2012-07-07 / Columnists

Nature Notes

Soft, Sleek and Luxurious: American Minks Find Home on Mackinac
By Patricia Martin

It seems appropriate since this is Forth of July week to write about an American mammal, to be specific, the American mink (Mustela vison or Neovison vison). American mink are not uncommon on the Island and it seems that in the last few years more and more of them have been spotted. Recently I have had two reports of Mackinac minks. The first came earlier in the summer. Several members of a Road Scholars class that I teach reported that they saw a weasel-like mammal cross the road near Grand Hotel and someone from the hotel told them that it was a ferret. Odds are it was one of our minks. The second report came from a friend who said that he had seen an American mink swimming and diving near his boat.

For those of you not familiar with the American mink it is in the weasel family (Mustelidae). Most, but not all of that family, have long, thin bodies and short legs. They have five toes with non-retractile claws and have well developed annal sent glands. Skunks are in this family.

American Mink American Mink Adult American minks are in total about 19 to 27 inches long with a long tapering bushy tail about 5.5 to 8.7 inches long making up about a third of the total length. The males are larger than the female weighting in, in the winter (their weight fluctuates throughout the year), at between one and three pounds the females one to two. Minks are heaviest in the fall. It is similar in size to the marten but with smaller ears and feet, less bushy tail and a less prominent throat patch. The fur of the American mink is soft, sleek, and thick and more luxurious that the European mink. It generally appears to be dark brown throughout the year though it can range from dark blackish tawny to light tawny. They often have whitish blotches on the chin, throat and/or belly. The longer guard hairs, which help shloush off water, are generally dark approaching black. Generally the tail is usually darker than the trunk. The summer fur is shorter, more sparse and duller than the winter fur. They molt in spring and fall.

These semi-aquatic mammals usually lives near open water such as a stream, river pond, or one of the Great Lakes, though they will wonder inland for up to a half mile. The home range of the female mink is usually about 20 to 50 acres, but the male’s is larger. They mark their territory with the musky anal secretions and guard it from others of their same sex, although male and female territory may overlap.

The American mink are well adapted to water, having webbed feet and streamlined long body, which undulates when swimming or diving. While searching for food, this weasel can cruise underwater for up to 100 feet and dive up to 15 feet (some sources say nine feet). They are quite efficient in catching fish. Their usual chase is only five to 20 seconds long. When they dive, their heart shows rapid bradycardia, which probably helps them conserve oxygen. In warm water (75 degrees) they can swim for three hours, but in cold water, much shorter times. Besides swimming and diving they also climb trees and can bound across the land at a rate of about four miles per hour.

American mink are most active at night, especially at dawn or dusk, but you may see them at other times of the day. They are carnivores, but do not eat exclusively fish (minnows and sunfish principally), though it does make up part of the diet. They are active year around but their summer and winter diets vary. In the summer they love crayfish, but they also eat frogs and small mammals like shrews, rabbits, jumping mice, and muskrat. They also eat a variety of water fowl. In the winter their primary food is mammals, particularly muskrat, which they follow into the muskrat den or catch them in the water or on land.

These weasels are solitary in general. They generally build or adapt a den or underground burrow that had an entrance near the water. They always have several entrances and exits. The burrow is up to 12 feet long and a foot to three feet below the ground and contains a nesting chamber about a foot in diameter that is lined with dried grass and leaves. In late winter the female mink mates with one or more males. Like many of the weasel family, they have what is called delayed implantation, which means that the fertilized eggs does not implant in the female until the day length is correct so that the young will be born in April or May. The gestation period is 40 to 75 days, which includes 10 to 45 days that may be the developmental delay. The length of the delay will decrease as the mating season progresses. Eventually four to nine kits will be born. The kits will make their first kill when they are in their sixth to seventh week and leave the den at 12 weeks. They will be both physically and sexually mature at 10 months old and the female will remain fertile for seven or more years.

Because of their love of fish and crustaceans, the mink have had problems, like their cousin the River Otter, due to accumulations of mercury, phychlorinated biphenyls, and pesticides which may affect their life expectancy and reproduction.

Because of the richness of the pelt of the American mink it became a popular wild animal to trap for the trade. In general they were trapped in the months between November and April when the fur was at its best. In the late 19th century mink farms were established as there was a demand for the fur and they are easy to keep and breed. By the way, farmed American mink have a much greater variety of fur color than the wild ones. They also have about a 20% smaller brain, 8% smaller heart, 28% smaller spleen, and an overall larger body. In the 1920s and 1930s, the American mink was brought to the U.K., Europe, Iceland, South America, and Russia, mainly to be farmed, but over the years they have escaped, established fera; populations which have been linked to the decrease in vole and native European mink populations.

Even in our neighbor to the north, Canada, escaped or released farm mink have been linked to the decline in native American mink populations. Many farm mink do not survive in the wild but their presence puts a strain on the wild populations and food supply. Those that do survive may add their weaker genes to the wild populations, as well, so it is a concern.

Trish Martin is a year-around resident of Mackinac Island, has earned a master’s degree in botany from Central Michigan University, and owns Bogan Lane Inn.

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