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.]


  1. Jamie, do you think beavers should be classified as rodents?

    Some differences (I'm working on a more complete list):

    1. I've read that beavers have two distinct leg bones rather than fused leg bones, as rats and mice do. This sounds weird to me, but is it true?

    2. Beavers are vegans, unlike most other rodents, even if other rodents eat no meat besides insects or eggs.

    3. Beavers modify their habitat to hold water for protection and transport. They're graceful under water, with webbed feet, and clumsy on land. Not true of other rodents.

    4. Beavers don't breed like other rodents, which typically have multiple litters a year. (Might as well put cats in the rodent category for that trait alone.)

    5. Beavers are a keystone species that create habitat for hundreds of other species. No other rodent does that.

    6. Beavers in North America can live in the wild to around 24 years of age. Very long-lived for a supposed rodent. They're also quite large here - up to 60 pounds (27 kg).

    7. Beavers sometimes work together in teams to move logs and rocks and build channels. There's no chatter and there doesn't seem to be an overseer. Other rodents don't work in teams.

    8. Beavers, sometimes from different lodges, also work together to rescue endangered kits.

    9. Beavers care for their young for two years, teaching them building skills. Sometimes beaver parents accompany their two year olds to help them find a suitable lodge site. Most rodent babies are on their own within a matter of weeks.

    10. Beavers adopt orphans and others who've suffered the loss of family. There's a lot of noise about how territorial they are, but there are instances where they've been observed taking in whole families who've been displaced and immediately setting about expanding their lodge. There are reports of 30-some beavers living together.

    11. Beaver lodges harbor a number of other species. Also not a practice of most rodents. The only species I can think of who share their homes like this is the so-called wood rats (pack rats, Neotoma).

    PS: It's thought that there are now around 4 million beavers in North America.


    1. Some of those aren't correct (#1 and #4, for example) and I don't see any of the others as sufficient reason to separate beavers out as non-rodents. They're just part of natural variation in a very large order.

      Part of the problem is that beavers are related to gophers, so if beavers aren't rodents, neither are gophers, which would seem odd. Moreover, beavers are more closely related to mice than to many other animals normally considered rodents. So, if beavers are not rodents, then neither are squirrels, mole-rats, guinea pigs, dormice and... well, I could go on. Saying that dormice shouldn't be counted as rodents would seem even stranger than discounting gophers. (And, of course, the Why Rabbits Aren't Rodents post also explains what all these creatures have in common and why we consider them a single group).

  2. OK, but if rabbits were reclassified because they have two tiny incisors behind their top large incisors, seems to me there are more than enough reasons to reclassify beavers. Just the fact that they work in teams is huge.

    * It's like my filing pile. Most things have their own drawers, sections and folders, but the biggest folder is a hodgepodge of things that need to be better organized.

    1. They're not the only rodents to work in teams, though, because that's also true of mole-rats. In any event, rabbits can be considered non-rodents, because they are a separate (but related) group to the actual rodents, whereas beavers are well inside the rodent family tree. It'd be like saying your brother isn't part of your family, but your cousin is.

      Now, since there's no hard-and-fast definition of what an "order" is, one could promote the beaver-group to Order level if one really wanted to. This would split the rodents into four (or five) different orders. But that doesn't solve anything, for two reasons:
      1) These groups already have names, so why would we change them? Yes, the Myomorpha (mouse-like rodents) could be redefined as an order, but it probably still have the same name.
      2) Even if it didn't, and we're prepared to agree that e.g. dormice aren't considered rodents any more, we still need a name for the group that contains the four (or five) new orders we've just created. Sure, it's now a superorder, or whatever, but it already has a perfectly good name: Rodentia. So why change it?

  3. Please forgive me if this is coming across as argumentative! I don't mean to at all. I see you as someone who can actually answer my questions with good reasons, and I am truly grateful for your time.

    Another question:

    Castorimorpha splits in two: beavers and gophers/kangaroo rats. Why link them?

    Few rodents are aquatic, and those who are seem to be carnivorous. Few rodents have webbed feet, and those who do seem to be more related to beavers(back feet structure and size) than to gophers, etc, such as nutria and capybara.

    What other rodent is considered a keystone species besides beavers?
    What other rodent has only one litter a year?
    What other rodent works in teams?
    What other rodent dopts the displaced?

    I found a study called “The Beaver’s Phylogenetic Lineage Illuminated by Retroposon Reads“ and want to connect with the authors if possible. Here’s an excerpt: “Many relationships within the complex evolution of rodents were resolved by retroposon presence/absence screenings based mainly on the three available genomes of mouse, guinea pig, and ground squirrel, and subsequent PCR verification of potential diagnostic loci in other rodents3,4,5. However, in these studies the phylogenetic position of one of the most iconic rodents, the beaver (Castoridae), could not be resolved. While molecular sequence support for placing beaver together with kangaroo rat within the mouse-related clade, a group comprising Myodonta (e.g., mice, blind mole-rats, jerboas), Anomaluromorpha (scaly-tailed squirrels and springhares), and Castorimorpha (beavers, pocket gophers, kangaroo rats/mice), has accumulated6,7, so far only a single retroposon marker supported for placing the beaver inside this clade5.

    Another study called “Castorid Phylogenetics: Implications for the Evolution of Swimming and Tree-Exploitation in Beavers“ and want to connect with the author, Natalia Rybczynski. “This study uses morphological and behavioral evidence from the fossil record to investigate the evolutionary history of tree-exploitation and swimming in beavers. The findings suggest that both behaviors appeared within a single castorid lineage by the beginning of the Miocene, roughly 24 million years ago.”

    PS: Beavers have a stout body size with
    legs that have separate lower leg bones, unlike rats and mice that have fused leg bones:

    Long bone partly fused to the fibula and forming the inner limb between the femur and the tarsus.

    Long bone partly fused to the tibia and forming the outer limb between the femur and the tarsus.

    Long bone partly fused with the radius and forming the inner limb between the humerus and the carpus.

    Long bone partly fused with the ulna and forming the outer limb between the humerus and the carpus.

    1. The retroposon study you quote explains why beavers and gophers/kangaroo rats are linked, and has been supported by many other studies.

      I'll note that capybara and nutria are not particularly close to beavers, since the former is a giant guinea pig, and the latter is at least more related to guinea pigs than to beavers and mice.

      Also, I thought you meant 'completely fused', hence the misunderstanding there, so my apolgies for that. But in the case of rodents, that's less significant than the tooth structure and dental formula. We see variation in the fusion of leg bones among the deer, for example, although it does at least divide them into two subfamilies, in their case.

    2. Answering your "why?" I'll tell you my interest: We need beavers here in the American west to mitigate drought and control wildfires, as well as slowing flood surges that come about because so many waterways have gone dry and any rain we do get washes quickly out to sea, periodically flooding roads and eroding the waterways even more. Last summer we heard a lot about a fairly new category of wildfires called 'complex fires', where individually ignited wildfires join. Most were lightning fired, but some were not. The fires just got so big that they joined up. It's a big problem and it looks like it will happen again this year.

      I'm working with a group of volunteers to bring beavers back home, and calling them rodents may currently be scientifically correct, but it doesn't help our cause because people consider "rodents" to be destructive and disease-ridden, like rats.

      And here's how accurate science can be: beavers weren't even considered native to California until a few years ago, because so many had been killed before and during the gold rush that almost all populations throughout the state had been extirpated before 1900. Now we have evidence that beavers are native throughout California, not just through the dating of dams that are over 1000 years old but the fact that native people throughout the state have words for beavers, even though their languages were also extinguished during that terrible time in California (1800s) when the state actually offered money for their scalps.

      Beavers cannot be relocated in California, but they can be killed - both for trapping (which is declining, thank goodness) but also with a permit issued by the Fish & Wildlife Dept. to deal with problem beavers. We're working on changing that, of course. You have some very successful examples of beaver relocation in the UK. I love Chris Jones (Cornwall Trust) and have referred many to his YouTube videos. He talks about having much more water for his livestock, which is THE main interest of ranchers here.

      The beaver group I'm working with isn't working to reclassify beavers though. It's a personal crusade. I'm a student of social marketing (marketing for the common good, not social media) and a big part of that is the language used to persuade people to take care of the environment or whatever the cause is. Heck, I'm trying to get people in our group to quit calling a group of beavers 'beaver' because it's a hunting/trapping term that does not recognize them as the important individuals they are. They still call our wildlife cams 'game cams'. I'm now calling them BeaverCams and because I do the graphics, and that's what is starting to stick. But it just goes to show how stuck people are in these outdated terms - and outdated scientific classifications.