Thermoperiodic influences on plasma melatonin rhythms in the lizard Tiliqua rugosa: effect of thermophase duration.

Rhythms of plasma melatonin levels were determined in lizards (Tiliqua rugosa) subjected to a 12 h photocycle (12 h light: 12 h dark) at constant 33 degrees C, and at 7 different thermoperiods (33 degrees C thermophase and 15 degrees C cryophase) whose thermophase duration ranged from 1.5 to 21 h. The melatonin secretion rate, as measured by the amplitude and duration of elevated melatonin levels and the area under the curve, was maximal at thermoperiods whose thermophase was between 9 and 18 h in duration. The results indicate that in ectothermic vertebrates the prevailing thermoperiod as well as the photoperiod may influence melatonin rhythms and hence the timing of annual physiological cycles.