The Pattern and Timing of the Molt

A complete molt cycle lasts from the beginning of the active phase of the follicles to the end of the shedding of the old fur. Since these are quite separate processes, even in one follicle, the cycles of growth and shedding can overlap. The biology of weasels offers two quite different methods of working out the timing and length of the cycle, depending on the species and location.

In places where weasels reliably turn white in winter, the simplest technique is to arrange a set of skins collected all year round, ranging from full brown through all the intermediate pied stages to full white, in order on a table. Glover (1942a) was one of the first to do this, using northern longtails, and van Soest and van Bree (1969) did it for European stoats from the collections of the Amsterdam Museum. Both studies showed that the autumn molt starts on the belly, and the new white hair with its thicker underfur appears there first. Then new fur grows on the flanks and back, and finally on the head. In spring the pattern is reversed; the old white coat is replaced first on the head, then across the body, and finally underneath. The spring molt passes as a distinct, sharp-edged wave of new brown hair across the body, whereas the autumn molt is somewhat more diffuse, producing a "salt-and-pepper" effect (Figure 3.2). Either way, the belly is the last part of the body to lose its extra insulation in spring, and the first to acquire it in autumn, which seems a practical arrangement for a short-legged animal living in a cool climate.

The disadvantage of this simple technique of describing the molt from dead weasels is that every observation is necessarily of a different individual, and if the variation between individuals, climates, and capture locations is extensive, any estimate of timing could become blurred. This problem can be resolved using another technique, watching and photographing captive weasels every day as their coats change. Here we meet a different set of disadvantages in estimating normal seasonal patterns, which could arise if the number of animals is small and their behavior affected by captivity, but at least the individual variations through each molt cycle can be documented.

Figure 3.2 The annual cycle of coat color of the northern weasels.

Feder (1990) watched the molt cycles in captive stoats collected from Oregon and from Alaska. She confirmed that their molt patterns were much the same as expected from the previous work, but the Oregon animals molted more slowly, and began later in autumn and earlier in spring. That is because the difference between day length in summer and in winter increases toward the poles, and so the rate of daily change in day length from one solstice to the other speeds up in the same direction. Therefore, the higher the latitude of a weasel's home is, the more rapid the transition is from summer to winter coats: at lower latitudes, the transition from one coat to the other is slower. Also, the longer the part of the year during which the day length is too short to maintain the summer coat is, the longer the winter coat is worn. The lower the latitude is, the slower the transition from one coat to the other.

Most studies of molt and color change in weasels have used one or other of these techniques, which means that their observations have been done only from the outside, and in climates where all or most individuals turn white every year. But these studies really only observe the last stage, shedding, and the vital early stages of preparation by the follicles and most of the phase of growth of new hair are not detected this way. Also, observing molt only by color change compounds the effects of two quite independent processes, and cannot be used in places with mild climates, where most weasels and stoats do not turn white.

Fortunately, there is an alternative technique by which the molt process can be studied quite easily, even in places where both summer and winter coats are brown, from the inside of the skin. On a flat weasel skin, which has been scraped clean of fat and dried in air without preservative, the distribution of the dark active follicles, the small black flecks along the inside of the summer-brown area, show what phase of molt that animal was in when it died. In the belly fur, the follicles do not accumulate pigment because they are growing white hairs, so the onset of the molt there cannot be detected by this method. But on the back and sides, the active follicles appear just before the new brown hairs grow, and fade as their melanin is used up, so it is possible to plot the progress of the molt by following the migration of the active follicles across the summer-brown area and estimating the period at which molt activity might have reached the white underparts (Figure 3.3).

This method has been used to show that the pattern of molt in common weasels in England is the same as in northern longtails and in Dutch stoats (King 1979). The spring molt begins in March on the head, then spreads along the spine and down across the flanks, ending in May or June on the underside. The autumn molt follows the same route in the opposite direction, beginning in about September or October and ending in about November. The spring molt ends, and the autumn molt begins, in the white fur, so the period during which the summer coat is worn is not exactly known. In King's sample, a few common weasels were still completing their winter coat in December and January: These were all young ones born late in the season, which were probably late in starting.

Figure 3.3 Active hair follicles on four common weasels killed at successive stages in the spring molt (left to right, dorsal views; far right, ventral view), drawn as if the follicles could be seen from outside through the skin. Active follicles (dark) first appear on the back; then the activity of follicles migrates down each side to the belly. (Redrawn from King 1979.)

Figure 3.3 Active hair follicles on four common weasels killed at successive stages in the spring molt (left to right, dorsal views; far right, ventral view), drawn as if the follicles could be seen from outside through the skin. Active follicles (dark) first appear on the back; then the activity of follicles migrates down each side to the belly. (Redrawn from King 1979.)

The actual color of the fur is found mainly in the long guard hairs, whereas the underwool is a nondescript grey at all seasons. The winter fur is thicker because it contains more underwool, and by the spring the guard hairs are worn and broken and less intense in color, and more underwool shows (Powell 1985a). Hence, even in weasels that stay brown all winter, the winter fur often appears to be slightly paler in shade. The difference is best appreciated during the spring molt. Among King's common weasels were several that showed quite a marked change in color as the incoming new fur, noticeably darker, contrasted with the old, paler fur. This was obvious only on animals caught at just the right moment, when the new fur had extended along the middle of the spine as a dark dorsal stripe, but before it had widened and spread across the curve of the flanks. On over a quarter of 122 skins, the dark molt patches on the inside of the skin could be seen spreading along in front of the new fur, while the skin under the area of the new fur itself was clean again.

When the same technique was applied to a large collection of stoat skins from New Zealand, the same pattern appeared again, appropriately adjusted to the southern hemisphere seasons. In New Zealand, the spring molt begins in August and ends in October (although in the cooler parts of the country the old hair might still be hanging on into December), and the autumn molt begins in March and ends in June (King & Moody 1982).

The control over molting by day length is so powerful that stoats in captivity can be induced to molt into their summer coats in midwinter and vice versa, merely by manipulation of the lighting over their cages. For example, five Wisconsin stoats kept on 18-hour "days" molted into their summer coats at a temperature of -6°C (Rust 1962). They took longer to get started than a control group kept at 21°C, but they could not postpone the change for more than 3 weeks when their eyes were insisting that spring had arrived. Conversely, Feder (1990) induced eight stoats to start their autumn molts out of season by switching them to short days.

The same conclusion was clear from the large collection of New Zealand skins, which came from 14 study areas representing a wide range of habitats, altitudes, and latitudes. Analysis of these skins confirmed that the timing of the molt is controlled by day length. On average, stoats in the far south of New Zealand (44°S to 45°S), which is cooler and reaches a given day length later than the north (39°S to 40°S) in spring and earlier in autumn, were always the last to start molting in spring and the first to start in autumn. Stoats from high and low elevations at the same latitude, whose homes differed in temperature but not in day length, molted at the same time (King & Moody 1982).

The molt of the wild stoats in New Zealand was much slower than that of Feder's captives in Alaska, although comparison is hampered by the different observation methods. On the other hand, climate probably influences the speed of molt much more than does observation method. The molt cycles in New Zealand stoats and in the common weasels of England were both studied by the same method, and both were slower and more diffuse than that of the stoats in Alaska. New Zealand and Britain both have mild maritime climates influenced by warm ocean currents, although New Zealand is nearer the equator (35°S to 47°S) than Britain (50°N to 59°N). Both are much more temperate than Alaska.

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