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Thursday, June 28, 2012

The Slinky Mink

This is the second in a series of articles on the ten members of the weasel family that are found in Michigan.  We have covered the three smallest members of mustelids, now we will look at the two aquatic weasels:  this issue the mink, and next issue, the otter.
Some predators are highly specialized, honed by evolution to efficiently hunt certain prey in distinct habitats and situations.  And then there’s the mink (Mustela vison).  The sleek, dark- bodied weasel is about as versatile as predators come – taking a wide variety of prey on land and water, day or night.  If a mink played baseball, it would be the utility player who could step in at almost every position. 

Mink are found throughout North America except in the extreme northernmost reaches of Canada and the arid southwestern U.S.  Much larger than the short-tailed and long-tailed weasels (see the March - April 2012 issue of The Wildlife Volunteer), adult males reach 28 inches in length and can weigh up to 3.5 pounds.  Females are smaller, but are still big enough to prey on muskrats, rabbits, small woodchucks, chickens, a host of smaller animals, and birds’ eggs.

A mink’s foot has five toes that are slightly webbed and with semi-retractile claws.  That combination lets the animals swim well and keep its claws sharp enough to grab fish and other slippery prey.
Mink can dive 15 feet and swim fast enough to catch muskrats underwater as well as in muskrat houses and burrows.  They stalk lakeshores, river banks, and wetlands, matching hunting times to prey availability.  This past winter, I watched a mink follow a lakeshore, then walk the edge of open water on ice in broad daylight far away from cover.  Yet, mink also frequently hunt at night, slinking in and out of thick brush, cattail stands, log jams, or rock piles.
Mink kill the same way as the smaller weasels with bites to the head or neck.  The prey is either eaten on the spot or carried to a den, often a former muskrat or beaver burrow, or a hollow log.
Like all other weasels, mink will fight viciously with each other or to defend themselves against foxes, bobcats, coyotes, owls, and hawks.  Mink exhibit delayed implantation with actual pregnancies occurring 9 to 45 days after mating.  Young are born blind and fur-less, usually in spring.  The number per litter is highly variable; just one or two when food is scarce, and as many as ten when food is abundant and other environmental conditions favorable.  Of course, many predators take young mink, especially when they are still in nests.
Mink are trapped in Michigan, but because of a long-term decline in the fur industry, most of the fabled mink stoles and coats now come from animals raised on ranches.  There they breed mink to yield some fur colors much lighter than that of wild mink.
Mink can be secretive, but at times seem almost oblivious to the presence of a human.  I’ve gotten within a few feet of mink several times.  A few of the animals seemed to see me, but did not run or hide.
Many long-time fishermen and duck hunters have stories about close encounters with mink bent on stealing fish, raiding the bait bucket, or grabbing a duck left aside during a hunt.  And as with most other weasels, farmers tell of mink raids on chicken pens far from water.
Occasionally, mink draw the wrath of landowners by stuffing drain pipes full of excess prey.  Dr. James Harding, a biologist at Michigan State University, remembers having his sump pump pipe plugged with dead frogs stored there by a mink.
Currently, YouTube has video of mink attacking prey including a young swan much too large for the mink to kill, trying to steal trout off a fisherman’s stringer, and other antics in full view of one or more humans.  The last couple of years, a mink family has been a minor tourist attraction on a Lake Michigan breakwater in the City of Ludington in Mason County.  In spring, the young mink dart in and out of the large stones just a few feet from walkers, bringing to mind the song, “Pop Goes The Weasel.”
Dr. Patrick Rusz
Director of Wildlife Programs – Mink Stealing Rainbow Trout (Good Close Action) – Mink Attack on young swan

Thursday, June 21, 2012

Hey, You Little Weasel!

This is the first in a series of articles on some of the ten members of the weasel family that are found in Michigan.
An adult least weasel (Mustela nivalis) weighs just 1-2 ounces and can fit its supple 6-8 inch-long body into a coffee cup.  It doesn’t spend much time in water.  A river otter (Lutia canadensis) can reach nearly 5 feet in length and weigh 30 pounds, and is a terrific swimmer, easily catching fish and other prey underwater.  The two animals might seem much different, but both are members of the weasel (mustelid) family and have many common traits.  And like several other close relatives, they are poorly understood.
Michigan is home to ten members (see sidebar) of the weasel family.  As a group, mustelids are known for their fierceness, big appetites, and several unique adaptations.  Mustelids have characteristic anal glands used both for defense and for marking territory.  And many members of the weasel family can postpone pregnancies until long after mating.  In the larger weasels, like wolverines, this “delayed implantation” can postpone births for years.  This can help reproduction when harsh environments are often unsuitable for raising young.
The least weasel is found throughout Michigan but is not common anywhere.  Even experienced naturalists seldom see least weasels.  The animals turn white in winter in the Upper Peninsula and northern Lower Peninsula, but some individuals stay brown in winter in the southern part of the state.  The least weasel is active both day and night and in all seasons, searching almost constantly for prey over home ranges of just an acre or two.  It may have several dens it regularly visits, and will also temporarily use abandoned burrows of other small animals.
The least weasel’s primary prey is meadow voles, mouse-like mammals which moves in distinct runways in grassy areas.  However, like other weasels, it will eat anything it can kill.  Mice, birds, eggs, and insects are all eaten regularly, and excess food (including prey parts) are often stored (cached) in the dens.  The least weasel’s appetite is legendary, as it consumes almost half of its own body weight daily.
Two larger “cousins” of the least weasel are also found in our state.  Michigan is near the southern edge of the range of the short-tailed weasel (Mustela erminea), also called the ermine or stout.  Adults weigh 2 to 6 ounces (males are twice the size of females) and are 7 to 13 inches long.  Michigan is near the northern edge of the long-tailed weasel’s (Mustela frenata) range.  That species tips the scales at 3 to 9 ounces and is 11 to 22 inches in length.  The three species can usually be distinguished fairly easily if captured as the short-tailed weasel has white feet year-round.  The least weasel is not only smaller than the other two, but has an even shorter tail that lacks a black tip.  All three species can be confused if merely glimpsed while moving in and out of cover.
Of course, the short-tailed and long-tailed weasels take much larger prey than does the least weasel, and they generate far more tales of hunting and fighting prowess.  They occasionally kill prey much larger than themselves such as young rabbits, rats, and snakes.  The long-tailed weasel will also take small woodchucks and even baby pigs, and has a reputation as a chicken killer.  If a weasel can get on the back of an animal, wrap its legs around it, and make a bite at the base of the prey’s skull deep enough to reach the brain or spinal cord, it can and will kill it.
“Surplus killing” of more animals than it can eat is common among all of the weasels.  This behavior is an adaptation crucial to survival in winter and other times when prey can become scarce.  Weasels have such high energy demands that fasting for more than a day or so is detrimental.  A weasel’s slim body lets it follow prey through burrows and other small openings, but the high surface-to-volume ratio works against it in winter.  So, weasels can’t simply curl-up to stay warm and wait-out cold weather (like raccoons, for example).  They must have stock-piled food or find prey quickly.  (If humans had the same per pound energy requirements as weasels, a person would have to eat more than 50 pounds of food a day.)  Not surprisingly, starvation in winter is a major cause of weasel mortality.
Weasels are killed by foxes, coyotes, cats, hawks, owls and many other predators.  Camouflage and hiding are the most important defenses, but individual weasels are sometimes able to put up amazing fights when attacked.  A naturalist in Europe reported seeing a hawk fly off with a weasel only to come crashing lifeless to the ground after the still-alive weasel worked its way free enough to deliver a mid-air killing bite to the hawk.  There are many reports of weasels using their quickness to fight off dogs; however, cats can usually kill small weasels without too much trouble. 
The delayed implantation exhibited by short-tailed and long-tailed weasels and some other mustelids is of much interest to biologists.  After mating, the resultant fertilized eggs don’t implant themselves in the uterus as occurs in most other mammals.  The eggs stay free but are somehow protected until biochemical clues, poorly understood by scientists, trigger implantation.  Then, the “true pregnancy” is fairly short (less than a month in the smaller weasels).  Many scientists are puzzled because it would seem that animals that eat so often and don’t live very long would get no advantage from delayed implantation.  Indeed, delayed implantation has been gradually lost in the least weasel and certain other mustelids as they evolved.  Least weasel reproduction is more reactionary.  They will have two or three litters of (usually four or five) young a year if prey is abundant (but just one if prey is scarce.)  In the weasels in which the adaptation persists, it is likely that delayed implantation allows the females to be ready to breed at all times.  If males are not always available that helps optimize timing of births.
Also poorly understood are the unique vocalizations of weasels.  These include shrill shrieks made not only when the animals feel threatened, but also sometimes when they spot prey.  In the latter situation, it might seem more advantageous to launch a silent attack.  But apparently, a shrill shriek may cause prey to “freeze in position” and might make them easier to catch.
Dr. Patrick Rusz
Director of Wildlife Programs

Thursday, June 14, 2012

Swimming on the Reef

The Michigan Wildlife Conservancy completed its first sturgeon spawning reef in the Detroit River at Fighting Island, located in Canadian waters. The Conservancy was the only private organization involved in the reef; we worked with numerous US Federal, and Canadian agencies. The reef was completed in 2009 and is now a spawning site for Great Lakes sturgeon, whitefish, walleye, northern madtom (endangered) and many other species.

In June of 2012 the Conservancy completed a larger reef on the American side, in the delta of the St. Clair River, by partnering with numerous Federal agencies. The reef was still being finished when divers from the U.S. Geological Survey went down to document the specifications of the new reef with video-cameras. This short video shows the many large sturgeon that were already spawning on the not-yet-complete reef that the divers encountered. It is rare to have such a significant, and immediate, impact on a "threatened" species. Enjoy!

To view the sturgeon video please go to and click on the “Sturgeon Reef Video” under “Community Events” or click here

Thursday, June 7, 2012

Monarchs Take a Hit

Many readers of The Wildlife Volunteer have expressed interest in the long term status of the Monarch butterfly.  In recent years this beautiful insect has been declining in numbers throughout Eastern North America and has become a concern of many wildlife conservationists.  Because Michigan is close to the northern terminus of the butterfly’s breeding grounds all our citizens should take a special interest in this animal.

The numbers are in for flies on their wintering habitat of oyamel fir trees in Mexico.  Scientists census the Monarch by measuring the amount of land the population occupies during the critical wintering period.  This is made possible by the animals themselves, because they “pack” as close as is necessary to keep warm during winter nights.  Measuring the acreage they occupy provides a relative population estimate, or index.  The winter population census began in 1994 and the long term average acreage of monarchs is 17.27 acres (6.99 hectares).   There are millions of individuals in that packed area so absolute numbers are impossible to determine.

The measured area the Monarch occupied in the winter of 2011-2012 was 7.14 acres (2.89 hectares), a decline of 28% from last year’s numbers, and 59% decline from the eighteen year average.  The sad news for Michiganders is – expect a 30% decline in butterfly numbers this summer.

It would be easy to blame the vagaries of climate for the Monarch’s decline - Exhibit A – 86o in Lansing on March 21.  But there appears to be so much more going on here.  Researchers in Texas and Kansas point to an inverse correlation between the number of acres planted to herbicide tolerant row crops on the Monarch’s flight path and the fly’s numbers.  Crops are now being bred to tolerate spraying of Round-up and other herbicides, so that fields can be sprayed to kill weeds after the crop is growing.  The later spraying is very effective to control weeds, even milkweed, in and around crop fields.  Without milkweed there will be no Monarch butterfly, the animal can’t exist without it to lay eggs on.

Herbicide tolerant corn and soybeans were introduced in 1996, the year Monarch numbers peaked at 51.82 acres in Mexico.  The butterflies have been declining as more herbicide tolerant row crops have been planted.  By 2004 genetically modified corn and soybeans accounted for 51% of planted acreage.  That figure reached 81% of all planted row crop acreage by 2010.  This trend seems very ominous for the Monarch, unless management strategies are adopted to accommodate the insect in the agricultural landscape.

One possible step forward is to encourage additional research on alternatives to widespread herbicide use.  Organic farmers are experimenting with sandblasting of corn fields early in the growing cycle.  Sandblasting takes ground-up corn cobs (available at most farms) and sprays them under pressure at the undesirable weeds.  Early research found that weeds can be controlled in corn fields if multiple treatments are done at the one-leaf, 3-leaf, and five leaf stage of the corn plant.

Other sandblasting materials are being considered such as nut shells and even fertilizer, or crushed limestone.  These alternatives may help with soil fertility or structure, which are important in agriculture.

If sandblasting weeds replaced even some herbicide spraying in agricultural crops on the Monarch’s migration route it may be very beneficial for the butterflies.