Shrewd Observatons

As one of our volunteers and I were standing at the window this morning, watching the birds at the feeders (he was hoping for some redpolls), I saw a small, dark, furry body scamper across the snow toward the sunflower seed hulls scattered on the ground.  As quickly as it appeared, it disappeared behind a pile of snow. 

“A vole!” I declared.  Then I corrected myself.  “No, a short-tailed shrew!”

Neil put up his binocs as the animal reappeared, continuing its foray toward the motherload, and indeed, it was a shrew.  Blarina brevicauda – the only venomous mammal in North America (technically there are two species, the northern and the southern short-tails, but for the purpose of the article, I shall lump them together).

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Now I know what you are thinking:  venomous mammals?  Really?  Truly!  There are not many worldwide:  the duck-billed platypus, the Cuban and Haitian solenodons (which are foot-long, shrew-like animals), the Eurasian water shrew, and the short-tails.  Unlike the others, the platypus isn’t going around biting and “poisoning” animals – it has a spur on its hind foot that it uses in self-defense to inject venom into its enemies; the rest of them, however, use their venom to incapacitate their prey.

If you came out to our Science Saturday last month, you would’ve seen a short-tailed shrew specimen at the table where we were talking about the subnivean zone, for these small mammals are active all winter, scurrying around between the ground and the snow layer in a constant search for food.  Their metabolism is so fast that they must eat every two or three hours or else starve to death.  Therefore, they do not hibernate, and they are constantly on the move.  Invertebrates (insects, worms) make up the bulk of their diet, but thanks to their venomous bite, they can also immobilize mice and voles, taking them into their tunnels to consume later at their leisure (if such frenetic animals are ever “at leisure”).

What kind of venom are we talking about?  Is it dangerous to people?  What if one bites my cat, my dog, my kid?

The shrew’s venom is apparently similar to that of the cobra, chemical-wise.  It contains two kinds of toxins:  a neurotoxin that causes paralysis, and a hemotoxin that decreases the prey’s blood pressure and wreaks havoc on the circulatory system.  It seems that the shrew’s “purpose” for applying the toxin is not so much to kill its prey (for it is fully equipped tooth- and claw-wise to take out most of the food it finds), but rather to allow it to put prey into storage for later consumption – a paralyzed beetle or mouse will stay “fresh in the larder” until needed later on, when perhaps a quick snack cannot been found.

Knowing all that, you might still be worried about Fluffy, Fido or Freddy.  But fear not – if your cat or dog is bitten by a shrew, it will most likely not come to any harm.  Infection from the bite is probably of more concern than any reaction to the toxin.  Ditto for your child. 

Many years ago, I took a class on Winter Mammalian Ecology, and the guest instructor was Dr. Joe Merritt, who probably knows more about short-tailed shrews than anyone, thanks to his years of research at the Powdermill Biological Station in Rector, PA.  He claims to have been bitten many times by his research subjects, and has suffered little more reaction than what one would get from a bee sting.  However, like with bee stings, some people can react more strongly than others, so it is always wise to not handle wildlife unless it is absolutely necessary, and even then, use proper care and precautions (wear gloves).

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But I love these little guys.  They are energetic, easy to sneak up on (I was within a couple feet when I took these photos), they sing (yes – many years ago I heard one singing in the backyard while I was sitting on the grass, writing in my journal).  They have tiny little eyes, no external ear flaps, and fur that is unidirectional – all adaptations for a life spent primarily underground.

I have found many a dead shrew just lying on top of the ground in my lifetime.  Found two within about 15 feet of each other last summer right here in Rockford while I was walking the dog around our neighborhood.  Short-tailed shrews are very common (living ones, I mean, although deceased ones seem to be quite common, too).  I have often wondered why these small morsels remain uneaten by the predators that caught/killed them, and later on by scavengers, and after a bit of research I learned that birds will eat them (owls, hawks), but not so much mammals.  Supposedly this is because the shrews taste bad (and birds, apparently, are oblivious to this).  This could be from glandular secretions (located on the animal’s belly and nether regions), which turn a tasty mouthful into something that makes the predator (say a fox, or house cat) think twice about actually consuming it.

If you didn’t get to see the shrews last month, don’t fret the missed opportunity.  I have sent all the bodies from my freezer to a taxidermist and in a couple months I anticipate having specimens on hand to share with visitors.  In the meantime, keep your eyes peeled.  Watch for small furry bodies moving along the foundation of your house, or darting out to your birdfeeders.  Look for a pointy snout and plush grey fur.  Odds are, short-tails are in your back yard and you’ve just never had the opportunity to say hello.