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Effects of melatonin implants in spring on testicular regression and moulting in adult male raccoon dogs (Nyctereutes procynoides)

Six male raccoon dogs were treated with constant-release implants of melatonin on 29 March, when the animals still had mature spermatozoa and a winter coat. Six untreated animals served as controls. High serum concentrations of melatonin were found in the treated animals throughout the study period (182.2 and 38.5 pg ml^–1 in April and August, respectively). The high serum concentrations of prolactin in April and May (18.9 and 15.7 ng ml^–1, respectively) in the controls were not seen in the treated animals. Testicular regression, judged by width of the testis and stage of spermatogenesis, was slowed after melatonin administration. Testis width from April to June and stage of spermatogenesis in May and June were greater in the treated animals than in the controls; however, from July onwards the differences between the two groups were no longer significant. Serum concentrations of testosterone remained low (below the detection limit in most cases) in both treated and control animals throughout the study period. Melatonin treatment also affected moulting, which was assessed by changes in the number of growing and mature underfur hairs per bundle and the percentage of bundles with a growing guard hair. The treated animals showed a more rapid shedding of mature underfur hairs and growth of new underfur hairs 6 weeks after melatonin implantation compared with the controls. During the study period, there were significantly more growing underfur hairs per bundle in May to August (4–9 per bundle) and a lower percentage of bundles with a growing guard hair in May and August (38 and 74%, respectively) in the treated animals compared with controls (1–4 per bundle; 79 and 93%), indicating that melatonin treatment stimulated the initiation of growth of underfur hairs and inhibited that of guard hairs. Although melatonin administration in March slowed testicular regression and maintained an unchanged winter coat in the initial stage, earlier melatonin implantation may delay testicular regression and spring moult more effectively in raccoon dogs.