Red in tooth, not claws

A couple of weeks ago, as Dan stepped outside one evening to bring Raven in from her tie-out (which we’ve started having to put her on, as she disappears off into the darkness at night and ignores our calls otherwise), he discovered this creature laying on the step. It was tiny – that’s my thumb included in the photo, partially for scale, and partially to prop the poor thing up while I took a couple of photos. It was dead, of course; we presume Raven caught it and killed it, but if she had it would be the first thing she’s killed, to our knowledge. Even the chipmunk she managed to catch some time ago she just pinned under her paws, apparently confused about what to do next. Perhaps it was already dead and she just picked it up and brought it back, liking the way it smelled. Perhaps she caught it, but in doing so she literally scared it to death – small animals can sometimes become so stressed out when captured that the stress itself will kill them (I’ve had it happen once or twice over the years of bird banding and it’s distressing; fortunately, in the some 20,000+ birds I’ve handled, a couple of times is an exceptionally rare occurrence, and virtually all are just fine).

Wherever it came from, and however it died, the poor creature was now on our doorstep. Dan brought it in and presented it to me because he knew I’d be interested in it for the blog (he always gives me the sweetest, most thoughtful gifts). Taking a closer look at it, I identified it as a shrew, probably a Masked Shrew, Sorex cinereus, the most common shrew of four possibilities in our area, as well as through the rest of its range across Canada and the northern US (hence its other common name, Common Shrew). Contrary to its name, it doesn’t have a well-defined mask; the websites I visited didn’t give a reason for this label.

Shrew, mouse or vole, how can you tell them apart? Shrews belong to the taxonomic order Insectivora, while mice and voles are in the order Rodentia. Members of Rodentia, the rodents, have long, sharp upper and lower incisors that grow their entire life. Shrews, however, do not; they are born with a single set of teeth (they actually replace their baby teeth before they’re even born) which wear down over the course of their life. Shrews also have five toes on their feet, while rodents only have four.

But probably as you’re watching something scamper across your yard you’re not able to get a really good look at its incisors or its toes. That’s okay. At least here in North America, you can tell the shrews from the mice by their elongated nose and somewhat big-headed appearance. Voles are larger, chunky rodents, looking a bit like a cross between a mouse and a guinea pig. The ears are often nearly hidden in a shrew’s fur. Some shrews will have shortened tails, but the Masked has a longer tail more like that of a mouse.

I spent a great deal of time flipping back and forth on the ID of this critter while I was taking the photos. There didn’t seem to be a defining marking in the coloration that would identify it as either a Masked or Pygmy. The Masked Shrew didn’t really show a mask, and the Pygmy Shrew wasn’t significantly smaller than the Masked, on average. I gather the Masked is sometimes more brownish and the Pygmy more grayish, but it’s a subtle distinction, and hard to decide on when you’ve only got the one shrew. Finally, I found a site that indicated you could tell them apart by their teeth – the side teeth immediately behind the incisors, called the unicuspids (similar to our canines), are all the same size in Masked, but the back ones are smaller than the front ones in Pigmy. I’d say these look to be about the same size.

Shrews in the genus Sorex (as well as others in the subfamily Soricinae) have red-pigmented teeth. Members of the subfamily are actually known as “red-toothed shrews”. The colour comes from iron deposited in the enamel of the tooth, which serves as a strengthening tool, hardening the enamel against the constant wear of day-to-day life, which is why it’s mostly found concentrated at the tips, where wear is greatest. I don’t know why only some shrews have it and others don’t, though.

The shrew’s face is covered in long, bristly whiskers. In most animals, long whiskers are associated with sensory functions, either of the surrounding habitat, or playing a role in food acquisition in insectivores. Think of birds, for instance, where species that hunt and chase insect prey, such as flycatchers, have lots of bristly whiskers, while those that eat seeds or tend to just pluck relatively stationary insects from foliage, such as most warblers, do not. The Masked Shrew eats primarily invertebrates such as insects, worms and snails, but is opportunistic and will take small vertebrates such as salamanders if given a chance, or seeds if living food is hard to find. Shrews have small eyes compared to mice, which feed mostly on seeds and berries and therefore require good eyesight to locate their food. Shrews rely on their sense of smell and touch. Their long, prehensile nose probably also plays a role in this.

Shrews are active year-round, and their trails can often be seen in shallow snow in the winter, where they plow a channel-like path compared to the leaping footprints of a mouse. Usually their trails dive down into the openings of melted snow around the base of trees and rocks, rather than tunnel up (or down) through the snow. Ontario Wanderer posted some good examples early last year (one of the front-page results that came up when I googled “masked shrew Ontario”).