Monday, December 10, 2007

The Untamed Shrew

Idaho is known for its predatory animals. The state still has all of its original large predators, including wolves, grizzly and black bears, wolverines, mountain lions and more.

But perhaps the most ferocious predator in the state is not the one you think. As a matter of fact, it's probably darting around your backyard right now.

Shrews are among the most common mammals on the continent. They are found in nearly all habitats including suburbia. Six species are found in Idaho, out of 376 found around the world.

These little gray creatures may look like rodents, but they are actually members of their own family--and what a bizarre family it is!

The pygmy shrew is the smallest mammal in Idaho. According to Idaho Department of Fish and Game's excellent narrative on the shrew (which helped me verify a lot of this information from a memorable but distant college mammalogy class), the pygmy shrew weighs about as much as a pencil.

The water shrew has stiff hairs on its feet that apparently allows it to scamper across the surface of the water. Its stiff fur also traps air bubbles, allowing it to stay underwater for a long time. Theodore Roosevelt, on a hunting trip to the Selkirks of Idaho, was lucky enough to see one of these amazing critters. He watched it catch and devour a minnow, an account you can read in Roosevelt's book, The Wilderness Hunter.

Some shrews have the ability to echolocate--emitting sounds to produce sonar that helps them navigate their world. And find prey. The sound waves bounce back and tell shrews where their next target might be.

And shrews need to find a lot of prey. The reason you don't see many shrews, despite their abundance, is because they are always on the move--darting in the underbrush and down burrows so rapidly as to be undetected by predators. And, yes, prey. All this movement requires a lot of energy and an incredible metabolism--a typical shrew's heart beats 700-1000 times a minute.

So the shrew hunts, and eats. A lot. Some species must eat their own body weight in prey each day, or else starve. Most species eat insects, and many also eat mice--some twice as large as the shrew.

Of course, hunting is never a guarantee for even the most skillful of predators. To have to do so every day would seem to be a particularly risky evolutionary gamble.

That's why shrew species have some eating habitats that, to our human sensibilities, seem a bit, ummm, unsavory.

Some shrews are venomous (although none found in Idaho). These species bite their prey as they find it, paralyzing it. For the mouse (or large insect), this will be the beginning of a very bad day indeed. The shrew then urinates on its unfortunate victim, so it can easily locate by scent the decapacitated critter later. When it returns, its prey is still quite alive, if not well. The shrew can thus stash fresh, healthy meals throughout its range. Shrews have a thing for fresh food.

In Idaho, shrews aren't venomous, but will chew off the prey's legs, and crush its skull. The shrew does so in a way so the animal stays alive, but can't get away. The shrew then stows its extremely unlucky quarry in its den, to be devoured when the hunting isn't so productive.

The next time you see a little, long-nosed, darting gray form in your yard--or on a trout stream, in a forest, in a farm field--don't just assume it's a mouse. Take a closer look, and you may be rewarded with a glimpse of the untamed shrew. And be glad it is not larger!--Matt Miller


Anonymous said...

Thanks for the information!

kenwilken said...

Would like to know how to recognize
The presence of shrews in north Idaho wheat field. I saw a shrew(at least it was very fast and very small) Near it were small holes with the dirt in pellets It was in an area where there was vole or mice activity last winter but not now
Thank you,
Kenneth Wilken