February 7, 2012It is only when snow blankets the trails that we see evidences of many of the animals that live on the farm, and one of these is the tiny shrew. Its tracks are distinctive and give it away, however, as it tends to run rather than hop when it emerges from one of its tunnels and then disappears again underground.
It is possible that you have never seen a shrew, although there are some six species that live in Wisconsin, and several of them are quite common. Then, too, if you do catch a glimpse of one you may mistake it for a mouse as many are much the same size and shape. Surprisingly, the shrew is not a rodent at all but a cousin of the mole and is such a fierce predator that someone once said that if it were as big as a house cat, even the largest animal would fear it.
All shrews have round bodies tapered to a pointed whiskered nose at one end and a short tail at the other. They have poor eyesight but acute hearing and touch, perhaps because they usually hunt in the dark underground. Forty of the approximately 290 species of shrews worldwide live in North America, and most are terrestrial, living in ground litter in forests, grasslands, and marshes, and building their nests of leaves, grass, and hair beneath logs and rocks.
The heart rate of a shrew can be as high as 20 beats per second, and because of its rapid metabolism, the animal must eat almost constantly in order to survive. Shrews caught in "live" traps often die before release due to starvation as they can live only a few hours without food. They have a three-hour activity cycle, alternating hunting and feeding periods with sleep twenty-four hours a day all year long. Their high metabolism also requires ample water and despite the moisture in their food, most shrews live where water is readily available.
Female shrews have one or two litters of half a dozen young each year. The babies are born hairless and helpless, but grow quickly and are ready to leave the nest after about four weeks. Most are killed by other predators during their first year; still, shrews become sexually mature at two months and sufficient numbers survive to breed.
There are records of shrews in the earliest documents, including those of the ancient Egyptians. Along the Nile the Egyptians built many animal cemeteries, for which they used the term "resting places of the God Osiris-animal" and six species of mummified shrews have been found in these. On the other hand, some types have been known to be poisonous throughout history, and ancient Romans and Greeks believed that shrews were evil. One old text stated "it containeth in it poison like a spider, and if at any time it bite either man or beast the truth of this will be apparent...". The term "shrew-struck" was applied to lame horses or cattle in old England and it was a common belief that if a shrew ran over an animal’s (or human’s) leg, it would become paralyzed.
The Wisconsin species range from the Northern short-tailed shrew with a total length up 5 1/2 inches, including its one-inch tail, to the American pygmy shrew that is sometimes only 1½ inches long plus a ½ inch tail, making it the smallest mammal in the world. The Northern short-tailed shrew can weigh up to an ounce and must consume up to three times its weight in food each day, eating insects, earthworms, voles, snails, and other shrews for the bulk of the diet. The shrew often caches extra food, especially in the fall and winter, and one study found that it only ate 10% of the prey it caught, storing the remainder.
The Northern short-tailed shrew has another weapon in addition to its sharp teeth as its saliva contains a toxin strong enough to kill small animals, as well as cause considerable pain to any human brash enough to handle one. One of the venom components, a peptide called Soricidin, has been patented and is being investigated in Canada for pain control and as an anti-cancer drug, while another component is being studied in Japan as an anti-hypertensive agent.
The most widely distributed species in North America is known as the common or masked shrew and is somewhat smaller than the Northern short-tailed. It usually weighs less than 1/4 ounce and has darker coloring. It mostly lives in humid areas with high levels of vegetation in which to hide, and is active day and night year-round, digging tunnels or using those created by other small mammals. It can make very high frequency squeaks and is thought to use these as a kind of sonar to help it find its way in its dark underground world as the echoes bounce back from objects in its path.
Because the shrew is of limited economic interest, very little study has been done about its activities, and not many details are known about its life style – particularly in the smaller species. A few published observations of pygmy shrews kept in cages offer the only available information and these give only superficial views at best of these fascinating animals. The next time you spy a “mouse” at a feeding station or corncrib, look carefully; you may be seeing a shrew.