Sunday, 8 December 2019
Small British Mammals: Moles
As the name implies, European moles are essentially unique to Europe. They are found throughout much of the continent, from northern Spain, Italy, and the central Balkans in the south to southern Sweden and Finland in the north. They are not found in southern Russia, but do reach just beyond the boundary with Asia in the northeast. While they are not native to Ireland, they are found on a number of smaller islands off the coasts of England, Scotland, and Denmark.
The wild habitat of moles is woodland, which is where the soil tends to be richest in temperate habitats. Frozen or thoroughly waterlogged soils would present obvious barriers to their digging, but they avoid the acidic soils of heathland as well, since these tend not to be good for their favoured prey of earthworms. In more modern times, of course, agricultural land - not to mention gardens - tends to have particularly rich soil, and anything that attracts the earthworms necessary to keep it that way will also attract moles to feed on them. Indeed, earthworms form at least half of their diet, topped up with insect grubs and other invertebrates.
Moles spend almost their entire lives below ground, surfacing only when they are particularly hungry (perhaps during a drought) or when their tunnels become flooded - in the latter case, they tend to head for higher ground and start digging again as soon as they can. Young moles also travel above ground immediately after leaving their mother, seeking to find somewhere to establish their own home.
Despite this, they spend less time digging than one might think, preferring to use tunnels they have already constructed, hunting for earthworms that wander through into the surface network. Deeper parts of the tunnel system, which can reach as much as a metre underground, are mainly used for resting and shelter, or for caching food for later use. As with shrews and hedgehogs, moles are typically antisocial, and territorial, perhaps using secretions from their anal glands to mark their tunnels.
Living underground does have the advantage that moles are difficult for most potential predators to find, and protects them from all but the worst of weather conditions. It does, however, also present a number of problems, such as the complete absence of light and low levels of ventilation. Physically, moles are highly adapted for digging, with a conical head, stocky body and short tail, and significant adaptations to the bones of the arms and fore-paws to form powerful shovels. In comparison, the hind limbs are far less modified, and have smaller claws. Their short, velvety fur is typically black, but albinos and other colour variants are perhaps more common in moles than in any other wild animal, presumably because they make little difference if you can't be seen.
The expression "blind as a bat" is largely misplaced, since bats often have quite good night vision to supplement their sonar. Moles, on the other hand, really do have terrible vision, since a subterranean tunnel is darker even than a heavily overcast night. They can detect light, and typically don't like it, since light breaking through to their tunnel system usually means that something is coming in to get them. But the lenses of their eyes cannot form a focussed image and some of the neural pathways associated with vision in other animals just aren't there. On the other hand, the European mole can at least open its eyes, which is more than can be said for some other species, where the eyelids are permanently fused.
Possibly to help with streamlining underground, moles have no external ear, but their hearing is actually pretty good. While they are particularly sensitive to low-frequency sounds, of the sort that might be transmitted through the ground, they can hear higher-pitched sounds, too. Another adaptation to underground life is that the blood of moles is unusually effective at binding oxygen, something that's in short supply anywhere that lacks potentially dangerous ventilation shafts. The details can vary with the seasons, but moles typically have three sleep-wake cycles in every 24 hour period, not being affected by night and day in the same way as other animals.
European moles breed in the spring, although, if the weather holds, they are sometimes able to birth a second litter later in the year. The young are born hairless and helpless, but grow quickly and are able to leave home less than five weeks later, in search of their own territory.
Possibly the weirdest thing about mole reproduction, however, is that, in a certain sense, female European moles are actually hermaphrodites. Genetically, of course, they are entirely female, and they produce egg cells and get pregnant in the usual way, so "hermaphrodite" may be going a little far. However, the central part of their ovaries are composed of what's effectively testicular tissue, incapable of producing sperm but nonetheless secreting relatively high levels of testosterone. Perhaps because of the effects of these hormones, the female reproductive tract develops some unusually masculine characteristics, including a prostate-like structure and a penis-like clitoris, which they urinate through as males do.
There are at least 50 widely recognised species of mole across the world, with the exact number largely depending on how you define "widely recognised". Nor is it unusual for new ones to be discovered, considering that moles are difficult to watch closely in their natural habitat, and all look fairly similar anyway. Indeed, the exact species status of the moles of northern Italy is disputed, since they are genetically different from those elsewhere (possibly due to isolation of their ancestors during the Last Ice Age).
Leaving those aside, however, while the European mole is by far the most widespread in Europe, there are four other species on the continent, with one each in Spain and Portugal, southern/central Italy, and the southern Balkans. The remaining species is the blind mole of Italy and the western Balkans, which is so named because its eyes, while present, are entirely covered with skin; it's unlikely that it can do any more than sense the mere presence of light.
Elsewhere. there are a number of species of mole found across Asia, and a smaller number in North America, including the odd-looking star-nosed mole (Condylura cristata). Others include the so-called "shrew-moles", a number of species that look somewhat like shrews, but aren't particularly closely related to one another. Not all of them are particularly adept at digging, but nonetheless, they are usually considered to be true moles, if only for lack of a better term. Also belonging to the mole family, but usually not considered to be moles themselves, are the desmans, two species of freshwater semi-aquatic animal, both of which are native to Europe, and have webbed feet rather than digging claws.
That brings me to the end of the list of British rodents, lagomorphs, and terrestrial insectivores. Of course, these are by no means the only mammals - or even the only small mammals - living wild in the British Isles. So, for the sake of completeness, here are the others:
Britain is home to weasels, stoats, polecats, martens, mink, otters, and badgers, some of which are certainly small, but all of which I've covered previously. Outside of the mustelids, it is also home to red foxes, which I've covered, and Scottish wildcats, which I haven't. However, assuming we don't count the seals and dolphins off the coast, the only wild animals larger than foxes are the three native species of deer.
Which really isn't very much.
As I said at the beginning of the series, this is largely because Britain is an island, limiting the opportunities for animals to wander across borders, but also because of the relatively high human population density of the country compared to certain other parts of the world.
For example, the US state with the closest land area to the UK is Oregon. We've seen that Britain has fifteen species of wild rodent, including the grey squirrel. In comparison, Oregon has no less than 53 such species (again, including foreign interlopers). Indeed, the squirrel family alone has twelve species native to Oregon, not all of which live in trees. Similarly, there are eight species of lagomorph, eleven shrews, and four moles. And that's before we look at the deer, coyotes, cougars, bears, and so on.
There's a reason I went with Britain.
There is, however, one exception, and that's the bats. Although these are quite clearly "small mammals", I didn't include them because it would have made the series too long, but, for the record, seventeen different species of bat are found in Britain; Oregon has fifteen. When you can just fly over the English Channel, borders seem rather less significant.
Bats have had fairly heavy coverage here, albeit not at a species level, so I'm not going to be turning to them next. Something, perhaps, that's a little more... popular.
[Photo by Mick E. Talbot, from Wikimedia Commons. Cladogram adapted from He et al. 2017.]