White on white

They are hard to see against a backdrop of fresh snow, except for the large, dark black eyes, and the black tip at the end of their tail.

Meet Herman, the ermine, who is in the winter fur phase. Herman had just been feeding on part of a carcass left out for the overwintering birds at Sax-Zim bog, and the tip of his nose is bright red from the bloody meat. This little Short-tailed Weasel has been hanging out at the Visitor Center for several days, and shows up regularly every morning (and evening) for a snack.
It’s a cold day to be running around on the snow, and these ferocious little predators have huge appetites to keep their metabolic furnaces going. They need to consume about 30% of their body weight daily. Deer carcasses are for weasels what bird feeders are to our feathered friends — a ready source of rich nutrients in times of need.

The long, thin bodies of weasels are a great adaptation for hunting rodents in their burrows, but they are a disadvantage in the winter climate because thin cylindrical bodies lose heat much faster than more spherical-shaped ones do.

Weasels are excellent climbers, so this part of the carcass hung in a tree is no problem for Herman to get to. Time for a little fat to add to the meaty part of the meal consumed earlier.

Weasels usually retreat to a burrow, perhaps of some animal they have recently eaten, when they are not active, and they will often cache extra food there to tide them over on days when prey are scarce.

This is what Herman looked like last summer — but molting the summer hair to all white coat in the winter serves two purposes: being less conspicuous to prey they are stalking, and being less visible to predators that focus on the black tip of the tail instead of the head of the animal that is white on white against the snow.

What determines when weasels molt into their white fur? It’s thought that declining hours of daylight in the fall are the primary cue, just as it is for migratory birds to begin their preparation to fly south. Molting from brown to white fur coat takes about three weeks, but a drop in temperature can hasten the molt, and unseasonably warm fall weather can slow it. Usually the molt to the all-white ermine phase is completed just about the time of the first snowfall, so the animal is perfectly camouflaged.