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.

Pinealectomy, melatonin, and courtship behavior in male red-sided garter snakes (Thamnophis sirtalis parietalis).

Activation of courtship behavior in male red-sided garter snakes is independent of androgens. Only exposure to extended periods of low temperature with subsequent warming stimulates courtship in males. The pineal gland is thought to transduce temperature as well as photoperiodic information in reptiles. Therefore, we explored the relationship of the pineal and melatonin to sexual behavior in this species. Pinealectomy of male garter snakes disrupted sexual behavior upon emergence from a 17-week period of low temperature in approximately 60% of treated individuals in each of the 3 years of study. However, 40% of the males were unaffected by the pinealectomy, engaging in vigorous courtship. Administration of exogenous, chronic melatonin did not significantly modulate the effect of pinealectomy. Upon pinealectomy in the autumn (before hibernation), plasma levels of melatonin fell. However, upon emergence from hibernation, melatonin levels in pinealectomized (PINX) and sham-treated (SHAM) animals were equivalent, indicating extrapineal source(s) of melatonin. However, PINX males did not exhibit a diel cycle in melatonin levels upon emergence. Instead, melatonin remained elevated through the subsequent 24-hr period. SHAMs did exhibit a diel cycle. Ten days after emergence, PINX animals either had a disrupted/abnormal melatonin cycle and were non-courters or had a cycle similar to SHAM males and courted. Therefore, a normal diel cycle of melatonin appeared necessary for the proper expression of courtship behavior. These results suggest that the pineal in snakes 1) is part of a complex, multi-oscillator system as it is in birds and lizards and 2) may play a role in maintaining polymorphism in timing of reproductive behavior.

Seasonal changes and annual variability in daily plasma melatonin in the red-sided garter snake (Thamnophis sirtalis parietalis).

We report seasonal and annual variation in the daily cycle of plasma melatonin levels in male red-sided garter snakes, Thamnophis sirtalis parietalis. In autumn of 1989 and 1990, levels averaged a maximum of 210 pg/ml during scotophase and a minimum of 45 pg/ml during photophase and had a similar diel pattern. Under hibernation conditions (4 degrees, 0:24 L:D), melatonin was undetectable and a diel cycle could not be determined. In Spring 1990, melatonin levels rose rapidly and precipitously within an hour of emergence (while in photophase), peaked at levels significantly higher than those seen in the autumn (approximately 900 pg/ml) and remained significantly high for 24 hr after emergence (though the majority of animals did have decreased levels at the 0400 sample). By the 10th day after emergence, a diel cycle was reestablished and absolute melatonin levels had decreased. The next spring (1991), melatonin again rose within an hour after emergence, while in photophase, but not as high as the previous year. Also unlike the previous year, a diel cycle was observed within 24 hr of emergence. Melatonin levels at emergence were significantly higher than those observed 10 days later. Disruption of a diel rhythm of plasma melatonin (by pinealectomy the previous autumn) inhibits courtship behavior by males the next spring, implying a role for melatonin in the stimulation of sexual behavior. Males in 1991 (with quickly established melatonin cycles) courted much sooner after emergence than did males in 1990. Therefore, the initial day/night difference in melatonin levels at emergence (i.e., establishment of a normal diel cycle) may function in synchronizing and modulating reproductive behavior in male red-sided garter snakes.