In wild-taken specimens of the species Mustela frenata, subspecies included, the spring molt begins on the mid-dorsal line and proceeds laterally, producing, at almost any given time, a relatively sharp molt line separating the white winter hair from the incoming brown summer coat. However, in autumn the change takes place first on the belly, then on the sides, and finally makes its appearance over all the upper parts at about the same time, with the result that the upper parts have a salt-and-pepper appearance without at this time any sharply defined molt lines. In general, the molt pattern can be said to be reversed in the two seasons; in spring, it begins on the back and in autumn, on the belly. The difference in spring and autumn color pattern is better illustrated on plate [39] than by additional description. Swanson and Fryklund (1935:123) have observed that the "spring molt proceeds differently" than the fall one in Mustela rixosa, and Barrett-Hamilton (1903:309) in commenting on the European hare (and the stoat?) remarks, "In spring the moult, and with it the brown colour, progresses in exactly the opposite order . . ." as compared with the white color of autumn, which that particular writer thought resulted from removal of pigment from the hairs rather than from molt.
The tail, excepting the black tip, lags in the molt in many instances, with the result that, especially in spring, it may retain a few white hairs as late as does the belly. In autumn it is less tardy and so far as I have observed, becomes white at about the same time that the general area of the back changes color. On the tail, the black tip itself, as clearly shown in more than a score of specimens, is molted at approximately the same time in autumn as is the pelage of the body. However, the long black hairs, which appear in, say, November, appear to increase in length until January. In spring, the long black hairs of the tip of the tail seem not to be shed at the same time as the rest of the winter pelage, but remain approximately six weeks longer and then are replaced by long black hairs of the summer coat. At any rate, this is the picture presented by a half dozen specimens of M. f. nevadensis and M. f. longicauda which do show a spring molt to be in progress on the black tip of the tail. Schwalbe similarly (1893:536-537) has suggested that the black tip of the tail in Mustela erminea in spring is not molted until about two months after the pelage on the rest of the body is changed. Schwalbe (loc. cit.) thinks also that in M. erminea studied by him, the black tip of the tail in autumn is replaced approximately one month in advance of the pelage on the rest of the body. As indicated above, my specimens of Mustela frenata, subspecies longicauda and nevadensis, do not show this discrepancy in autumn. I have considered the possibility that the black tip of the tail, in some species of Mustela, is molted only once while the remainder of the coat was undergoing two molts. My inconclusive data lend but little support to this possibility.
The difference in pattern of color between specimens taken in autumn and spring is known to some fur-trappers of my acquaintance who have suggested that molt occurs in spring, whereas the individual hairs change color in autumn. Reference to plate [39] will show how gross comparisons might lead one to this erroneous explanation of the color change.
As to time of molt: In eight subspecies of Mustela frenata, namely, noveboracensis, occisor, primulina, spadix, longicauda, arizonensis, nevadensis and effera, material is available to indicate that the autumnal molt begins in October and is completed in November, and that the spring molt occurs in March or April. A condensed list of specimens providing basis for this statement is as follows:
M. f. noveboracensis: 26 specimens in transitional pelage taken in autumn and 14 taken in spring; M. f. occisor: One topotype has acquired one-fifth of the winter pelage on October 22, 1896; M. f. primulina: 2 in November, one in March, and 2 in April are in process of change; M. f. spadix: 6 autumnal specimens and one in April show pelage change; M. f. longicauda: 7 autumnal specimens and one in April show pelage change; M. f. arizonensis: 12 specimens in autumn and 3 in spring are in process of molt; M. f. effera: One November-taken male has acquired four-fifths of the winter coat and another taken on April 21 at Fort Rock, Oregon, is half finished with the spring molt.
It may be added that no marked difference in time of either autumnal or spring molt is apparent as between the more northern and more southern localities from which the mentioned specimens come. With more complete material I would expect to find a difference in this regard.
The material of the other, more southern, subspecies of Mustela frenata has not been adequate to show the time of molting or the number of molts which occur in one year.