Spot the Difference

Common European shrew

The shrew family contains what are, by the usual method of measuring such things, the smallest of all living mammals. Yet they are actually quite a large group, with well over 300 species so far identified, and, given their small size, probably quite a lot that we don't yet know about - especially in the tropics. Indeed, in the grand list of mammal families, they come fourth in terms of sheer number of species, beaten only by the rats and mice, the cricetids (most, of which, frankly, are also mice, although the group also includes the voles and hamsters), and the vesper bats. They may not have the greatest range of physical variation, at least to human eyes, but that's still quite a lot of diversity, and they certainly aren't "all the same".

It's perhaps worth pointing out, though, what a shrew is, and what it isn't. Firstly, while they may look kind of like mice, shrews are not rodents. Rodents are defined by the presence of great gnawing incisor teeth, and by not having the second set of incisors that rabbits have. Shrews don't look even remotely like this, having lots of small sharp teeth ideally suited for biting into insects. In fact, shrews' closest relatives are actually the moles and hedgehogs - relatively small animals that also eat a lot of invertebrates.

Secondly, the term "shrew" is a vague one, and there are a number of animals referred to as "shrews" in common parlance that do not belong to the shrew family. So we're not talking about such creatures as elephant shrews or tree shrews here, which are rather a different sort of thing. Even so, there is actually more variety among the "real" shrews than you might think.

At the highest level, we can divide the shrew family into two main groups: red-toothed and white-toothed. The former have iron pigments in their teeth, apparently to make them stronger, much as rodents do. These are the only sort of shrew found in the Americas, and are also common in Europe and Asia. White-toothed shrews, on the other hand, have... well, white teeth. They are found in Europe, Asia, and Africa, although there are, for example, no native species of white-toothed shrew in Britain.

Well over 90% of shrew species belong to one or another of these two groups. But there is a third group, as well, and it's one we know much less about. Diverging from other shrews about 19 million years ago, these go by the somewhat awkward term of "myosoricines"... largely on the grounds that nobody has come up with a better one. They have white teeth, but that name is already taken, and, while they're only ever found in sub-Saharan Africa, we can't just call them "African shrews", because there are actual white-toothed shrews in Africa, too. So, myosoricines it is, I fear.

There are at least two dozen different species of myosoricine, but they, in turn, can be placed into three groups. Most numerous are the mouse-shrews (Myosorex spp.) These are fairly typical looking shrews, with moderately long tails, visible ears, and so on. Second, however, are the African mole-shrews (Surdisorex spp.) As their name suggests, these spend a lot of time underground, although they are not truly subterranean as most moles are. Fitting with this, their ears and eyes are tiny, almost invisible, and they have short tails and feet that appear adapted for digging.

Finally, there are the Congo shrews (Congosorex spp.), which look somewhere in between the other two in general appearance, but whose habits we know little about. Studies on these, and the other myosoricines, are few and far between. However, studies on the better known kinds of shrew have often shown variations and differences between species that are not entirely obvious at first glance. Could the same be true of these obscure African species?

What we do have, as we have for pretty well any species that has been described sufficiently to actually get a scientific name, are external descriptions of what the animals look like. We also have, as is also typical for mammals, highly detailed measurements of the shape of their skulls, including such things as the interorbital breadth and the condylobasal length. (You probably don't need me to tell you that the latter is the distance from the anterior point of the premaxilla to the posterior surface of the occipital condyle. Obviously).

Such information, technical though it may be, certainly has its place. But if the main difference we know of between the mouse-shrews and the African mole-shrews is how much digging they do, there is also a good case for looking at the shape of the feet. Specifically, the fore-feet, because those will be the main ones used for digging, But how adapted are they, what can they tell us about the mysterious Congo shrews, and just how much like a mole is a mole-shrew anyway?

Examining X-rays of the front feet of a number of preserved specimens of myosoricine shrews, and comparing them with each other and with those of certain kinds of mole, gives us some idea of the answers. The pattern that is revealed is that the Congo shrews resemble the mouse-shrews much more than they do the mole-shrews. This is not particularly obvious, since Congo shrews do have the small ears and eyes that you would expect for an animal that digs a lot, has to avoid getting soil in its ears, and doesn't need to see particularly well.

But, while the rest of the body looks like it might be suitable for a burrowing animal, the feet just aren't that well adapted for the digging that would require. Specifically, the toes are relatively long and slender, and the bones within the paw are also narrow - fine for walking, but less effective for repeated strenuous activity. In contrast, the mole-shrews have short, sturdy bones, with the exception of the tips of the fingers (that is, the bit beyond the last joint), which are longer in order to support relatively powerful claws.

There are differences, however. The fore-feet of moles are highly symmetrical - the thumb and little finger are about the same length as each other, and so are the index and ring fingers. Furthermore, these fingers are not much shorter than the middle finger, giving an overall effect of a smooth, relatively flat curve, allowing all five claws to dig equally at the soil in a manner not unlike that of a spade.

The feet of shrews are not like this, in particular, tending to have thumbs shorter than their other digits (as humans do). Myosoricines turn out to be no different from other shrews in this regard, but, while one might expect the mole-shrews to be less so (and thus more mole-like) the opposite seems to be the case. In fact, what seems to have happened, over the course of their evolution, is that the middle finger became longer and the thumb and little finger shorter, focussing the digging power on the central digits. In compensation, the bones of the hand are even stronger and thicker than they are in comparable mole species, although not of those that spend their entire lives underground.

This perhaps shows a variation in how much time different species spend digging, searching for earthworms rather than insects on the surface. Congo shrews are, in this model, likely to be better diggers than mouse-shrews, but clearly fall far short of the mole-shrews, perhaps scratching at the surface rather than burrowing through soil, or perhaps digging through less compacted material. Even the mole-shrews have taken a different path than their subterranean cousins, developing something that is perhaps more like a pick or an adze than it is a shovel.

And, indeed, there is previously unsuspected variation here: one species of mouse-shrew has feet that, despite its other features, are quite well adapted for digging. This is the Kilamanjaro mouse-shrew (Myosorex zinki), a species found only on the mid to upper slopes of the eponymous mountain, and nowhere else in the world. That there is more variation than we thought, within a group of small and obscure mammals, shows how each of them has adopted a slightly different lifestyle, adapting their bodies to meet their differing biological and environmental needs.

Which may help to explain why there are quite so many different kinds of shrew in the first place.

[Photo by Sophie von Merten, from Wikimedia Commons]