PHOTOPERIOD EFFECTS ON BODY-MASS, BODY-COMPOSITION, GROWTH-HORMONE, AND THYROID-HORMONES IN MALE COLLARED LEMMINGS (DICROSTONYX GROENLANDICUS)

The purpose of this study was to examine the effect of photoperiod on serum levels of growth hormone (GH) and thyroid hormones (TH), body ma ss, and body composition in collared lemmings (Dicrostonyx groenlandic us). Collared lemmings undergo seasonal changes in body mass and body composition, being heavier, but relatively less fat, during the winter months. These physiological changes can be induced in the laboratory by exposure to short photoperiod. The present study tests the hypothes is that photoperiod-mediated changes in GH and TH occur simultaneously with, and therefore may regulate or be permissive to, changes in body mass and body composition. Juvenile (19 d of age) or adult (50 d of a ge) male collared lemmings reared in long photoperiod (LD), 22 h light (L) : 2 h dark (D), either remained in LD or were transferred to shor t photoperiod (SD), 8 h L : 16 h D. After 1, 2, 3, and 4 weeks (juveni les) or 1 and 3 weeks (adults) of treatment, animals were sacrificed. Serum levels of GH, triiodothyronine, and thyroxine were determined. C arcasses were weighed and analyzed for absolute and relative (analysis of covariance) fat-free dry mass, fat, and water. Exposure to SD resu lted in an increase in GH, a decrease in TH, an increase in body mass (due to an absolute increase in all body components), a relative decre ase in fat mass, and a relative increase in body water. Results are co mpatible with the hypothesis that a SD-mediated increase in GH is invo lved in the greater body mass gain observed in SD animals. Decreases i n TH resulting from SD exposure, if associated with a decrease in heat production, may be permissive to mass gain.