfamiliar term to a shrew. With such a hectic hunt-and-eat pace, it seems a wonder that shrews survive in arctic and sub-arctic regions at all.
These smallest of mammals (the pigmy shrew is the second smallest mammal in the world) are not well insulated and they can perish from exposure within a few short minutes. Yet, in the Yukon alone, there are five different species and all are thriving and active year-round.
Characteristics
Worldwide, shrews are a very successful family. With a fifty million year history and a wide distribution across North America, their lifestyle works. The key lies in living at a fast and furious pace.
A Yukon shrew, like all North American shrews, is active day and night in a near-constant game of hunt and eat. Being the size of a thumb or less, and weighing no more than a nickel, a shrew is like a tiny jet engine: it must consume a relatively large amount of fuel and quickly convert it into energy. A shrews high rate of metabolism is characterized by a heartbeat of 1200 beats per minute -- the same as that of a hovering hummingbird. To maintain that rate of living, a diet of easily digestible, high energy food, is essential
One best source of such food is insect life. But how do shrews find insects in winter? Surprisingly, insect eggs, larvae, pupae and dormant adults are still present, often at ground level where the shrew hunts. In most circumstances, such insect food is more than ample for a shrews needs.
Ample in a shrews perspective means eating ones own weight in food each day. For a pregnant female, an adequate food supply can amount to three times her weight per day. This task is accomplished by being fast and aggressive. With front teeth that are designed for grasping and quickly cutting up its prey, this little insectivore scurries through natural or mouse-made passageways, attacking and eating any suitable object in its path.
Nowhere is efficiency a more important word than when it is applied to shrews living through the seasons in the Yukon. Whether arctic tundra or boreal forest, minimizing energy and water loss is an essential. One way of doing this is to live in sheltered habitats that reduce the loss of soil warmth and moisture.
The under snow environment, with a ground temperature near zero degrees Celsius, provides that habitat in winter. Even in summer, shrews are usually found in damp, sheltered locations. Nevertheless, a shrews life is very short. It will be over by the end of the shrews second summer. But by that time, it may have produced as many as three litters (all in the second summer), each containing as many as ten young.
Similar in Appearance But Different in Species
When it comes to shrews, you almost need to be a shrew to know one species from another. Of the five Yukon shrew species, three are very similar in outward appearance
and differentiation is made on the basis of tooth and skull features. Unfortunately, there have been few studies of our shrews in their natural habitat that would help to identify unique ecological adaptations.
One such study done in eastern Canada showed that one species chose insect larvae, while another had a flattened head which allowed it to poke its mouth into places unreachable to other shrews. Perhaps someday an ardent shrew researcher will examine Yukon shrews this way and consequently provide more obvious clues to their differences. In the meantime, the differences are subtle.
Masked Shrew (Sorex Cinereus)
One of the most common of Yukon shrews is the masked shrew. It is found in a wide variety of habitats, including forest and tundra, and at all elevations other than bare rocky mountain tops. Look for this species along forest edges, in clumps of willow and dwarf birch, or in wet sedge meadows containing cotton grass (Eriophorum sp.) tussocks.
The key to identification is body size and characteristics of the teeth in the upper jaw. The masked shrew is about 10 cm long from nose to tail and has five single-cusped teeth (like human eye teeth) in the upper jaw. Its larger size and five single-cusped teeth instead of three, separates this shrew from the pigmy shrew.
Dusky Shrew (Sorex Obscurus)
Equally widespread throughout the Yukon, the dusky shrew is about 12 cm long and otherwise difficult to distinguish from the masked shrew. The dusky shrew is somewhat larger and more apt to be found closer to water.
Water Shrew (Sorex Palustris)
Unique among other shrews, the water shrew is a swimmer and diver most commonly found under overhanging vegetation and banks bordering fast moving streams. To date, this species has been found onIy in the southern Yukon, including such well-known sites as McIntyre Creek, Nisutlin River, Canol Road, and Dezadeash Lake.
This is the largest of Yukon shrews (15 cm nose to tail) and is distinctive in its pelage which is black on top and silvery underneath. It also has tiny hairs fringing its feet that help during swimming. When diving underwater in search of aquatic insects, small fish and tadpoles, the water shrew has a silvery sheen caused by the millions of air bubbles trapped in its fur.
Arctic (Tundra) Shrew (Sorex Arcticus)
The arctic shrew is found across the northern half of the Yukon in somewhat drier sites than those of the masked and dusky shrews. It is larger than the northern subspecies of the masked shrew and has a distinctive dark back, brown sides, and grey belly. Dry hillsides and well-drained tundra ridges bordering on bogs, marshes and wet tundra are the typical habitats.
Pigmy Shrew (Microsorex Hoyi)
This tiniest of shrews is the smallest of North Americas mammals and competes with a South American shrew and bumblebee bat of Thailand for the title of the worlds smallest mammal. Yet this miniscule creature is widespread across nearly all of sub-arctic and temperate Canada and Alaska. It is found in the southern two-thirds of the Yukon. Excluding tail length this shrews body is only 51/2 cm long and weighs a little more than a penny (2.5g.).
Thought to inhabit meadows and sphagnum bogs or their edges, the pigmy shrew is rare in most of Canada including the Yukon, although this may be due to lack of research into the species.
Viewing Opportunities
Shrews are widespread throughout the Yukon and likely to be found in almost any moist habitat. But their minute size, earthy coloration, and habitat preference means that you are likely to see them only by accident. More than likely, the shrew you are lucky enough to see will be a dead shrew and, even then, that luck will probably come in winter when a dark shrew shows up well on white snow.