American mink (Mustela vison).
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There are two living species of "mink," the American Mink and the European Mink. The extinct Sea Mink is related to the American Mink, but is much larger. All three species of mink are dark-colored, semi-aquatic, carnivorous mammals of the family Mustelidae, which also includes the weasels and the otters. The American Mink is larger, and more adaptable than the European Mink. It is sometimes possible to distinguish the European and American species because the American Mink usually has a large white patch on its upper lip, while the European Mink always has one. Any mink without such a patch can be identified with certainty as an American Mink, but an individual with such a patch, if encountered in continental Europe, cannot be certainly identified without looking at the skeleton.
The American Mink's fur has been highly prized for its use in clothing, with hunting giving way to farming. Its treatment has also been a focus of animal welfare activism. American Mink have found their way into the wild in Europe (including Great Britain) and South America, after being released from mink farms by animal rights activists or otherwise escaped from captivity. They are believed by some to have contributed to the decline of the less hardy European Mink through competition (though not through hybridization -- native European mink are in fact closer kin to European Polecats than to their North American cousins). Trapping is used to control or eliminate feral American Mink populations.
Mink oil is used in some medical products and cosmetics, as well as to treat, preserve and waterproof leather.
If something like a large weasel or small otter is seen, near a lake or a river, or on the sea shore, it may well be a mink. Unlike the otter, which is only active at night when there is no danger of human disturbance, the mink is about at all hours, even when people are in evidence.
It is difficult to estimate the number of mink in Britain today. A mink needs several miles of waterside to make its home and, considering the thousands of miles of waterways and courses throughout Britain, there must be thousands of mink in Britain.
Minks are very territorial animals. A male mink will not tolerate another male within its territory, but appears to be less aggressive towards females. Generally, the territories of both male and female animals are separate, but a female's territory may sometimes overlap with that of a male. Very occasionally it may be totally within a male's.
The territories, which tend to be long and narrow, stretch along river banks, or round the edges of lakes or marshes. Sizes vary, but they can be several miles long. Female territories are smaller than those of the male.
Each territory has one or two central areas (core areas) where the mink spends most of its time. The core area is usually associated with a good food supply, such as a pool rich in fish, or a good rabbit warren. The mink may stay in its core area, which can be quite small, for several days at a time, but it also makes excursions to the ends of its territory. These excursions seem to be associated with the defence of the territory against intruders. It is likely that the mink checks for any signs of a strange mink and leaves droppings (scats) redolent of its personal scent to reinforce its territorial rights.