Graduate Thesis Or Dissertation
 

Pheromone mechanisms and behavioral strategies for maximizing courtship success in a terrestrial salamander, Plethodon shermani

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  • Darwin devised the evolutionary theory of sexual selection to account for the manifold extravagances of courtship behaviors and displays. Mating interactions represent a major evolutionary process driving the elaboration, vibrancy, and peculiarity of these courtship traits. For my dissertation research, I strived to elucidate the dynamics that constitute the complex and delicate interplay between the satisfaction of the male's and female's reproductive interests. Recent work on the red-legged salamander, Plethodon shermani, has established that a pheromone blend from male plethodontid salamanders increases female sexual receptivity as measured by reduced time to insemination. These courtship pheromones work as "releasers," stimulating centers of the brain to produce near-immediate effects on behavior. Pheromones have the potential to bypass the more traditional methods of mate assessment and directly activate centers of the brain implicated in the control of motivated behavior. I hypothesized that there were three candidate mechanisms by which male pheromones may alter female receptivity and therefore influence mate choice: (1) increased sexual motivation, (2) suppressed competing motivations, and (3) enhanced general (non-specific) arousal. Chapters 2 through 4 describe a series of behavioral experiments in Plethodon designed to uncover patterns in female responses to pheromones. Chapter 2 examines pheromone-induced modulation of female choice as an indirect consequence of suppressing competing motivational forces. These studies demonstrated that pheromones inhibit feeding activity, but not the tendency to flee from an alarming stimulus. Chapter 3 examines pheromone-induced modulation of female choice as a direct effect of enhanced sexual motivation, or as a secondary consequence of general nervous system arousal. The results supported a specific pheromone mechanism in which attraction to male scent is enhanced, but not attraction to other appealing stimuli. Chapter 4 examines the role of individual pheromone components. I hypothesized that these different pheromone components may work in tandem to increase female receptivity by activating or suppressing specific behavioral systems. My studies supported pheromone component-enhanced attraction to male olfactory stimuli as a mechanism to enhance sexual receptivity. For Chapter 5, I conducted a meta-analysis designed to: (1) quantify variation in female willingness to engage in courtship, (2) quantify variation in male effort expended during courtship, and (3) determine whether these male and female characteristics interacted in a manner to shorten courtship or affect the probability of insemination. The meta-analysis uncovered associations between the time spent in the mutual participation phase of courtship and: female receptivity, male-female compatibility, and male effort. Furthermore, insemination success was associated with the interaction between male-female compatibility and male effort. In light of these results, a male would do well to be attuned to his potential partner's motivational states and intrinsic preferences, and to adjust his courtship behavior accordingly. I conclude my dissertation by discussing the findings from Chapters 2 – 5 within the larger contexts of the potential role of pheromones as the substrate for sexual selection, or as a form of manipulation when the reproductive interests of the courter and courted are not in perfect accordance, and finally, the implications for behavioral studies and interpretations of evolutionary processes.
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