Stress and reproduction in male garter snakes Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/gt54kr30d

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  • Animals respond to stressful situations with increases in plasma levels of glucocorticoid stress hormones. These hormones originate from the adrenal cortex in response to stimulation of the hypothalamic-pituitary-adrenal axis. Glucocorticoids (corticosterone in reptiles) function to mobilize energy and suppress unnecessary functions until the stress passes. Among these immediately unnecessary functions is reproduction. Suppression of reproduction by glucocorticoids is often manifested as decreased plasma sex steroid levels and diminished mating behavior. However, animals living in extreme conditions with limited reproductive opportunities will often suppress their stress response during the breeding season in order to maximize their reproductive opportunities. The purpose of these studies was to investigate seasonal and environmental adaptations of both the behavioral and hormonal responses to stress in the garter snake, Thamnophis sirtalis. The red-sided garter snake, T.s. parietalis, of Manitoba Canada, is active for only four months of the year and mates during a brief period in the spring. Males respond to capture stress during the mating season with an increase in corticosterone and a decrease in testosterone but this has no effect on sexual behavior. Thus, these animals exhibit a novel response to stress in which sexual behavior is uncoupled from the physiological response to stress. However, treatment with exogenous corticosterone does suppress mating behavior in a dose-dependent manner. This suggests that corticosterone levels are maintained below a threshold level above which mating behavior is suppressed. When compared across seasons, males display a suppressed hormonal response to capture stress during the breeding season relative to the non-breeding season. In contrast, the red-spotted garter snake, T.s. concinnus, of western Oregon is active for ten months of the year and mates during an extended period in the spring. Plasma corticosterone and testosterone levels cycle together through the year in a positive relationship. These animals exhibit a consistent stress response of increased corticosterone and decreased testosterone throughout the year. In summary, these studies demonstrate that both the hormonal and behavioral stress responses are seasonally and environmentally adapted in these conspecific garter snakes inhabiting very different environments.
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