Sensory ecology of tropical crickets

This study combines field and laboratory investigations on the ecology, acoustic behavior and sensory physiology of neotropical crickets in Panama. Most crickets achieve pair formation by acoustic signaling and phonotactic approach. Due to call frequency overlap and masking interference, the air-borne sound channel represents a limited resource for communication in a species-rich cricket community. The overall aim of this study is to describe behavioral, sensory and central nervous adaptations which allow acoustic communication and mate localization despite strong competition for call frequency. Two behavioral adaptations which might contribute to acoustic niche segregation are temporal and spatial separation of species. We expect a reduction of niche overlap or an expansion of niche space for the more species-rich community in Panama compared to temperate habitats described for European field crickets. A quantitative calculation of channel breadths and overlaps along these dimensions will allow us to determine their respective contribution to reproductive isolation. Sound mediating the pair formation function is separated along the frequency dimension in a low-frequency channel, and represented in the afferent spike discharges of a prominent interneuron (AN1) which can be recorded under both laboratory and field conditions. We will use this homologous interneuron in all cricket species to test a number of hypotheses regarding sensory and central nervous adaptations in tropical crickets, on a comparative basis. The neurophysiological studies will be complemented by behavioral tests in which the ability of tethered flying crickets to detect and orient towards a sound source can be studied quantitatively. We focus on the consequences of these adaptations for intraspecific acoustic communication and reproductive isolation between distinct species.

In species-rich biomes such as tropical rainforests the efficiency of acoustic communication will strongly depend on the degree of signal overlap. Signal interference with background masking noise deteriorates detection, recognition, and localization of conspecific signals. Thus, the communication space should be partitioned sufficiently to reduce masking interference and to promote intraspecific communication.In the finished project we used a comparative approach of an acoustically communicating tropical cricket community to explore both physiological and behavioural mechanisms contributing to enable successful communication under high background noise. Neurophysiological experiments on the peripheral auditory system in tropical cricket species revealed three mechanisms that contribute to an excellent neuronal representation of conspecific signals despite the masking background of heterospecific senders: (i) a sharply tuned frequency selectivity, (ii) spatial release from masking, and (iii) gain control or selective attention.The community organization of a tropical cricket assemblage showed significant reduction of signal overlap of calling frequencies between species, suggesting that frequency partitioning of calling songs might be an evolutionary outcome of selection against interspecific frequency overlap. Moreover, the spatio-temporal distribution of senders further reduces the chances for masking events.The second important task of the hearing system in crickets is related to sound source localization. Directional hearing in these insects is achieved by an acoustic tracheal apparatus, a sound mediating mechanical system working as a pressure difference receiver. We focused on the role of morphological differences of such a sophisticated system between a large number of species comprising hearing and non-hearing taxa and try to draw conclusions about the evolution of the structures which determine the pressure difference receiver.