The teething of the shrew

Last year, I spent three months identifying and cataloguing small mammal skulls in the Canadian Museum of Nature’s mammal collection.

Most of these skulls were from mice, voles, and shrews. These are the little animals that sound a lot bigger when heard at night rustling underneath leaves, though they are rarely seen due to their nocturnal habits — unless you have the eyesight of an owl!

A shrew, vole and mouse.

Three small mammals in Canada that are often confused: the shrew (Sorex), the vole (Myodes) and the mouse (Peromyscus). The vole and mouse pictured here have ear tags used in mark-recapture studies. Images: Shrew and Vole: Patrick Moldowan, © Patrick Moldowan. Mouse: Jonathan Gagnon, © Jonathan Gagnon.

So, using just a skull, how do you distinguish a shrew (Sorex), from a vole (Myodes), from a deer mouse (Peromyscus), and go on to identify the particular species?

Look at the teeth.

As furry little creatures they may look pretty similar, but when it comes to a dental perspective they’re distinct, particularly shrews.

Unlike mice and voles, shrews are insectivores — they feed primarily on insects, rather than the seeds, stems and leaves mice and voles consume. This difference in diet is reflected in shrew’s teeth. They have pointed canine teeth that are used to catch and eat worms, beetles, and spiders.

Ok, so this quick ID tool narrows an identification to “shrew”. But this is just getting started. There are 19 different species of shrews in Canada. A number of these species, including the barren ground shrew (Sorex ugyunak), live as far north as the Northwest Territories, Yukon and Nunavut.

illustration of 19 shrews

The 19 species of shrews found in Canada. Shrew images: Brenda Carter, Julius Csotonyi and Paul Geraghty, © Canadian Museum of Nature

These diverse Canadian shrews can be hard to tell apart using external characters, but again, the teeth tell the species tale.

For example, the cinereous shrew (Sorex cinereus) and the dusky shrew (Sorex monticolus) are almost indistinguishable when placed side-by-side, and they’re often found together since their ranges overlap throughout most of western Canada.

However, a careful look at their teeth, especially their unicuspids, teeth with a single point, tells them apart. As you move back from the snout, the unicuspids in cinereous shrews gradually decline in size, while a dusky shrew’s third unicuspid is clearly smaller than the fourth.

Photo montage of two shrew specimens and their teeth.

A. Side-by-side comparison highlights the close outward resemblance of the cinereous shrew (Sorex cinereus, left) and dusky shrew (Sorex moticolus). B. The upper teeth of the cinereous shrew. C.  The upper teeth of the dusky shrew. The tell-tale dental identifier of the dusky shrew is that its third unicuspid is much smaller than the fourth. Image: Elliott Schmidt, © Canadian Museum of Nature.

So, after an autumn of looking at small mammal skulls, I became very familiar with their different teeth, and was also glad I wasn’t identifying shrews based on another unique characteristic — they mark their territory using pungent scent glands which give off a strong, unpleasant odour.

 

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One Response to The teething of the shrew

  1. Pingback: Biting into the Past | Canadian Museum of Nature – Blog

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