Vertebrates communicate with one another and coordinate intraspecific reproduction by using a variety of sexually dimorphic signals, such as plumage, ornaments, sounds, and/or scents. These sexual dimorphisms are maintained by physiological factors, typically sex-specific hormones (though see Chapter 3 for an exception). The purpose of the research in this dissertation was to explore the mechanisms regulating sexual dimorphism in the sex pheromone blends of red-sided garter snakes (Thamnophis sirtalis parietalis ).

The reliance on sex pheromones (both identified and unidentified) in the coordination of snake reproduction appears to be conserved in all groups of the ophidia (Chapter 2). The red-sided garter snake is a model reptile for studying the mechanisms regulating expression of chemical signals because both abiotic and biotic factors have been shown to shape the peculiar biology of this vertebrate. Temperature is known to regulate male garter snake reproductive behavior, and I found, in accordance with this, that females are maximally attractive upon emergence from hibernation (Chapter 3). Thus, low temperature dormancy is critical for both sexes in this species for optimizing reproduction (behavior, pheromone production).

The relationship between sex steroid hormones and pheromone production was incomplete prior to the work in this dissertation. I first discovered that estrogen implantation induced female pheromone production in males, suggesting that estrogen is the primary steroid hormone inducing female pheromone production (Chapter 4). Further, the effect of estrogen is purely activational since implant removal abolished attractivity (Chapter 5). Also in Chapter 4, I showed that castration induced female pheromone production in males. My last data chapter revealed that testosterone actively inhibited female pheromone production, which I saw after supplementing castrates with testosterone (Chapter 6). Further, aromatase inhibition in castrates changed at least one property of the pheromone blend that abolishes attractivity.

My research into the control of pheromone expression in garter snakes has revealed a pattern of interaction between steroid hormones and sexually dimorphic signal production at the level of the skin that corroborates findings in their closest relatives (birds). The presence of estrogen promotes expression of the female trait (female plumage in birds; pheromone production in garter snakes). However, the absence of testosterone is also sufficient for expression of the female trait in male garter snakes, suggesting that, unlike any avian species studied thus far, the balance between testosterone and estrogen is critical for proper expression of sexual phenotype. Thus, garter snakes may be a model group for exploring the evolutionary origin of hormonal control of sexually dimorphic signals.