Signal production and function in weakly electric fish: A comparative investigation of sexually dimorphic communication behavior in Apteronotus.

摘要

Sex and species differences in signal production constitute a significant portion of the complex phenotypic variation observed in animals. The sheer diversity of signals and patterns of phenotypic divergence suggest evolutionary patterns and constraints likely common to signal production in most taxa. Weakly electric fish provide a remarkably unique and accessible model system for investigating the proximate and ultimate causes and consequences of phenotypic complexity, structural plasticity, and functional evolution of signal production. I used two closely related species, Apteronotus leptorhynchus and Apteronotus albifrons to comparatively study the neural and social control of communication behavior. These two species produce constant-frequency quasi-sinusoidal electric organ discharges (EODs) that are modulated during social encounters to produce signals known as chirps and rises. I was specifically interested in identifying the relevant stimuli, and possible sources of neuromodulation, responsible for the expression of sex and species differences in chirp production. By using artificial stimuli simulating the presence of conspecifics, I recorded electrical and physical behaviors produced in response to playback stimuli across a range of stimulus frequencies. Both A. leptorhynchus and A. albifrons responded vigorously to playback stimuli and expressed many previously undescribed dimorphic behaviors. Chirp structure was highly variable and sexually dimorphic in both species, and there was evidence for functional differentiation across homologous chirp types. Using immunocytochemistry, I also identified at least one potential source of neuromodulation of chirp structure, the neuropeptide substance P. Although stimulus frequency differentially affected chirp production in both species, there was no effect of stimulus frequency on most physical behaviors of A. leptorhynchus. The results of these experiments suggest that, as sexually-dimorphic signal structure diverges, so too may signal function. This provides evidence that complex sex differences may aid in social organization and speciation. If so, neuromodulators such as substance P may act as mediators between the development of sexually dimorphic behavior and the evolution of species-specific signal function.