Sunday, 18 August 2019

Small(ish) British Mammals: Beavers

There are, arguably, a couple of reasons why beavers don't really belong in this series on the small native animals of Britain. So far, I have looked at mice, voles, rats, and squirrels, all of which are undeniably both small and British... but they are also all rodents, and so I may as well complete the set of such animals by including the one that isn't really all that small.

Indeed, weighing in at around 15 to 20 kg (33 to 44 lbs) on average, and occasionally a fair bit larger, the Eurasian beaver (Castor fiber) ties with its American cousin for the honour of being the second-largest living rodent, after the capybara. It is also, of course, a very distinctive animal, and one whose general appearance most people are surely familiar with.

The Eurasian beaver once lived across almost the whole of Europe and the temperate regions of Asia. Today, they are really only common and widespread in European Russia and the Baltic states, although there are significant populations in Norway and Sweden, and scattered groups across Poland, Germany, and France, along with a few neighbouring countries. Even the Russian populations, however, are smaller and more isolated on the eastern side of the Ural Mountains (which mark the traditional boundary between Europe and Asia)... although, to be fair, those populations do reach a considerable way east, almost to the Pacific, and there are even small beaver populations in northern China.

Given this huge range, it's unsurprising that many subspecies of Eurasian beaver have been named down the years, with the assumption that each must have sheltered in a separate geographic refuge during the Ice Ages. However, while it is just about possible to distinguish these populations genetically, the differences are sufficiently small that they may post-date the end of the Ice Ages, and that, at best, only two subspecies are distinct enough to qualify as such - and probably not even that.

The reason that beaver populations are so scattered, and often small, across their vast former range is, of course, overhunting. This reached its peak in the 19th century, and it is estimated that as few as 1,200 Eurasian beavers were alive in 1900. They had vanished from Spain and Portugal in the 17th century, and from much of the rest of Europe in the 19th, surviving only in Russia and as tiny relict populations in France, Germany, and Norway.

Over the course of the 20th century, a combination of hunting bans and active efforts at restoring beaver populations meant that, by 1998, the worldwide population was estimated at 430,000; it's now probably over a million, and the species hasn't been considered as even close to "threatened" status since 2008.

Individual populations are a different matter, however, especially in Asia. The tiny beaver populations of China are the most endangered, with only a few hundred individuals still thought to live in the country, mostly along a single stretch of the Ulungur River in the Altai Mountains close to the Mongolian border.

This, of course, brings us to the other reason why beavers might not fit in my British Mammal series: officially, they're extinct in Britain.

It's not that they've never lived there, because they may well have been quite common in medieval times. But they seem to have died out in the mid 16th century, with the last populations probably surviving in Scotland. However, with reintroduction efforts so successful in other countries, why not Britain?

Outside of countries that would have been, as of 1900, part of the Russian Empire, the most successful reintroduction was probably that in Sweden, where beavers went extinct in 1871 - two years before they were granted legal protection there. This was also one of the first such efforts, starting in the 1920s, and with the advantage that beavers still lived in neighbouring Norway at the time. Other countries followed through the 20th century - although in a few cases, that was due to expanding populations wandering across a border of their own accord. Particularly notable examples include the Netherlands in 1988, Slovakia in 1995, and, more recently, a small area of northern Spain in 2003.

Since the turn of the 21st century, small numbers of beavers have been brought across to England, and allowed to live in secure patches of land not generally open to the public. These enclosures, which are located in Kent, Gloucestershire, and Lancashire, are large enough that the beavers live much as they would do in the wild, but they are still enclosed and beavers cannot truly be considered to live wild in England.

Scotland, however, is a different matter. A family of beavers were spotted living wild along the River Tay in 2001, although they must have escaped from somewhere else, since nobody would admit to putting them there deliberately. Nonetheless, a formal reintroduction took place at Knapdale Forest in 2009. The starting population was twelve, although they have started breeding successfully, and more beavers have been imported from Germany since. Taken together, these represent the only truly wild beavers in Britain.

But, hey, it's greater than zero, and there are plans to expand the populations in both England and Scotland - and possibly Wales - in the near future.

So long as they are not being hunted to extinction, beavers seem to be fairly tolerant of human presence. For example, following their reintroduction to Austria after a century-long absence in 1970, a number of beavers now live along the densely settled Danube River, and are apparently a regular sight where the river passes through Vienna. In fact, beavers will inhabit most temperate rivers or other substantial waterways, avoiding only those that are particularly steep and fast-running, although, given the choice, they will colonise unspoiled areas first.

One thing they do need, of course, is food. Beavers are strongly herbivorous, feeding on mainly leaves, water plants, and tree bark. Willow seems to be a particular favourite, followed by aspen and birch, and beavers inhabit areas where these trees are common more frequently than other types of woodland. On the other hand, willow is rarely the primary tree in any given forest so that, in practice, the bulk of their diet often consists of something else, and there's evidence that they do like a bit of variety, perhaps to ensure a good mix of nutrients.

Beavers live in family groups, led by a mated pair, accompanied by a few young adults as well as their most recent young. Although they reach adult size at around a year, they typically don't leave home to find a mate of their own until the age of two years, although it can be earlier under the right circumstances. As is well known, the first thing they do on finding a new territory is build a lodge in which to live.

In this respect, Eurasian and American beavers are very similar, cutting down trees by gnawing away at their trunks, and then darting out of the way once they hear it starting to fall. These form the foundation of the dam, which is filled in with smaller branches and constructed so as to form a number of internal chambers, accessed from below the water line. In fact, dams aren't essential, since if the water is deep enough to start with, there isn't really any point in building one. Many beavers don't even build a lodge, instead using a burrow, perhaps with the underwater entrance propped up by tree branches, and with the air-filled chambers lined with bark, but otherwise just dug into the ground.

On average, a beaver family occupies a stretch of river that's about a kilometre (0.6 miles) long, although this does vary, and spend most of their lives within 30 metres (100 feet) of the bank, with occasional trips further afield. They mark the edge of their territory with large piles of scent-marked mud, and do so more frequently the more potential neighbours they need to ward off. Both sexes scent mark, although the males do so more frequently, especially at times of the year when the female is too busy suckling her young to have time for such things.

The scent used to mark the mounds comes from two sources. Firstly, beavers do have two anal scent glands, much as many other mammals do, which produce the usual mixture of odiferous substances. But they also possess two sacs around the anal and urinary openings, which can store scented urine, but which are not themselves glandular. This specially stored urine is referred to as "castoreum", and has been used (among other things) as an ingredient in perfume since ancient times - it's this, as much as the obvious value of their fur, that led to the massive overhunting that almost wiped out the Eurasian species in the 19th century. The castoreum clearly carries some sort of individual signal, since beavers are able to tell whether it (although, oddly, not the glandular anal secretions) comes from a member of the local population or not, as well as probably gaining some information on the physical fitness of its originator.

Given their large size, it's not surprising that beavers breed relatively slowly by the standards of rodents. They give birth to just one litter a year, in the spring, following a pregnancy that lasts around 110 days, and don't become sexually mature until the age of two. They can also live for well over a decade in the wild, a feat that's quite impressive as rodents go.

The beaver family as a whole contains just two living species - the other one being, of course, the North American sort (Castor canadensis). The two species have radically different numbers of chromosomes, making cross-breeding impossible, but in other respects, it's quite difficult to tell them apart, the few physical differences being quite minor. They were first recognised as distinct in 1820, but this wasn't widely accepted until well into the 20th century. As a result, when beavers were first re-introduced into Finland in the 1930s, rather than use the Norwegian population as a source (as had happened in Sweden), it was decided to use the more numerous Canadian population.

While similar mistakes in France and other countries just resulted in the interlopers dying out, the Finnish population survived, and, today, the beavers of Finland all belong to the North American species, and not the original native one. In present-day Russia, both species are found, sometimes quite close to one another.

While there a number of fossil species of beaver and beaver-like animal that once inhabited the world, today, only the two species remain, and they have no close living relatives. Probably the closest relatives they do have are the gophers and kangaroo rats, which, together with the beavers, form a side-branch to the rodent lineage that eventually led to the mice and their relatives.

With this, the largest and locally rarest example, I have reached the end of the rodent species living wild in Britain. But there are other small mammals also found on the islands, and, next time, I'll begin with one that, as my most popular current blog post discusses, is often mistakenly identified as a rodent itself...

[Photo by Bohuš Číčel, from Wikimedia Commons.]

No comments:

Post a Comment