Photoperiodic effects on serum glucocorticoids and fur growth in mink

The relationship of glucocorticoid secretion to artificial light-induced, early winter priming of pelage in ranch mink was investigated. On December 13, 1977, 16 mature female mink were exposed to natural light. On June 26, 1978, the animals were assigned randomly to one of two groups. Animals in Group 1 were housed in a light-control facility and subjected to 6 hr of artificial light daily and ambient temperatures until November 27, 1978. Animals in Group 2 served as controls and were exposed to natural light. Samples of blood and measurements of the fur and vulva were taken biweekly throughout the year. Concentrations of total glucocorticoids in serum were determined by competitive protein-binding assay. The pelage of mink reared under reduced artificial light was fully prime by October 31, while that of control animals did not become prime until November 27 (time x light regimen interaction, P less than .05). A seasonal effect (P less than .01) on total glucocorticoid concentrations in serum of control mink was revealed by a pattern of three peaks over the 12-month period. The pattern of glucocorticoid secretion was not affected by exposure of mink to reduced artificial light. Maximum concentrations of total glucocorticoids in serum occurred on the same date (September 5) in both groups, although levels were significantly higher in the light-treated mink (44.5 +/- 10.4 ng/ml) than in the controls 26.9 +/- 5.7 ng/ml). Fur growth was positively correlated with total glucocorticoid concentrations in serum during the period from June to December (overall r = .14, P less than .05). Both length and width of the vulva increased (P less than .01) during February and March in animals reared under natural light. These data suggest that the increased secretion of glucocorticoids induced by exposure of mink to reduced artificial light may be involved in promoting early priming of the winter pelage of these animals.