My dissertation focuses on the evolutionary forces that have shaped the chemical signaling system of plethodontid salamanders. Pheromones mediate two phases of plethodontid reproduction: mate attraction prior to courtship and female persuasion during courtship. Substrate-borne chemical signals are believed to play an important role in mate attraction for these animals. A behavioral assay of sex- and species-specific odor preferences in closely related species of Plethodon indicates that: (1) there is asymmetry on the receiver side of the system, wherein male salamanders invest more energy searching for mating partners than do females; (2) substrate-borne chemical signals are sexually dimorphic and male preference strongly favors the female odor versus the male odor; (3) females of allopatric salamander species have evolved distinctive chemical cues; and (4) female chemical signals of sympatric species are divergent and may play a role in sexual isolation. Sexual persuasion is another phase of plethodontid reproduction that is mediated by chemical signals. Plethodontid Receptivity Factor (PRF) is a proteinaceous pheromone that is produced by the male and delivered to the female during courtship. Female receptivity increases following application of this pheromone. PRF is a recent innovation of the male courtship signal, originating in eastern Plethodon (~27 MYA). Codon-substitution models indicate that several amino acid sites along the PRF protein have experienced positive selection. Structural modeling suggests that many of these selected residues are important for receptor binding. Adaptive change in this pheromone is likely driven via a coevolutionary association with female receptors. The male plethodontid courtship signal, however, is comprised of multiple chemical components. I investigate micro- and macro-evolutionary divergence in plethodontid chemical communication by surveying two additional pheromone component genes, PMF and SPL. Like PRF, regions of the PMF and SPL genes have experienced adaptive change. At least one of these components (SPL) has been retained in the signaling system for ~100 MY. However, significant differences in the composition of the courtship pheromone are evident in some plethodontid lineages. It appears as if at least two lineages (Desmognathus and eastern Plethodon) use different major pheromone components and yet achieve the same behavioral response in the female (increased receptivity). / Graduation date: 2005

Sexual selection can influence the mating system of an organism through multiple mechanisms. These mechanisms result in variation in reproductive success among individuals, and include scramble competition, endurance rivalries, contests, mate choice and cryptic choice, and sperm competition. Understanding the mating system of a species requires the identification of which processes are occurring, and to what degree. In this thesis, I explored the influence of mate choice mechanisms on the mating system of the terrestrial red-legged salamander, Plethodon shermani. I also documented the potential for post-copulatory processes (such as sperm competition and cryptic choice) to influence mating system dynamics. The evolution of mate choice requires (among other factors) variation in the reproductive value of potential mates. This variation is made apparent to choosy individuals through cues. Most animals use multiples cues incorporating many modalities to assess the reproductive quality of potential mates. In Chapter 2, I tested the contribution of two cues (chemical and visual) to mate choice by female P. shermani. I found that a male visual cue ("foot-dancing") increased courtship success. In contrast, delivery of non-volatile pheromones during courtship did not influence courtship success in the laboratory setting, but did affect the duration of one of the courtship stages. In Chapter 4, I identified a tactile cue that was significantly correlated with male reproductive success. Thus, P. shermani females could use at least three modalities to assess the reproductive quality of potential mates. Mate choice can also evolve in males. In Chapter 3, I tested this possibility in P. shermani. I found that males vary the reproductive effort they invest in a particular courtship based on the reproductive value of their partner, indicating male mate choice is occurring. A male invested most when paired with a female with large, well developed ova, and invested less with females that were non-gravid or had small ova. In addition to documenting male mate choice, I showed that the male visual display ("foot-dancing") that affected female mate choice was correlated with male condition, implying foot-dancing may be an honest indicator of male quality. Finally, in Chapter 5, I explored the potential for post-copulatory processes to influence the P. shermani mating system. The opportunity for sperm from multiple males to overlap in the female reproductive tract (i.e., the opportunity for females to mate multiply) is necessary for post-copulatory processes such as cryptic choice and sperm competition. The capacity for long-term sperm storage by females can increase the likelihood that this overlap in sperm from multiple males will occur. I found that females can store viable sperm for at least 9 months and in some cases beyond oviposition. In addition, I documented one female with sperm in her sperm storage organ from a mating that occurred 17 months earlier. Such lengthy sperm storage allows the possibility of sperm from one breeding season to interact with sperm from a subsequent season. Thus, the potential for post-copulatory sexual selection within this salamander system is high. / Graduation date: 2012