The breeding cycle in longtails (Figure 9.4B) is similar to that in stoats (Wright 1947, 1948, 1963), except in one respect. The young are born in April or May, but postpartum estrus in female longtails is inhibited by lactation. Unless she loses her litter, a female longtail does not return into heat until 65 to 104 days after parturition (i.e., in June, July, or August, according to location) (Wright 1948), later in the season than does a female stoat. Females that give birth but lose their litters very early in the season can be ready to mate again earlier, 39 to 71 days after parturition, but still cannot produce the litter until the following spring. Males are fertile from April to August (Wright 1947).
As in stoats, the spring molt is an accurate herald of the reproductive season for longtails. Both males and females are sexually inactive while in their white coats. From the time their brown summer hairs begin to appear, it takes about 21 days to the implantation of the blastocysts and about 47 days to the birth of the young. Any female remaining white unusually late in the season may be confidently assumed to have resorbed her litter. In males the spring rise in the weight of the testes correlates closely with the beginning of the spring molt. For these and other reasons, longtails have proved themselves useful research animals, especially in studies of endocrinology and photoperiodism.
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