Courting and noncourting male red-sided garter snakes, Thamnophis sirtalis parietalis: plasma melatonin levels and the effects of pinealectomy.

Previous studies found that pinealectomy of male Canadian red-sided garter snakes (Thamnophis sirtalis parietalis) in the autumn, before prolonged exposure to low temperatures (hibernation), significantly impaired the expression of courtship behavior upon emergence in the spring. Additionally, pinealectomized animals with a disrupted diel cycle of plasma melatonin did not court while those exhibiting a more typical diel pattern did. These results suggested that the pineal gland functions in the transduction of a temperature cue which stimulates courtship. To test this hypothesis, we pinealectomized males in the spring after they had undergone a normal hibernation but were still courting. Pinealectomy of courting males in the spring, in each of the 3 years of study, had no effect on courtship. This result suggests that once the cue is transduced, the pineal gland no longer has a modulatory effect on courtship behavior. Finally, we took advantage of the fact that, in the laboratory, there is always a small percentage of males that do not court upon emergence. Pinealectomy of these noncourters greatly increased the percent of males expressing courtship behavior in each of the study years. Plasma melatonin levels of unmanipulated courting and noncourting males was measured after emergence in successive years. In both years, courters had a typical pattern of melatonin secretion (low in the photophase, high in the scotophase) while persistent noncourters displayed the opposite pattern.