Endocrine mechanisms mediating temperature-induced reproductive behavior in red-sided garter snakes (Thamnophis sirtalis parietalis).

We investigated the mechanisms by which temperature induces seasonal reproductive behavior in red-sided garter snakes (Thamnophis sirtalis parietalis). Specifically, we addressed whether elevated temperatures during winter dormancy influence (1) diel melatonin and corticosterone rhythms; (2) sex steroid hormone and corticosterone profiles; and (3) the expression of reproductive behavior following emergence. Elevated hibernation temperatures (i.e. 10 degrees C versus 5 degrees C) significantly increased overall melatonin and decreased corticosterone concentrations of snakes. The temperature-induced differences in melatonin rhythms between the 5 degrees C and 10 degrees C treatment groups persisted even after both groups were again acclimated to 10 degrees C, indicating that cold temperature exposure has a lasting influence on melatonin rhythms. Elevated hibernation temperatures also significantly altered androgen and corticosterone profiles of snakes, providing a potential mechanism to explain reported annual variation in steroid hormones. Although previous studies indicate that male red-sided garter snakes exhibit a dissociated reproductive strategy, we demonstrate the presence of intersexual variation in sex steroid hormone profiles, as estradiol concentrations of female snakes increased significantly prior to spring mating activity. Importantly, the percentage change in body mass did not differ significantly between snakes in the hibernation treatments, indicating that the observed changes in hormone profiles are indeed temperature induced and not simply an indirect result of significant changes in the energy balance of snakes. Finally, in males maintained at 10 degrees C during winter dormancy the onset of courtship behavior following emergence was delayed. Our results suggest that environmental temperatures induce reproductive behavior, in part, via changes in diel melatonin and/or corticosterone rhythms in this seasonally breeding reptile.