The end third of the brown tail is black. In winter an ermine is white, tinged with yellow on the underparts and back. The tail tip remains black. An ermine molts twice a year, in spring and autumn. The molts are triggered not by temperature but by amount of light per day, increasing in spring and decreasing in fall.
Molts usually begin on the belly and spread to the sides and back, finishing with the tail.
Aside from the varying hare, the weasels are the only Pennsylvania animals to turn white in winter. A molting ermine looks mixed brown and white. Like all weasels, ermines are alert, curious and bold. When annoyed, they stamp their feet or emit musk from their anal scent glands. An ermine can leap five or six feet and run about 8 mph for short distances.