Spatial and temporal variation in three call traits and preferences of the tree cricket Oecanthus forbesi

Abstract

Multiple selective forces act on the evolution of mating preferences. While mating preferences are central to pre-zygotic isolation, certain preferences and traits may make greater contributions. For some traits, females may exhibit preferences, but accept heterospecifics trait values when preferred values are scarce. For other traits, females may fail to reproduce before accepting heterospecifics trait values. Understanding patterns of variation and divergence in this later class of traits is particularly relevant to understanding divergence and speciation. Here, I focus on three call traits of Forbes’ Tree Cricket (Oecanthus forbesi) to quantify their capacity to produce reproductive isolation and to compare patterns of variation and divergence in these traits. By generating female preference functions and measuring male call parameters, I test two hypotheses: (1) traits and preferences vary in their capacity to contribute to reproductive isolation and (2) traits that are important to reproductive isolation have lower intrapopulation, interpopulation, and interannual variation and weaker correlation with male body size. I find that female response to one trait (pulse rate) decreased sharply when trait values fell within the heterospecific range. This trait had low variation and no correlation with male morphology. For two other traits (pulse duration and dominant frequency), females responded to many values, including values characteristics of co-occurring heterospecifics. Trait variation was higher and pulse duration correlated with male leg length. These results indicate that the evolutionary dynamics of a low-variation trait (pulse rate) may be more important to speciation than changes in more conspicuously variable sexually selected traits. Animals often attract and assess mates using complex signals. This paper tests whether some signal components contribute more to preventing mating between species. The data show that changes in a single trait [pulse rate of cricket calls] can eliminate female response to males, while other traits [dominant frequency (pitch) and pulse duration] can be changed to match the values produced by other species without reducing female response. Consequently, some traits may diverge without contributing to reproductive isolation between species. The paper then tests for correlations between trait function and patterns of trait variation within and between populations and species. Pulse rate has low variation within and between populations, but differs substantially between species. Dominant frequency and pulse rate are more variable within and between populations. Pulse duration also correlates with male body size, indicating that pulse duration could reflect male condition even if it is relatively unimportant for reproductive isolation.