Montana's Mysterious Mink

By Jan L. Wassink

As the sun sinks toward the horizon and the shadows lengthen along the river, a dark brown form emerges from a log jam and begins exploring the shoreline.

A nearby rustle in the grass attracts the mink's attention. Balancing on its short, sturdy legs, the animal uses its long, slender body to raise its head above the grass. Another slight sound betrays the exact location of the vole and the predator launches itself. The end comes quickly. After delivering the vole to the mink's young beneath the log jam, the female mink continues to explore the shoreline for other prey. This time, she ventures into the shallow water and soon emerges with a small fish. After several hunting sessions throughout the night, she retreats to the shelter of the log jam and waits for the coming of nightfall once again. Leaving the drier uplands to the smaller, terrestrial weasels and the deeper waters to the larger and more aquatic otter, mink frequent the shallow waters and tangled habitats along the shorelines of rivers, streams, ponds and lakes throughout Montana. 

Because they spend their days hidden in dens under log jams, in old muskrat burrows or tucked into a rock crevices and are active primarily at night, mink often live unnoticed by humans. Even sharp-eyed naturalists may become aware of their presence from seeing their tracks or latrines rather than the animals themselves. An adult male mink weighs between one and three pounds and reaches approximately 28 inches in length, including an eight inch tail.

The females are noticeably smaller, reaching 22 inches in length and weighing from one to two pounds. Both sexes are a rich, dark brown in color and have soft luxurious undercoats covered with long, glossy guard hair. The dense pelage protects them from the cold water and is their only obvious aquatic adaptation. 
When actively hunting, mink venture along the shorelines, exploring almost every nook, cranny and crevice in their path. Whether this behavior is simply curiosity or an innate hunting behavior is probably irrelevant because this habit of investigating everything in their path is what inevitably leads them to their prey. Opportunistic in their hunting behavior, mink catch whatever is available. As a result, their diets change throughout the year and with the relative abundance of prey species. 
Small mammals are the most common prey of mink, with male mink able to catch and kill mammals up to the size of rabbits. The smaller females take fewer rabbits but seem to take more aquatic prey - fish, amphibians and invertebrates. Aquatic prey make up a higher percentage of the diet in winter than in summer, presumably because there are fewer alternative prey species available with the coming of snow and cold temperatures. 
During summer, mink will also take waterfowl. Like most of the other mammalian predators, mink are solitary animals. Home ranges generally stretch along shorelines and vary in size and length with the relative abundance of prey, the thickness of the cover and the sex and age of the mink. Studies in habitats similar to Montana have found between three and five mink frequenting each mile of shoreline. As they move through these home ranges, mink rarely stay in the same den two days in a row. A notable exception is females with young. Even that only lasts about eight weeks when the young begin following the female on her hunts and the entire family group moves from den to den.

The females become receptive to breeding from January through March and the overlapping home ranges between the sexes allows more opportunity for encounters between the sexes while the female is receptive. The actual breeding is often a noisy, rough-and-tumble affair that seems to involve the male eventually subduing the female. Because mink are induced ovulators, which means the females ovulate in response to the breeding process, the vigorous activity may help stimulate ovulation in the female. Females may mate with more than one male and ovulate each time so litters may contain kits from different fathers.

Following fertilization by the male, the eggs suspend development and remain dormant in the female for up to six weeks. Eventually, perhaps in response to increasing day length, the fertilized eggs attach to the uterine wall and continue their development. The young are born in May or early June and weigh around only .2 ounces at birth. Their eyes open at three weeks and they begin eating meat at 37 days. By seven weeks, they are fully capable of regulating their own body temperature. Shortly after, they begin accompanying the female on her hunting trips. By fall, they have learned to hunt and kill prey on their own and disperse to find home ranges of their own. 

Although mink frequent a variety of shoreline habitats, permanent water is the most important variable in predicting the presence of mink. The best way to insure that mink will move in or remain on your shoreline habitat is to encourage or preserve shoreline habitat and vegetation. Activities like removing trees, shrubs, log jams and emergent or shallow submerged aquatic plants will remove the cover needed by prey species sought by mink. Without prey species, mink move on to find more productive hunting grounds. With proper habitat management, you greatly increase your chances of an occasional glimpse of this intriguing predator.