Evolution of Signal Divergence and Behavior in Cyprinella galactura, the whitetail shiner

Abstract

Acoustic signaling in anurans, insects, birds, and mammals has played an important role as an interspecific isolating mechanism and is also believed to be a mechanism of sexual selection in intraspecific mate choice. A similar mechanism in North American freshwater fishes has been virtually unexplored. Before the evolutionary significance of signal divergence in fishes can be determined, a detailed description of variation within and between species must be made. Cyprinella galactura, the whitetail shiner, is known to produce sounds during the breeding season, and due to its disjunct distribution and complex acoustic repertoire, was the perfect model for a study of this type. Acoustic signals were thoroughly described detailing a complexity in signal structure never before documented in freshwater fishes. In addition, the role of acoustic signaling in agonistic and courtship behavior was examined. Sound was found to be frequent during low and moderate level male displays and decreased during the highest levels of male motivation for both contexts. Geographic signal variation was examined in detail among four populations of C. galactura separated by the Mississippi embayment. Significant population-level differences in both temporal and spectral parameters were found, however, pulse parameters in every method of analysis contributed to the most divergence. Adjacent populations are more similar for courtship signal parameters, but not for agonistic signal parameters. A combination of geographic isolation and genetic drift may contribute to these differences. Furthermore, acoustic variation was examined at five different levels: within a signal, within individuals, within a population, within a species, and among species. Agonistic signals were more divergent and more variable than courtship signals both within a population and within a species. In addition, static and dynamic signal properties were defined. Burst duration and burst interval were more variable at the within-a-signal level and are considered dynamic properties. Pulse duration, pulse interval, and pulse rate are more variable at the within a species level and are considered static properties. The role these variations may play in the evolution of acoustic signaling is discussed. This study paves the way for further studies of signal variation within the genus Cyprinella.