Winter White

March 2, 2018

Going out to the woodpile this time of year, we are always on the lookout for a short-tailed weasel in its winter white. Taking advantage of a place filled with crevices that may house mice and voles, a weasel can sometimes find a whole grocery store full of food in a woodpile!

Short-tailed weasels are members of the weasel family Mustelidae, which includes the Long-tailed and Least weasels, Mink, Fisher, Pine Martin and Wolverines.  The short and Long-tailed weasels in NH look very similar except for size, and the Least weasel is the smallest of the three. All three go through a fall molt in which they lose their normally brown coats and grow a white winter version, even in a year without snow.  Triggered by the shorter day lengths their molt happens whether or not there is snow on the ground; in the spring the longer days will trigger the return to their summer browns.

I am always amazed at the speed and agility of weasels; the long narrow body and short legs are perfect adaptations for their hunting lifestyle.  They are built for exploring stone walls and downed logs, other mammal tunnels and burrows and brushy habitats, for the small mammals that make up most of their diet.  Though they eat mostly mice, voles, chipmunks and shrews, they will also eat animals larger than themselves including birds if given a safe opportunity. During summer months their diet may also include invertebrate’s, reptiles and amphibians, though small mammals still make up the largest part of the diet.

Unfortunately weasels often get the reputation of being vicious and wasteful in their hunting.  I’ve heard them accused of killing for “fun”, and there are myths about weasels which include “blood sucking”.   In fact they are just aggressive towards their prey, fast and efficient. Killing more than they need stems from the strategy of taking prey when it is available and caching what they don’t eat for later.    Most of us have a few days worth of food stored in our kitchens; why not the weasels?

Weasels are active all winter, above and below ground or under snow cover.  Their long slim bodies with large surface area are great for moving in and out of tight spaces but not efficient for retaining heat.  When not actively hunting they return to a den site or tunnel, often one that was dug by some other mammal, to rest and stay warm. Using the leftover feathers and fur from their prey provides added insulation.

Weasels are not social, coming together only during the breeding season for mating.  Females give birth in March or April having bred the previous spring. All the members of the weasel family have a reproductive strategy that includes delayed implantation; after mating the fertilized eggs become dormant in the uterus, until the following spring.  Once the eggs implant in the uterus, development happens over the course of a month. The young are born with eyes closed and a fine layer of white fur. The female cares for the young on her own for 6-7 weeks until they are old enough to make excursions outside the den.

Short-tailed weasels have large territories considering their size; a male’s territory may be as large as 100 acres, females less.  Territories of same sex weasels rarely overlap but males and females may share parts of the same. The size of a Short-tailed weasels’ territory,  as with many animals, depends on habitat and food availability.

Though the Short-tailed weasel is an efficient hunter, it is also an animal that must be constantly on the lookout for predators.  Larger hawks, owls, fox, and coyote will take advantage of a weasel meal if they have the opportunity. The white coat in winter (when we have snow) is an advantage as they are hunting or being hunted. The black tipped tail is thought to be an adaptation to distract predators to the wrong end of the body and allow escape more easily. For the Least weasel with such a short tail this trick wouldn’t work quite as well; scientists believe that may be the reason why this weasel lacks the black tip.

Keep your eyes open for weasels the remainder of this winter while they are in their winter pelage, and after a fresh snow check the fields and forests for their tracks. Their bounding prints in pairs are easy to distinguish by the chaotic and exploratory looking pattern.  The track pattern makes it look like the weasel was just having fun romping in the snow!

A good reference book for the weasel family and other North American mammals is: Mammals of the Eastern United States;  by John Whitaker and William Hamilton. ISBN 98-11962

Not a field guide but a good reference for reading more about our wild mammal neighbors!