Ecology and Evolution of Anuran Communication
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Inheritance of calls and preferences
Selective responses to mate-recognition signals serve as a strong isolating barrier between closely related sympatric species. One controversial theory about the co-evolution of signal structure and receiver selectivity, genetic coupling, postulates that species-specific signal properties and receiver selectivity for those properties are under common genetic control. This research seeks to provide behavioral support for genetic coupling by testing the signaler-receiver relationship in hybrids of parental species whose females discriminate mates based on different call parameters. This work may help us understand how sexual selection via female choice plays a role in the evolution of sexual signals.
I am using two closely related treefrogs, Hyla chrysoscelis and H. avivoca, to investigate this research question. The two species have similar advertisement call structures, but differ in fine temporal and spectral characters. Female H. chrysoscelis key in on the conspecific pulse rate, while H. avivoca require that the pulses in calls have a minimum duration. Females of H. chrysoscelis also prefer signals with two spectral peaks (as in conspecific calls), whereas females of H. avivoca actually discriminate against two-peaked calls in favor of calls with a single, high-frequency peak (as in conspecific calls). I have raised artificially produced H. chrysoscelis x H. avivoca hybrids (F1) in the laboratory, and have recorded the calls of males, and measured the selectivity of females with regard to the parental and hybrid call types. Since females of either species are highly selective for the conspecific call, and this is the only sensory modality used for mate choice, we can use hybrid individuals to see if this correlation persists in the hybrid generation, resulting in hybrid females who prefer hybrid calls over either parental species. The distributions of the acoustic phenotypes of these hybrids will provide critical information regarding the inheritance of signal properties that serve to isolate two closely related species in nature and upon which female choice operates within each species. This behavioral data may serve as a precursor for more sophisticated studies using advanced genetic analyses (i.e. QTL localization) to determine if and how genetic coupling affects calls and preferences.
Dynamic signalling strategies in Hyla femoralis
For many species, localization of an appropriate mate is dependent on the broadcast of courtship signals by one sex and the appropriate response to that signal by the other. Intense competition in lekking species can, however, drive individuals to modify signals in ways that result in an emergent group-signaling dynamic. For instance, many insects and anurans adjust the duration, rate and/or frequency of their acoustic signal relative to calling neighbors. These adjustments by individuals can result in the formation of synchronous choruses, leader/follower dyads, or alternating signals (in which neighboring males avoid signal overlap). I am interested in understanding how the social context (male density) influences the calls of H. femoralis, which exhibits a unique irregular call pattern.
Female preference for signal structure and timing in Hyla femoralis
Sexual signals are often classified as either static (low within-individual variation) or dynamic (high within-individual variation). These “rules” of signal structure have been extensively studied in anurans, with evidence suggesting that fine-scale temporal patterns are generally static. Variation in signal structure is presumed to be critical for species identification and mate choice because females show strong preferences for a narrow range of values near the population mean. The Pine Woods treefrog, Hyla femoralis, has evolved an atypical advertisement call in which variation in pulse period is extremely high. Preliminary results indicate that despite strong conspecific female preferences for fast, static pulse periods, males rarely produce such calls. How do females select mates in this species? Several studies have documented female preferences for species-specific signal traits (i.e. call rate, call duration, pulse number, etc.) or signal timing (i.e leader preference, preference for synchronous choruses); however, few studies have investigated the interaction between these distinctive preference types. The goal of this research is to investigate the significance of this interaction in order to enhance our understanding of the evolution of acoustic communication.