Senders & Their Sensory Environments: Acoustic Lessons From a Radiation of Ancient Grasshoppers

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Communication between individuals is ultimately dependent on the functioning of sensory organs, such as eyes or ears, and is affected by a variety of factors, ranging from the physics of signal transmission in the microhabitat, to the neural characteristics of peripheral receptor organs. Although mate choice scenarios have been thoroughly investigated, the evolutionary links between the environment and sensory systems remain virtually unexplored. Notable exceptions occur in fairly advanced vertebrate groups such as frogs and fish. However, the vertebrate central nervous system is almost certainly more plastic than is the peripheral nervous system, and changes in sensory characteristics at this level are difficult to evaluate. In contrast, sensory change in invertebrates is considerably more concentrated at the periphery, thus invertebrates may provide useful model systems. Bladder grasshoppers (Orthoptera, Pneumoridae) are highly specialized for long distance (~2 km) acoustic signaling and presently occupy a range of contrasting sensory-ecological environments. This small family, in which each individual has a dozen ears of two distinct types, offers a unique opportunity to investigate the role of sensory biology in evolutionary processes. The contributions of environmental constraints and ear structure and function will be explored in this ancient group. To determine whether habitat is a significant selection pressure for divergent calling songs, a series of acoustic playback experiments will be performed to compare the efficiency of sound transmission in forest, grassland and desert habitats. The detailed morphological structure of both a tympanate- and pleural ears will be compared across species, and microscanning laser vibrometry will yield relative estimates of ear tuning and sensitivity. These will indicate to what extent ears are matched to the frequency spectra of species-specific calls, and the potential for sensory partitioning across different habitats. Finally, the evolutionary history of the pneumorid family will be reconstructed using DNA sequencing of the mitochondrial 16S rRNA gene to provide the infrastructure for integrated comparative analyses. This project involves international travel, and will be carried out in collaboration with several undergraduate, graduate, and post-doctoral associates. The results of this study will provide insight into the complex associations that exist between the environment and sensory systems, and the potential role of these interactions in the speciation process. Moreover, evolved features of these unique invertebrate ears may be relevant in the design of small, highly sensitive hearing devices.