An experiment was conducted to determine the effects of a reduced daily photoperiod and exogenous melatonin on serum prolactin levels of mink during the autumn molt and growth of the winter pelage. During the last week of June, adult standard dark female mink (Mustela vision) were exposed to natural changes in daylength (controls), a reduced photoperiod of 6 h light: 18 h dark (6L:18D) or exposure to natural changes in daylength and treated with melatonin (10 mg) in a Silastic implant inserted subcutaneously over the scapular area. Beginning July 2, and continuing through October 22, blood samples were collected at nine biweekly intervals, and serum prolactin concentrations were quantified by a heterologous double antibody radioimmunoassay. Both reduced photoperiod and exogenous melatonin caused serum prolactin levels to decline rapidly after mid-July, resulting in concentrations that were significantly lower than those of controls 6 to 8 wk earlier. These data suggest that growth of the winter pelage of mink is strongly associated with declining prolactin levels. It appears that part of the photoperiodic-induced effects on fur growth of the mink are mediated through melatonin and its effects on prolactin synthesis and/or secretion.

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Adult and kit standard dark male and female mink were treated with 0, 5 or 10 mg melatonin, or a reduced photoperiod of 6 h light: 18 h dark (6L: 18D, initiated during the last week of June) to determine the effects of treatment on winter fur growth. Melatonin was administered in a Silastic implant inserted sc over the scapular region during the last week of June. Regardless of sex or age, mink treated with melatonin or a 6L: 18D photoperiod molted the summer pelage and grew the winter pelage earlier than controls (P less than .001). Winter pelage was fully prime by mid-October, 6 wk earlier than normal. Weight gain of mink, regardless of sex or age, was not affected by melatonin treatment. These findings indicate that photoperiodic effects on the growth of winter pelage in mink may be mediated through the pineal gland and its secretion of melatonin.

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