RESPONSE OF COLLARED LEMMINGS TO MELATONIN .1. IMPLANTS AND PHOTOPERIOD

We examined the effect of constant-release melatonin capsules on the p hysiology and morphology of female collared lemmings exposed to either chronic long (22L:2D) or short (8L:16D) photoperiod, or to a change i n photoperiod. When animals were maintained on unchanging long or shor t photoperiod, subcutaneous melatonin implants were without effect. Ho wever, when animals were reared on either 22L:2D or 8L:16D and transfe rred to the alternate photoperiod at weaning, melatonin (implanted at weaning) prevented most photoperiod-related responses. At sacrifice (a fter 8 weeks of treatment), melatonin-implanted animals exposed to a c hange in photoperiod did not differ from animals remaining in the orig inal photoperiod with respect to pelage color, bifid claw size, uterin e mass, or serum prolactin (PRL). In contrast, regardless of treatment , animals exposed to a photoperiod transfer developed a body mass that partially or fully reflected that characteristic of the secondary pho toperiod; i.e., both control- and melatonin-implanted animals transfer red from long to short photoperiod developed a large body mass. These results indicate that masking the endogenous melatonin rhythm via cons tant-release melatonin implants renders collared lemmings unable to re spond to a change in photoperiod with respect to most physiological pa rameters. However, the striking seasonal change in body mass experienc ed by collared lemmings appears to be at least partially independent o f a melatonin signal.