all weasels molt twice a year, in spring and autumn. In mild climates the incoming fur is always about the same brown color as the old, so it is usually difficult to tell from the outside, on cased museum skins or on living animals, whether a particular individual is molting or not. But in the autumn in cold climates, the incoming fur is white, and then the patterns and timing of the molt process become easily observed.
The process of molting is really a compound of two other processes, which quite independently control the growth of the new hair and the color it will be. The growth of new hair is stimulated when the lengthening days of spring, or the shortening days of autumn, reach a certain critical number of hours of daylight, which acts like a trigger. The color of the new coat (always brown in summer, white or brown in winter) is controlled mainly by heredity.
Cold temperatures also affect a third process, the shedding of the old hair, the final stage in the replacement of the old coat. So in the arctic, where the shortening days of autumn always herald the rapid onset of a severe winter, the old coats are replaced quickly, within a few days. In mild climates, the shortening days of autumn set off the molt process just as predictably, but it is slower, spread over a month or 6 weeks. In spring, cold temperatures delay the shedding of the old fur, so that in changeable temperate climates, weasels do not lose their winter coats too early if the spring is particularly cold or late (Rust 1962). So there is a lot of variation possible in the molting patterns shown by individual animals in different places and years.
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