Shrew Business

In the gray, gathering gloom of an imminent February snowstorm, I stopped to watch a northern short-tailed shrew foraging on the edge of our powerline right-of-way. On this day it was a breezy 22 degrees Fahrenheit and patches of bare earth alternated with patches of frozen snow.

The shrew had scuttled past a mere five feet away. Then it paused and used its long, mobile, cartilaginous snout to poke in leaf litter and dried grasses in search of food. Next it pushed its snout under a snowy patch for ten minutes and busily ate whatever it had found.

That was when I slowly eased myself down on my “hot seat” to watch it. The shrew was too close to focus my binoculars and remained oblivious to my presence. It pursued its prey vigorously, its pointed snout questing, its clawed back feet pumping, its front feet digging like a frantic terrier. Once it pulled what looked like a caterpillar from beneath the leaf litter and chomped it down.

A small, plush, charcoal-gray, furry ball, it scuffled over the snow. Its pink nose constantly sniffed while its naked, pink feet scratched the thin snow layer or the open turf. The little creature ate so much that it even paused to excrete.

After almost 45 minutes of high-octane hunting and eating, the shrew ran under a log at the edge of the woods. Probably it was returning to its apple-sized resting nest. Constructed of grasses, sedges, and leaves in the shape of a hollow ball, the nest is located as much as six to 16 inches below ground or beneath logs, stumps or old boards. From the nest, openings lead to a complex underground burrow system that includes separate food-caching locations and latrine areas.

Mostly northern short-tailed shrews (Blarina brevicauda) sleep in the winter to reduce their need for food. But such periods are alternated with intense, active hunting periods that usually occur below the snow cover where it is warmer. Researchers claim that northern short-tailed shrews spend only brief periods above ground during cold weather, but this one, at least, was undeterred by the damp cold.

On a much warmer day in late May I heard a rustle in the dry leaves beside First Field Trail. Again it was a northern short-tailed shrew. This time it ranged over the forest floor, in and out of the leaf litter, along the sides of fallen logs, and atop beds of mosses as it looked for food. Once it tried to grab a centipede but missed. It also lunged twice at a small toad. Finally, it disappeared underground.

I have been enamored with these small, fierce creatures ever since I discovered my first northern short-tailed shrew dashing frenetically around the bottom of an old bucket I had set in our basement sink one winter almost three decades ago. I knew it was not a mouse but just what it was puzzled me. After a little research, I identified it and discovered to my surprise that it was only one of seven species of shrews living in Pennsylvania and 312 species worldwide! Found on every continent but Antarctica and Australia, shrews make up 25% or more of the species richness in northcentral and northeastern North America, especially in wet sites.

Shrews belong to the family Soricidae and first evolved soon after dinosaurs disappeared 38 million years ago. Since then they have remained almost unchanged. Most have five clawed toes on each foot, a long, pointed snout that extends beyond their jaw, a wedge-shaped skull, and sharp, pointed teeth. Those in the eastern United States possess minute eyes and a highly-developed sense of smell and hearing.

Of the seven Pennsylvania species, five belong to the Sorex genus (Sorex being the Latin word for “shrew”) and, except for the larger water shrew (S. palustris), are difficult to distinguish in the field.

Water shrews are primarily denizens of rocky-bottomed, rushing mountain streams in forests of hemlock, spruce, and rhododendron although they also have been found in bogs, dry creek beds, and near small springs. At 5.6 to 6.2 inches in length, they are the longest of Pennsylvania’s shrews. However, they are outweighed by northern short-tailed shrews, the largest shrew species in the state. Water shrews are further distinguished by their long, bicolored tails and bodies that are gray above and white below.

Their large, broad hind feet are equipped with stiff hairs on the sides of their toes that hold globules of air. This allows them to perform the seemingly miraculous feats of walking and running on the surface of water. They are adept swimmers and divers even under ice in the winter as they pursue their small aquatic prey. These are shrews I would love to see in action, but they are rare enough to be listed as a threatened species in Pennsylvania.

One Sorex species I have positively identified on our property is the smoky shrew (S. fumeus). I found one dead in the woods, popped it in our freezer, and later took it to Dr. Joseph T. Merritt, shrew expert and Resident Director of Powdermill Nature Reserve, the biological field station of Pittsburgh’s Carnegie Museum of Natural History. (For more information on Merritt’s work with northern short-tailed shrews, see my February 1997 column.) Merritt was quickly able to distinguish my smoky shrew from other Sorex species because it is larger (4.3 to 5 inches) than all but the long-tailed shrew.

Also called “gray shrews,” they like damp, dark woods with dense ferns and other ground cover shaded by a canopy of second-growth timber–the kind of environment we have here–and they eat small invertebrates of the leaf litter. Smoky shrews live with or near deer and woodland jumping mice, red-backed and pine voles, hairy-tailed moles and northern short-tailed shrews, and they use burrows of other small woodland animals. When disturbed they are liable to throw themselves on their backs and wave their legs while emitting grating, high-pitched calls.

Long-tailed shrews (S.dispar) resemble smoky shrews except for their longer tails, slimmer bodies, and darker coloration. But they prefer to live under and among rocks or boulders, especially talus slopes, hence their other name “rock shrews.” In Pennsylvania their favorite food is centipedes.

Then there are the two smallest, look-alike Sorex species–the masked shrews and pygmy shrews. They can only be told apart by examining their teeth, even though the pygmy shrews (S. hoyi) have the honor of being the smallest mammals in North America. Although they prefer dry mountain habitats, they can also be found in open fields and along the edges of woods. They make their tiny burrows below stumps, fallen logs, and forest leaf carpets and eat twice their weight (that of a dime) in insect larvae, spiders and beetles every day. Unlike most shrew species that produce two to three litters a year, they have only one.

I’m almost certain we have masked shrews (S. cinereus) here and so is Merritt because of the behavior I have observed. Over the years I have often been stopped by rustling and twittering sounds in the leaf litter. Tiny bodies dash in and out of the leaf cover and under and along logs. Sometimes they are so intent on what they are doing that I can sit on a log while they run under my legs. It is difficult to get a good look at them and an accurate count of their numbers, but I’ve seen up to five at a time. Masked shrews are most famous for this behavior, yet so far no researcher has positively figured out why they do it. Some think it is connected with courtship (they do have three litters of four to ten young a year) and others believe it is connected with food gathering. C.R. Vispo observed running masked shrews in a mountain forest in western North Carolina and found that their stomachs were stuffed with fly larvae. Another researcher mentioned that the larvae of some flies may travel in long, snake-like masses over the forest floor. Shrews, with their keen noses, would easily detect such a phenomenon. On the other hand, the running I have observed has occurred only in the spring and summer when masked shrews are courting, mating, and raising young.

Masked shrews are the most widely distributed shrews in North American, living in Alaska, across Canada, and south into the northern half of the United States. They need a shaded, moist habitat and eat tiny mollusks, insects and their larvae, small worms, and the carrion of larger animals.

Finally, there are the endangered-in-Pennsylvania least shrews (Cryptotis parva) that look like smaller versions of northern short-tailed shrews. Also called “bee shrews” because they enter beehives in search of larvae and pupae, their major foods, they like drier habitats than the other shrews. A grassland species, they live in grassy, weedy and brushy fields and forage in the runways of meadow voles. Least shrews breed from March to November and are sometimes found together in nests, as many as 12 and 31 in Texas nests and 31 in Virginia. Although most shrew species are solitary creatures except during courtship and breeding, researchers believe that both least shrew parents care for their offspring.

Shrews in general communicate mostly through scent and vocalizations and the masked, northern short-tailed and water shrews use echolocation especially when they are exposed to strange situations. Researchers think that echolocation may be a way for them to explore a new habitat without attracting predators. Of those they have plenty–owls, house cats, hawks, opossums, raccoons, snakes, foxes, weasels, bobcats, and herons, to name a few. Because they possess high metabolisms, they must eat almost constantly and most small shrews die of old age after a year. Only northern short-tailed shrews live longer–2.5 years.

Much more needs to be learned about these tiny, voracious, pugnacious creatures. Over the last couple decades, shrew research has expanded. As the t-shirt of Pennsylvania wildlife biologist Jim Hart puts it, “There’s no business like shrew business!”