Sunday, 20 October 2019

The Smallest British Mammals: Shrews

Common shrew
For some reason that I no longer recall, a picture of a shrew cropped up while I was talking with a friend on the internet. "That's a mouse," she insisted. "No," I said, "it's a shrew." "Mouse!"

It was only later that I remembered that the German word for "shrew" is spitzmaus (literally "sharp/pointed mouse") and, since my friend was Austrian, she wasn't necessarily entirely wrong. From a certain perspective, perhaps to German-speakers, all these small, furry, long-tailed things are mäusen.

And it's certainly true that shrews do look, superficially, quite like mice. They tend to be smaller, with narrower snouts, and very small ears and eyes, but there is something quite mouse-like about them. Nonetheless, shrews (unlike, say, voles) are not rodents. In fact, as placental mammals go, they aren't even particularly related to rodents - humans are more closely related to mice than shrews are.
The distinguishing feature of rodents is the presence of a single pair of large, ever-growing incisor teeth in each jaw (this is what separates them from the lagomorphs, where the incisors are slightly different). Indeed, they don't have many more teeth than this; the number varies between species, but just 16 teeth in total is typical. Shrews typically have around 28 teeth and, while their incisors are also unusually adapted, it's a completely different adaptation to that of rodents.

The first pair of upper incisors in the mouth of a shrew are elongated and sharp, stabbing down like daggers. They often have additional spikes behind the main one, or grooves to promote the flow of venomous saliva, and the first pair of incisors in the lower jaw also tend to be sharp and double-pointed, if not quite so large. These are not the gnawing teeth of rodents, but teeth intended to deliver a fatal blow to a victim, or at least hold it fast once it is bitten into.

Most rodents are herbivorous or omnivorous, but shrews are tiny killing-machines.

The most common shrew in Britain is, appropriately enough, the common shrew (Sorex araneus). These are found throughout Great Britain, but are not found in Ireland. They are also absent from most of France, being found only in the Pyrenees (where they can cross into Spain), the eastern border regions and a few isolated patches elsewhere. They are also absent from subalpine Italy and the southern Balkans, but are otherwise found throughout continental Europe east of France, and across much of Russia, into eastern Siberia.

Shrews are mostly known for being small, and the common shrew is no exception, with full-grown adults reaching a maximum of 8 cm (3 inches) in length, ignoring the tail, and weighing from 5 to 15 grams (0.2 to 0.5 oz.) They live in broadleaf forests, often venturing into open patches of tall grass nearby, but avoid dense pine forests, which presumably lack suitable undergrowth. While they can hunt just about anything that's small enough, their preferred food appears to be adult beetles and their grubs. Woodlice, fly maggots, spiders, and earthworms are also a significant part of their diet.

Common shrews are highly territorial, preferring not to remain around other members of their species any longer than they absolutely have to, even during the breeding season. Their eyesight is relatively poor, but they have been reported to possess echolocation abilities similar to, if much less highly developed than, those in bats. They use these to find tunnels or other cover to hide in, especially during the night. 

One of the main features of shrews is that, because of their small size, they need to eat constantly in order to maintain their high mammalian body temperature - small animals lose heat more rapidly. Even allowing for this, however, their metabolism is remarkably fast, and they are active both day and night, taking only short naps, presumably because they become too hungry otherwise. For the same reason, they obviously can't hibernate, and literally shrink the size of their brains and skulls over winter to reduce the caloric requirements of maintaining them. 

It's something they only need to do once because they don't live for longer than a year. This is common for small animals, but in their case, their teeth also wear down with the constant use, especially against hard-shelled beetles. As a result, a year-old shrew would starve to death even if it was otherwise still healthy at that age. The effect is compounded by the fact that shrews are born with their adult set of teeth already in place, although it is also partially offset by the presence of iron in their tooth enamel, making the teeth harder... and also red, just like those of many rodents.

Among biologists, the common shrew is also known for another unusual feature: the number of chromosomes varies significantly within the species. What happens is that all the main, gene-bearing arms of the chromosomes are always present, but they arrange themselves in different ways, so that they have anything from 20 to 33 chromosomes in total. This is called Robertsonian translocation and, while it doesn't significantly affect fertility or do anything bad in and of itself, the children of humans with this condition often end up with more serious chromosomal abnormalities (Down's syndrome, mainly). Clearly, this doesn't happen in shrews.

As if that's not weird enough, you may have noticed that 33 isn't an even number, meaning that some common shrews must have an extra chromosome that isn't paired. This is because males of the species have two different Y-chromosomes, making them XY1Y2 rather than the usual XY pattern of most other mammals. How and why that happened is unclear...

Pygmy shrew
Closely related to the common shrew but (mostly) lacking its chromosomal pecularities is the pygmy shrew (Sorex minutus). This is the smallest species of mammal in Britain, measuring just 4 to 6 cm (1.5 to 2.5 inches) plus tail, and weighing 2.5 to 7 grams (0.1 to 0.25 oz.) when fully grown. Otherwise, they look physically similar to common shrews, and they have a generally similar habitat. However, they apparently do not earthworms (perhaps they are too large) or fly maggots, as their larger cousins do, but do eat a significant amount of spiders in addition to their main diet of beetles. In some parts of the world, small centipedes have also been reported as a popular prey item. This slightly different diet allows them to survive in some places where common shrews cannot.

Pygmy shrews are found through almost the whole of mainland Europe, apart from southern Spain and Portugal, and as far east as central Asia. They are the only species of shrew found in Ireland; genetic evidence suggests that they first arrived five or six thousand years ago, probably with unintentional human assistance.

The habits and life history of the pygmy shrew are similar to those of the common species and they have, if anything, an even higher metabolic rate. Both species breed whenever there is sufficient food to make raising young a viable proposition, and pregnancy lasts 25 days. They seem to be sexually promiscuous, something likely made more likely by the reluctance of the male to stay with the female for any longer than strictly necessary, leaving him unable to drive off rivals. As with other similar animals, this means that not all the members of a litter will share the same father.

Many other closely related species are found in continental Europe. The Appenine shrew, found only in Italy, and the Iberian shrew of Portugal and parts of western Spain are the most localised, while the crowned shrew is found in France, Germany, and the Low Countries as well as parts of Spain and Switzerland. The Alpine shrew, which is also found in the Carpathian and Balkan mountains prefers to live at the upper edges of forests, close to (and sometimes slightly beyond) the tree-line.

Finland is home to no less than five species of the genus Sorex. In addition to the common and pygmy shrews, there are three species adapted more specifically to colder lands, with some crossing over into neighbouring Sweden and Estonia, and all of them common across European Russia and well into Siberia - one even reaches as far east as northern Japan. The taiga shrew is almost entirely a forest dweller, but Laxmann's shrew and the least shrew also venture out into frozen bogs and the southern tundra, impressively for a small mammal that can't hibernate. The least shrew (Sorex minutissimus) has the honour of being, by physical dimensions, the smallest species of mammal; it's 3 to 5 cm (1 to 2 inches) in length, although there is a bat that's lighter.

But these are far from the only species of shrew in Europe. One of the others also lives in Britain, so it is to that I will turn next...

[Photos by Agnieszka Kloch and "Polandeze", from Wikimedia Commons.]

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