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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


  1. Just saw what looks like a least weasel here in Grand Haven. It had killed a vole and was dragging it under our deck. I couldn't believe it. He/she looked right at me and proceeded with its task.It seemed almost smaller than its prey.

  2. I just saw a least weasel here in Grand Haven. It had killed a vole and was dragging it under our wooden deck. It looked right at me and did not seem afraid. It was amazing. I saw its little eyes and white underbelly. The vole was bigger than the weasel.