Graduate Thesis Or Dissertation
 

Phenotypic variation in the sexual attractiveness pheromone of the red-sided garter snake, Thamnophis sirtalis parietalis

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  • Pheromones are chemical cues produced by organisms that affect the behavior and/or physiology of conspecifics. The orchestration of reproductive behaviors in many animals depends on the expression of sex pheromones. In insects, intraspecific variation in sex pheromone expression is commonly observed and often influences social interactions between individuals. To what extent similar variation is present in vertebrate sex pheromone systems is not well understood. This thesis investigated the occurrence and significance of phenotypic variation in the sexual attractiveness pheromone of the red-sided garter snake, Thamnophis sirtalis parietalis. Composed of a homologous series of saturated and unsaturated methyl ketones sequestered in the skin lipids of females, this pheromone elicits male courtship behaviors during the breeding season. Variation in the expression of the sexual attractiveness pheromone was examined at three levels: seasonal, individual and populational. Seasonal variation in pheromone expression was examined with respect to trailing behavior. In field experiments, males followed female trails during the breeding season but not during the non-breeding season. Skin lipid analysis revealed marked differences in pheromone composition between the two seasons, suggesting a role for this pheromone in regulating male trailing behavior. Individual variation in pheromone expression was examined with respect to mate choice. In arena trials, males displayed a courtship preference for larger females and continued to demonstrate this preference when visual, tactile, and behavioral cues from the females were removed through the use of skin lipid extracts. Analysis of pheromone profiles from individual females showed size-specific variation in composition, demonstrating that this pheromone can function as a reliable indicator of female size. Finally, populational variation in pheromone expression was examined with respect to sexual isolation among hibernacula. In arena and trailing experiments, males preferred to court and trail resident (same den) females over non-resident (distant den) females. Chemical analysis showed significant inter-den variation in pheromone composition, suggesting that this pheromone represents the cue that males use to identify females from their own population. In summary, these studies demonstrate that significant variation exists in the female sexual attractiveness pheromone of the red-sided garter snake and suggest that this variation has functional significance for this species.
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