Effects of photoperiod, melatonin administration and thyroid block on spontaneous daily torpor and temperature regulation in the white-footed mouse,Peromyscus leucopus

Summary Cold exposed (13°C) mice maintained on a short day photoperiod (9L:15D) became torpid 9 times more frequently than long day photoperiod (16L:8D) animals (Table 1). Mice on a short day photoperiod also exhibited a 26% increase in nesting behavior, a 9% decrease in food consumption and a 49% increase in norepinephrine induced thermogenesis (Table 2). No change in resting metabolism was observed. Similarily, chronic melatonin administration (subcutaneously implanted beeswax pellet containing 3.5 mg melatonin) elicited a 2.5 fold increase in spontaneous daily torpor relative to shamimplanted mice (Table 3). Mice treated with melatonin exhibited a 33% increase in nesting behavior and a slight decrease in food consumption. Although thyroid block (propyl-thiouracil) increased the incidence of daily torpor only slightly, it did effect an 11% decrease in resting metabolism, a 42% decrease in norepinephrine induced thermogenesis and a 5% decrease in food consumption. Thyroid block did not alter nesting behavior.