Weasels on the Riverbank: Mink

European mink

While most are still terrestrial, the long sinuous bodies of weasels are easily adapted to an aquatic lifestyle. Within the lineage that led to the modern weasels and their relatives, semi-aquatic habits have evolved at least three times. The first of those lines led to the otters, but the most recent led instead to the European mink (Mustela lutreola), a much smaller animal that is clearly not an otter, and is, in fact, most closely related to the ferrets and polecats.

The European mink was once found throughout central and eastern Europe, from Germany in the west to European Russia in the east. It is almost as aquatic as an otter, and is never found far from fresh water, preferring dense vegetation along the banks of fast-flowing streams and small rivers. Its feet are partially webbed, and it is a good swimmer in comparison to most other members of its family, although not as skilful as the otters. Mink den in natural hollows, such as those beneath tree roots, and will also take over the burrows of their favourite food, water voles.

Even so, while they are undeniably well adapted to a semi-aquatic lifestyle, they are no match for otters. For example, mink do not see well underwater, and only dive after fish, crayfish, and so on, after they have spotted them from above the surface. Given that otters and mink both inhabit the same parts of Europe, why aren't the latter simply out-competed? It turns out that it's their very lack of adaptation that helps them survive.


Otters are superbly well adapted to the riverine life, and if both animals compete only to catch fish, the otter is going to win hands down. Which is good news for the otter when there are lots of fish about. But when there aren't, for whatever reason, mink can easily switch to catching prey on land, eating voles, small lizards, and birds. True, they rarely travel more than a hundred metres or so from the riverbank, but that's generally further than otters are happy going, and the mink can readily live on the boundary, taking food from both water and dry land, allowing them to live alongside both the otters and their more terrestrial relatives, such as polecats.

Like most members of the weasel family, mink are solitary animals, with each individual staking out a claim to a stretch of river anything from a few hundred metres long to a mile or more. Although there is some overlap between the territories of different sexes, they meet up mainly to mate, when the male may travel long distances in an effort to mate with as many females as he can find. Indeed, this seems to be the main way that the male shows his physical prowess. Females don't seem to care who they mate with, and will happily do so as many times during the breeding season as they can, so, to start with, everyone gets a look in. But the males' journeys are long and exhausting, so that, as the season draws to a close, only the most physically fit are still at it - and it's whoever the female mates with last that gets to father her young.

Females also seem able to control exactly when they give birth. It takes around five weeks for the young to develop, but the female is able to delay the start of that development for up to another five weeks, keeping the embryonic ball of cells inactive in her womb until she senses that it would be a good time to start the pregnancy proper. As a result, pregnancy can last anything up to ten weeks, before a litter of about four to six young are born in April or May. Like many mammalian carnivores, the young are born blind, and, in mink, they don't open their eyes for four or five weeks. They leave home in the Autumn, establish their own territories along the riverbank, and are ready to reproduce by the time the next mating season rolls around in February.

When European colonists first arrived in the North America, they encountered an animal that looked remarkably similar to the one they were already familiar with, and, unsurprisingly, they also called the new animal a "mink". Indeed, so similar were the two species that, for a long time, it was assumed that they were close relatives, perhaps only diverging from one another when the Bering Straits formed at the end of the last Ice Age. However, modern genetic studies have shown that that this isn't the case.

It turns out that the American mink (Neovison vison) represents a third semi-aquatic lineage within the family, diverging from the other weasels long after the otters did, but also long before the European mink. Indeed, the two species last had a common ancestor, not a few thousand years ago, but five million, long before even the first Ice Age. That they look so similar is due, not to a close relationship, but to parallel evolution, where both animals have adopted essentially the same lifestyle.

The American mink is slightly larger than its European namesake, and the white markings on the chin usually (although not always) do not extend to the upper lip as they do in the other species; other than that, they really are quite hard to tell apart. The American species does seem the more adaptable of the two, and it is more ready to live on lake-shores or in marshland, as well as along rivers. This has allowed it to spread across the North American continent, from Florida and the Mississippi delta as far as central Canada and Alaska; aside from some islands and the high Arctic, about the only place it isn't found is in the southern deserts.

Like the European species, American mink often take over the burrows of their favourite prey animal - in this case, the muskrat - but they are also much more willing to dig burrows of their own. Females, in particular, can dig tunnels up to three metres long, with nests at the end in which to raise their young. In many other respects, the two species are near-identical. They have similar habits and swimming abilities, eat more or less the same kinds of food, and have similar breeding patterns and lifespans. However, there is another factor that has led to a dramatic difference between the fates of the two species.

Like many other members of the weasel family, the European mink had long been hunted for its fur. The American mink, however, turned out to have even more luxuriant and valuable fur, which made it a favourite target for trappers - it is this animal that is currently used to make mink coats. As it turned out, the American mink was widespread enough and common enough that this was never a threat to its continued survival, and, today, only one of the fifteen subspecies - that in southern Florida - is considered endangered. Indeed, wild trapping began to decline in the nineteenth century, as, from 1866 onwards, fur farms were established to raise large numbers of the animals in captivity.

Although never as domesticated as the ferret, farmed American mink have nonetheless been bred to develop fur in a range of different colours, from pure white, through tan, brown, and grey-blue, to near-black. So successful were these fur farms that new ones were established outside America, and, starting in the early twentieth century, farmed mink were introduced to Europe, and, much more recently, also to China and Japan. The problem was, the mink proved pretty difficult to keep penned up, and, from at least the 1920s, they began to escape into the wild.

American mink

Being adaptable animals, the newly escaped mink soon found new habitats to colonise, and they were happy to feed off prey such as water voles instead of muskrats. In places such as Britain, that had never had European mink, this wasn't much of a problem, and American mink are now found across Britain, Ireland, Scandinavia, parts of Italy, and even Iceland. But, where the European mink already existed, it was not the same story.

European mink were able to survive alongside otters because of their more generalist lifestyle, but against the very similar American mink, it was another matter entirely. Larger, and perhaps slightly more adaptable, the American mink began to out-compete the natives, and they went extinct across much of their range - the mink now found in places such as Germany and Poland are all the American species, not the native one. The majority of European mink alive today are found in Russia, and even there, they occupy only a few fragmented areas, and their population seems to be declining. There are also a few in Latvia, Estonia, Belarus, and around the Danube delta in Romania. From the 1950s, a few even fled westward, and colonised parts of France and Spain, where they had previously been unknown, but even these populations have not thrived.

The European mink was already in trouble before its American namesake arrived, largely due to destruction of its native habitat by increased agriculture, and, in more recent times, river pollution has certainly not helped. The species is now considered critically endangered, with less than 25,000 left in the wild, and those numbers declining rapidly. There have been attempts at conservation, including some introductions to isolated islands, and even implanting embryos into polecat/mink hybrids, but, so far, the animal remains in danger of extinction. Still, it could have been even worse.

The sea mink (Neovison macrodon) once lived along the rocky shores of eastern North America, from Newfoundland down to Massachusetts, where it apparently fed on fish, and possibly also on shellfish and other animals. It was much larger than either of the other two species of mink, at over 60 cm (not counting the tail), closer in size to an otter than to the 35 cm of a typical American mink. It's fur was more reddish in colour than that of its neighbouring species, and that larger size made it's pelt even more valuable. We don't know exactly when fur hunters finally killed the last of the sea mink, although it was probably some time around 1860. More recent sightings, as late as the 1890s, were probably mis-identifications of American mink, which subsequently inhabited the same area.

Either way, while animals such as the black-footed ferret have come perilously close, the sea mink remains the only member of the weasel family to have gone extinct in recent historical times, and it was not even scientifically described until 1903, long after it had vanished. Conservationists may be struggling to save the European mink, but all of that has come far too late for the sea mink, an animal now entirely consigned to the history books.

[Pictures from Wikimedia Commons. Cladogram adapted from Koepfli et al. 2008.]