Coordinated communication: an analysis of signal and preference phenotypes in the genus Hyla
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Pre-mating isolation is often accomplished through the use of communication signals. Generally, species differ in their mate-attracting signal and conspecific receivers show strong discrimination based on these signal features. This coordination aids in the prevention of potentially costly hybridization because individuals that produce signals with values that differ greatly from the population mean are unlikely to attract a mate, and receivers with extreme preferences may not find a mate producing the preferred signal values. Selective pressures shape both male and female reproductive behaviors, often mediating the degree of variation they contain and ultimately resulting in a tight association between the two. Over the past several decades, scientists have argued about how a tightly matched communication system can be maintained between sender and receiver. The first goal of my dissertation was to investigate how unlearned signal and preference phenotypes are inherited from one generation to the next. Using the F1 hybrids of two closely related treefrogs species, I quantified the behavioral phenotypes of male and female siblings in order to determine if the genetically determined male acoustic signal and female preference criteria aligned with each other, or if the coupling of these behaviors would be disrupted due to hybridization. I investigated the behaviors of hybrids between the sister species, Hyla chrysoscelis and H. versicolor. Males of both species broadcast a simple, rhythmic signal which follows a distinct temporal pattern. Females generally prefer the species-specific call properties, with minimal variation. The calls and preferences of these two species differ in several important ways, providing an opportunity to track both behaviors in F1 individuals. After raising the hybrids to sexual maturity, I recorded and analyzed the advertisement calls of males and subjected females to preference tests in which they were given a choice between a representative call of either parental species and the call of hybrid males. My results clearly do not provide evidence of genetic coupling in this system. While I would have expected to see intermediate phenotypes in both males in females simply to do additive effects in the F1 generation, I saw clear evidence of dominance from one parent, depending on the certain traits. Future behavioral work (beyond the F1) as well as genetic analysis is needed to further determine the path of inheritance of signals and preferences in closely related hylids. Despite the tight coordination of signals and preferences within species, variability of signal traits can occur among and within individuals. Variation is often seen in the presence of changing conditions, such as the level of competition, which can drive individuals to implement signaling strategies to increase their ability to obtain mates. These often involve adjusting the frequency, duration, or intensity of those signals, or switching to distinctive aggressive signals, in order to increase their receptivity to females or repel their competitors. My second goal was to learn more about the role of signal plasticity as it relates to male competition and mate choice. To study the role of social context on signal variability, I analyzed the behavior of males and females of the pine woods treefrog, Hyla femoralis, which broadcasts a highly variable signal and calls from densely populated breeding aggregations. I investigated the extent of behavioral variability in this species in the context of competition, and also how this signal plasticity influences mate choice. My behavioral results indicate that males are strongly influenced by competitor behavior, and that individuals modify their signals on a fine-temporal scale. Females showed strong preferences to fast, static rates, a behavior which is not commonly displayed by conspecifics for extended periods, indicating that selective pressure from females is probably weak for most acoustic signals in this species. Females also surprisingly show preference for the males in the following position when competitors overlapped, suggesting that physical masking may be an important perceptual process driving mate choice. This work speaks to the relative strength of female preferences for signal timing, specific signal properties and their interaction in an effort to enhance our understanding of signal plasticity in the dynamic social context of breeding aggregations.
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