Sunday 16 June 2019

Small British Mammals: Ratty and the Water Voles

Four different species of vole live wild in Britain, with, perhaps, the field vole being the one that is most typical of the wider group. However, it's perhaps a different species that has the most claim to fame, appearing as one of the central characters (confusingly named "Ratty") in the popular children's book The Wind in the Willows.

Ratty is a water vole (Arvicola amphibius), an animal with a number of differences from common and field voles, despite being fairly closely related to them. Before discussing the species in more detail, we should acknowledge that, over the years, there has been considerable confusion as to what the scientific name of this animal actually is - and, for once there's quite a good reason for it.

The water vole was originally given the name Mus amphibius by Linnaeus, back in 1758. On the very same page of the magnum opus in which he described it, however, he also named another animal as Mus terrestris. While this animal was similar to the water vole, they were easy enough to tell apart, since the latter was quite a bit smaller, and didn't like living particularly close to water. In 1799, the French naturalist Bernard Germain de Lacépède moved both species into his newly created genus Arvicola, thus formally distinguishing voles from mice. Two years later, A. terrestris was split into two species, which have, over the intervening centuries been merged back together, and then split apart again in a different way.

Nothing unusual there; this sort of thing happens all the time. It's been 260 years after all, and we're learning new things all the time. No, the problem is that it turns out that A. amphibius and A. terrestris were actually the same species all along, And, because they were named simultaneously, there has been a good deal of confusion as to which of the two possible names is the "official" one. I'm sticking with A. amphibius as that's the one that's generally accepted these days (for reasons too tedious to explain), but the alternative name can also be found even in relatively recent sources.

But hang on, didn't I just say that the animals were easy to tell apart? Well, yes, because they are. It was thought for a while that this must be because they were different subspecies of the same thing, but it turns out that's not true, either. Detailed genetic analysis has shown that we simply cannot identify a single evolutionary group that defines either type on its own. What we have instead is one species that exists in two, distinctly different, physical and behavioural forms that are perfectly capable of interbreeding and can't be reliably distinguished genetically.

It could be that they are part way through the process of splitting into multiple new species - something that would take hundreds of thousands of years. And there are subspecies of water vole, some of which fall into one type, some into the other, and some seemingly into both, so that, quite frankly, we don't even know how many subspecies we're talking about. (Probably at least a dozen, but 30 or more is entirely possible.) Nature is obviously sticking two fingers up at taxonomists.

So, bearing all that in mind we can say that water voles, as currently understood, live from Britain and southern France in the west, through most of Europe north of the Mediterranean, and across a wide swathe of Asia from Turkey and Russia in the west as far as Mongolia and the margins of northern China in the east. Although they do live in some islands in the Baltic, and off the German and Dutch coasts, they have not, however, reached Ireland.

Water voles are significantly larger than common or field voles. The majority are 16 to 20 cm (6 to 7 inches) in length, and weigh 130 to 200 g (4.5 to 7 oz.) - about five times the weight of a field vole. They are heavily built, with a large head, strong jaws, and a tail that's proportionately longer than in more typical voles. They are typically brown in colour, although black, and even piebald, individuals are known.

But I say "the majority" precisely because there are those two different physical forms. The less common type is noticeably smaller, although still with the smallest adults having a similar body length to the very largest field voles (and being heavier, because of their stockier build). Compared with the standard type, their tail is slightly shorter in proportion to the body, they have shorter, less glossy fur, a different shape to the skull, and smaller pads on the soles of their feet. The gnawing incisor teeth so typical of rodents have a thicker coating of enamel in the less common form, although the same is not apparently true of their other teeth.

Why? Well, the other key difference is that the two forms have different lifestyles. As a result, they can be technically referred to either as "morphotypes" (because they look different) or "ecotypes" (because they live differently). Take your pick.

The common form, referred to as the "aquatic morphotype", lives close to slow-moving year-round sources of water, which can be anything from ditches to lakes or marshes. Here, they can either dig short burrows into the muddy banks or construct nests from grass above the water surface, as needs dictate. Excellent swimmers, with a fringe of fur around their toes in lieu of webbing, they often swim out into the water to feed on the stems of reeds and other water plants. On land, they feed on a range of plants, sometimes digging roots out of the ground, and are almost (but not quite) exclusively vegetarian. They seem to avoid forests, but do like plenty of long grass in which to hide from predators - which means that they will avoid areas that are heavily grazed by livestock.

The second form is the "fossorial morphotype". These are only found in high, mountainous terrain (and so are not seen in Britain), up to just over 3,000 metres (10,000 feet) in elevation, where they inhabit open meadows and heathland, often quite far from substantial sources of water. While they have much the same plant-based diet as aquatic water voles, they dig larger and more complex burrow systems, and many of their physical differences are adaptations to this more subterranean lifestyle.

While it may not be possible to define these fossorial water voles as a single genetic group, the difference is nonetheless inherited, with several populations in different mountainous areas having independently developed the physical changes over time. Elsewhere, aquatic water voles will move into drier land if they really have to (at least temporarily) and take up a fossorial lifestyle when that happens. It may be worth noting, for instance, that the ears of water voles seem to be intermediate in form and hearing range between those of entirely subterranean animals, such as moles, and, say, mice.

Water voles are active both during the day and at night. While each occupies a particular stretch of river bank (or equivalent, in the fossorial forms) they don't seem to be very territorial, and are willing to put up with close neighbours intruding on "their" patch. Indeed, while they mark their territories with latrines in which they regularly defecate and leave their scent, they seem to do this only during the breeding season, so this may be more of an advertisement than a warning to keep out. The males, on the other hand, produce scent from glands, and, since they also move about more, apparently feel less need to mark with latrines. Unusually for mammals, the females travel further from home when they reach adulthood than the males do, perhaps being more inclined to find unoccupied territory.

Like other voles, they can breed multiple times each year, leading to booms and crashes in population from year to year as conditions change; this seems to be something that is particularly noticeable in fossorial populations, possibly because they are not constrained by having to stay close to water. Breeding takes place from late spring to summer, with females able to become pregnant again as soon as they give birth. They are highly promiscuous, so that each female mates with multiple males, and is usually pregnant by two or three different fathers at the same time - it has been suggested that this prevents males from killing off young that might not be their own, since, honestly, they have no clue which might be which.

The young are weaned at around three weeks of age, but aren't usually sexually mature until they reach the end of their first year. While they can reach three years of age in captivity, in the wild, few survive a second winter.

Despite the complexity of the water vole's taxonomic history, it has been widely agreed that the similarly large "southern water voles" of Spain, Portugal, and western France do constitute a different species (Arvicola sapidus). Among other things, they have a different number of chromosomes to the regular sort, and, while nobody seems to have actually tried the experiment, this makes it unlikely that they could easily produce fertile offspring with the more widespread animal. More recently, some of the fossorial water voles of the Alps and Pyrenees have been considered to be a different species from those burrowing elsewhere (which scientific name they should have remains debated), and there's probably a separate one in non-Alpine Italy, too.

All of these seem to have arisen in the relatively recent past, with estimates based on DNA mutation rates suggesting that they first diverged no more than a quarter of a million years ago. Prior to that, so far as we can tell, there was only a single species. Since the Ice Ages ended more recently than that, water voles likely huddled in the south of Europe at the time, before returning to recolonise the continent (and Asia) when the ice sheets departed. Interestingly, it seems that the water voles of England and Wales are descended from some that were hiding out somewhere in Eastern Europe or western Asia, while those in Scotland represent an entirely separate colonisation event, having originally come across from Spain or Portugal.

While water voles are the most distinctive voles found in Britain, if only due to being closer in size to rats than to mice, there remains one more species of British vole that I have yet to cover, and I will turn to that next time...

[Photo by Peter Trimming, from Wikimedia Commons.]

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