Pheromonal Mediation of Intraseasonal Declines in the Attractivity of Female Red-Sided Garter Snakes, Thamnophis sirtalis parietalis

During the breeding season, female red-sided garter snakes ( Thamnophis sirtalis parietalis ) produce and express a sexual attractiveness pheromone that elicits male courtship behavior. Composed of a homologous series of saturated and monounsaturated methyl ketones, this pheromone is expressed in female skin lipids. Recent studies have shown that the sexual attractivity of unmated female garter snakes declines as the breeding season progresses. Here, we investigated whether temporal changes in the quantity and/or quality of the female sexual attractiveness pheromone are responsible for the observed loss of attractivity. Female red-sided garter snakes were collected immediately following spring emergence and held under natural conditions for the duration of the breeding season. Behavioral experiments confirmed that unmated females become significantly less attractive to males within two weeks of emergence from hibernation. Additionally, these females had lower estradiol concentrations at two weeks post-emergence. Subsequent chemical analyses revealed qualitative variation between the pheromone profiles of newly emerged females and those of females at two weeks post-emergence. Together, these results support the hypothesis that changes in the female sexual attractiveness pheromone are responsible for declining post-emergence female attractivity in garter snakes.