Natural History Series - Mink (Mustela vison)

Introduction

The mink is one of the most widespread carnivores of North America. They thrive throughout the continent except for the far north of Canada and the arid southwestern United States. Mink are valuable furbearers that are also aggressive, effective predators of muskrats and waterfowl, which can be an economic loss to trappers and hunters. Males are 1.4–1.8 times heavier than females. Weights of adult males may vary from 1.5–4.5 pounds, but this variation is between populations throughout its distribution in the United States rather than within a given population. Mating can occur with several males from January through March. Gestation varies from 39-76 days due to delayed implantation. Implantation in the uterus of the female is probably triggered by the lengthening daylight usually in March. Once development of the embryo begins, two to six kits, sometimes as many as 10, are born in 27-33 days in April or May. Young mink can fend for themselves in two months, and at five months of age they are as large as adults. Sexual maturity is reached in 10 months, and though some females might stay with their mother longer, most leave the den to establish their own territory in autumn.

September 11
The pair of young female mink was three miles upstream from the safety of their mother’s den and care. There had been seven kits in the family, four brothers and three sisters. Three of their brothers struck out on their own several days ago. She, and one of her sisters, left the den to find their own territory shortly after her brothers departed. The males went downstream, but she and her sister went upstream.
The two females had moved rapidly upstream during the first day. They lingered only long enough to catch some crayfish, and covered three miles before curling up together in the natural obstruction of a large logjam for some much needed rest. This is the farthest they had ever been from the abandoned muskrat burrow where they were born last May.
September 12
After several hours of sleep in the cozy confines of the log jam, both females stirred. The gurgling sound of the stream weaving its way through the logjam had been comforting to their peaceful rest, but their nocturnal instincts now awakened, and they were hungry. They often followed their mother on her night hunts for crayfish, frogs, rabbits and muskrats. Mother had been a good provider and teacher of the skills they needed to survive on their own. The entire family of young mink witnessed their mother’s swiftness and aggressive attacks on prey larger than herself.
After some stretching and yawning in the makeshift den, they emerged in the cool darkness on the top of the logjam. Each predator sniffed the air for any sign of prey or threats. Detecting none, they loped across the logs and bounded down to the streambed shore. The pair of females separated and began the search for crayfish along the shoreline as they continued their slow travel upstream.
Both females heard the rustling of the leaves on the river bank above them, but the female downstream also caught the scent from the slight swirl of air currents moving in her direction. She quickly recognized it as prey and darted a little further downstream where she climbed the stream bank and perhaps might catch the prey coming in her direction. Her hunting partner hadn’t caught the scent, but observed her sister climbing the river bank cautiously. Moving quickly upstream, she also climbed the river bank. Now, whatever made the sound in the leaves was between the two predatory mink. The young mink that was downstream couldn’t see the potential meal yet, but she knew from the scent that the leaves were being rustled by a rabbit.
The rabbit was feeding on small crabapples that had fallen to the ground. Several wild crabapple trees along the river bank often attracted deer, rabbits, and muskrats throughout the fall. The seeds of the crabapples were consumed by Pine Grosbeaks which regularly visited the small stand of crabapple trees. Both female mink silently closed in on the rabbit from two sides. The rabbit saw the mink on the downstream side of her and bolted upstream with the mink in pursuit. She ran directly into the second mink who quickly grabbed the fleeing mammal by the throat. The momentum of the heavier, swiftly moving rabbit lifted the smaller female mink off her feet and dragged her along for some distance. But the mink’s four sharp canine teeth punctured the rabbit’s jugular; she let out a high pitched squeal, fell on her side, and kicked frantically to no avail. Though the rabbit was larger than both of the young female mink, she was dead within seconds after the second mink arrived to help her sister.
The two females fed voraciously on the three pound cottontail that weighed more than the combined weight of the two young mink. They squabbled and snarled at each other as they fed, but it was the competition of den mates. This would to be their last meal together.
After an hour or so of feeding, one mink continued on her way upstream. Her sister stayed with what was left of the rabbit, and eventually made the log jam they had slept in as one of her dens. She established this area as her territory and would range from one-third of a mile to almost two miles up and down the stream from the log jam. While she often used other den sites in her travels, the log jam was the center of her claimed territory which she patrolled and protected.
The departing female traveled upstream in the darkness without stopping to hunt as her belly was full of rabbit meat. She only hesitated briefly to urinate and leave some scat on the rocks of the stream, though she would have killed any prey she stumbled into, because mink are surplus killers. Even if she wasn’t hungry, if prey was present, she would have killed it. But, she didn’t encounter any prey. She did detect several interesting places where another mink had urinated regularly.
High on an old snag next to the river sat one of the denizens of the forest. She had patrolled thousands of acres of the mixed hardwoods of beech, maple, birch, and oak and abundant evergreen trees lining the stream and hillsides for the past ten years. Her only natural enemies were others of her kind and in rare instances, Goshawks. This stalwart forest resident never built a nest; but used the nests of hawks, herons, crows, squirrels, and hollow trees.
Daylight was breaking as the young mink was crossing a fallen tree to the other side of the river when she instinctively detected danger. She stopped on the log to look around and sniff the air when she saw a large bird rapidly descending towards her. The young mink dove off the log into the water below just as the outstretched talons of a Great Horned Owl barely missed capturing her. The owl had descended from its high perch in the partially denuded fall forest. The predatory bird had a wingspan of forty seven inches, but kept her wings folded when she dove off the perch to capture the mink. The descent was silent at the speed of gravity. The owl extended its wings and its talons only when it was very near the mink. Had the mink not noticed the owl, she would have been killed instantly when grasped within the powerful large talons.
Safely under water, the mink swam downstream with the current using all four paws to rapidly put distance between her and the danger overhead. The owl glided silently through the open forest on the other side of the stream, and fortunately spotted and killed a red squirrel that was hunting acorns in the deep leaves of the forest floor. She then carried her prey to a high perch where she ripped it apart to feed.
The mink surfaced under some overhanging brush of the stream to scan the sky overhead for the danger that nearly ended her life. She waited there for almost an hour before cautiously swimming upstream. The young female remained in the stream for the next mile so she could submerge to escape danger before moving back to the edge of the stream where her travel would be faster. A little further on the four month old mink found a large abandoned muskrat colony. The stream was wider and deeper here. There were numerous burrows in the bank with entrances that were under water. The remaining odor of muskrats was weak as she entered one of the burrows below the river bank. The underground tunnel led to a large cavity under the river bank which had several exits to dry land above the river and one exit to the stream. It reminded her of where she was born, and she curled up in the dry den and slept.
September 13
She awoke during the night and decided to scout out her new surroundings, and search for food. Emerging from one of the exit holes to the dry stream bank, she immediately caught the active scent of field mice. The grassy bank of the stream was also home to hundreds of voles that were feeding on the forbs, grasses, leaves, and seeds of the extensive plant life. She caught and ate several of the mice then descended the stream bank to the edge of the stream where she discovered numerous minnows congregating in the shallow water along the edge of the stream. She chased and caught several of the minnows which were quickly consumed. The abundance of food, numerous dry dens, and the safety of the wide and deep water convinced her this would be her new home. She spent the rest of the night patrolling both sides of the stream picking up a few crayfish and minnows in her travels.
March 21
Mink can breed in their first year. She was ten months old and coming into breeding season. The young mink was a little larger than many other females at 18 inches and just over two pounds due to the abundant food source. Females attain adult size faster than males, but they are smaller. Males continue to grow in their second year.
A male mink was headed downstream in search of females. He was more than two miles beyond the normal range of his territory of 3.5 miles, when he caught the scent of the female at the abandoned muskrat dens. The male was 23 inches long, approaching the range of their maximum size of 28 inches, and he weighed just over three pounds.
He and the female met on the shoreline. They cautiously approached each other. The female was in the last week of her three week breeding season and receptive to copulation. No other males had previously approached her. Females are often bred by several males, though development of the embryo can be delayed in the uterus. Fertilization to implantation, when mating occurs with several males, can last 7 to 30 days. This delay accounts for much of the variation in the duration of pregnancy. However, the delay does not occur when mated later in the season as in her case.
Mating with the male was very aggressive and lasted several hours. After which, the male continued further downstream in search of other females. The female hunted a little more then retired to her den. In her case, implantation of her fertilized eggs would occur over the next several days and she would give birth in the next 27-33 days.
April 24
Late in the afternoon the female returned to her den where she gave birth to six kits. Three males and three females were born. The tiny kits were less than half an ounce at birth and were covered with a short, silvery-white hair. It would be several weeks before their coat would be replaced with a fluffy, reddish-brown coat. The kits were born deaf and blind, and would not gain the ability to see or hear for 21-25 days.
The new mother changed her behavior regarding how much time she would spend out of her den. She now left the den only every two to three hours and hunted for no more than 40 minutes before returning to her kits. This behavior continued for the next seven weeks when her kits would then attain the ability to keep themselves warm. Though the kits were able to eat mice and crayfish in five weeks, they would not be weaned until the eighth or tenth week of their birth.
June 13
The protective mother brought her six kits out of the den for their first hunting lesson for mice in the grass above the den. Several of the faster growing females clumsily chased a few mice but lost them, though one mouse was injured enough that one of the males caught him. Mother captured six mice which she shared with her kits.
They regularly left the dens in subsequent days for hunting trips as a family, and were gradually introduced to hunting for crayfish, mice, young birds, rabbits and minnows. The kits hunting skills improved very rapidly. Over the summer the kits would explore at further distances from the safety of their den, both up and down the stream in small groups, and occasionally alone. Some of the juvenile males ventured as far as several miles from the den, but the juvenile females rarely went farther than a half a mile from their birth place. A variety of temporary dens were used by all the mink throughout the summer.
September 19
Two of the males departed their mother’s den for the last time. They struck out on their own. When they didn’t return, the mother didn’t know whether they had been injured, killed, or left to establish their own territory. But, she didn’t look for them. Over the next week, all but one female left the den to establish their own territory.
December 21
The last kit, a female, might have stayed with her mother until the next spring, which occasionally is the behavior of the remaining female mink of a litter. However, a high school age trapper had placed a coil spring trap set on a log that crossed the stream about a mile or so from her permanent den during the winter trapping season. She didn’t return. The remaining female inherited her birthplace as her permanent den.

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