Why shrews are interesting

It has been said that our most distant primate ancestors, the mammal that gave rise to early primates but itself wasn't quite a primate, was most like the Asian tree shrew, which is neither a shrew nor does it live in trees. This is, of course, untrue. When the average American sees a shrew native to the new world scurrying past, he or she usually thinks of it as a form of mouse. Which it isn't. (In fact, there are no "mice" native to the new world, but even if we give our hypothetical observer the concept of "rodent" as in "eeek, a rodent" the shrew is not that either.) If you spend any time hanging out with the Efe Pygmies of the Ituri Forest, eventually there will be a sudden movement on the forest floor, a quick snap of a machete or other similar implement, and ... elephant shrew will be on the menu. And, most interesting, all three of the aforementioned shrews do not belong comfortably together in a single taxonomic group. The closest non-shrew relative to the most common North American shrew are moles, the closest non-shrew relative to the Asian tree shrew are flying lemurs, bunnies, primates, and rodents; and the closest non-shrew relative to the African elephant shrew could be, astonishingly, an actual elephant! (Or hyraxes, goldem moles, sea cows or the Aardvark.)

Why do we call all these things shrews? Well, they are similar: Small, furry, pointy nosed, mostly insect eaters, and they aren't something else. More likely, though, the word "shrew" has simply been called on to do more work than any word should be recruited for. And, all the shrews combined are not cattle, dolphins, antelope, monkeys, pandas, big cats or big dogs, or some other sexy mammal. I suppose we should be lucky that this diverse group of organisms aren't all called "mouse" (as in "eeek, a mouse!"). I should note, by the way, that I've not actually mentioned all the known animals called shrews: To do so, I'd have to mention the otter shrews and the extinct West Indies shrews.

And yes, I have eaten the elephant shrew, and no, it does not taste like chicken. In fact, almost nothing I've ever eaten that was not a chicken has tasted like chicken. Elephant shrews don't taste like elephants either.

The common mouse-like shrew is in the shrew family Soricidae (Order Soricomorpha), which includes over 300 species distributed among 23 genera. They are worldwide wherever you find small furry things other than Australia (they are not shrew-roos, after all) and not in South America. Some of these shrews, in the genera Sorex and Blarina use echolocation, like bats.

Six wandering shrews (Sorex vagrans) were trained to echolocate the position of a platform and drop to it. They preferentially directed their ultrasonic emissions at the platform. With their ears plugged, they were unable to locate it above chance levels even though they increased their emission rate. Shrews with hollow tubes in their ears performed as effectively as controls. Echolocating shrews, trained on an elevated Y-maze, detected a 15×15-cm flat metal barrier to a distance of 65 cm. The minimum detectable barrier at 20 cm was 3·5×3·5 cm. They detected a target with a 4×4-cm hole to 30 cm. Their dependence on echolocation was inversely related to familiarity with an area. Audition was important for the location of suitable cover in strange areas. The adaptive significance of echolocation to the wandering shrew is discussed.¹

Tree shrews have the odd characteristic of possessing human-like ears. They also have a tooth comb (the front teeth make up a comb used for grooming), which is found as well in many prosimian primates. The tooth comb and a few other traits caused early researchers to suggest that the tree shrew is a living version of the original primate. We now know that tree shrews share a common ancestor with primates and the flying lemurs to form an unresolved group of related mammals, but there is no particular reason to include or exclude the tree shrew as a model for the first primate. The key primate features that make primates primates (and I greatly oversimplify) are not really found in tree shrews. The tree shrew tooth comb, for instance, is made up of different teeth than the tooth comb of primates. To my knowledge, no one has explain why they have human-looking ears. Coincidence, most certainly.

Regarding the elephant shrew: The version with which I'm most familiar is the giant elephant shrew (Rhynchocyon cirnei). (Kingdon probably has the best pictures and descriptions generally available.) This creature has fawn-like markings on it even as an adult, as do many rain forest mammals. And it is called giant because it is HUGE. For a shrew. Small enough to fit in your shoe, but large enough to make a meal. Well, a side dish, anyway. The most interesting thing I observed with the elephant shrew is not so much the shrew but how the Efe Pygmies handled it. The Efe have a characteristic way that they butcher the antelopes and monkeys that make up most of the meat in their diet. But the giant elephant shrew is smaller than any piece into which an antelope is cut using this method. For many small animals like little song birds or small fish, the Efe just cook the animal on an open fire until it becomes crunch then eat it like you might eat funnel cake at the fair. But when the caught elephant shrew, always by chance as one happen to wander by in the leaf litter on the forest floor, they butchered it exactly as they butcher an Okapi or a Duiker. (That process of butchery is the subject of another blog post, which I've not written yet but someday will. It's quite interesting.)

There is one final thing I'd like to mention about shrews that I find interesting. There are a lot of underground rodents, and some non-underground rodents, that eat roots as their fallback food. This has been the subject of a fair amount of research by my colleagues and me. These rodents have specific characteristics of their jaw and teeth. It occurred to us one day that maybe these characteristics had to do with living underground, and not with eating roots. But the underground dwelling shrews (and other non-rodent mammals, like moles) don't have those characteristics at all, showing that big teeth and strong jaws are not caused by eating dirty insects or living in holes. I know, I know, that may not be that interesting to you, but ... well ... I guess you had to be there.

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¹BUCHLER, E. (1976). The use of echolocation by the wandering shrew (Sorex vagrans) Animal Behaviour, 24 (4), 858-873 DOI: 10.1016/S0003-3472(76)80016-4