Sensory Ecology Of Acoustic Communication In The Tropical Bushcricket Genus Mecopoda : Mechanisms And Evolution Of Synchrony
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In this thesis, I characterise five species of the bushcricket genus Mecopoda with respect to their acoustic signals and morphology and investigate the phenomenon of acoustic synchrony in one of these five species: Mecopoda ‘Chirper’. In several bushcricket species, individual males synchronise their chirps during acoustic interactions. Synchrony is imperfect and the chirps of the males lead or follow each other by a short duration of time. Imperfect synchrony is believed to have evolved in response to female preferences for leading chirps. This model of the evolution of acoustic synchrony, however, depends on assumptions about the physiological mechanism of acoustic interaction between individual males and also on the presence of selective attention to a restricted number of neighbours. Neither of these two assumptions has previously been investigated in the species Mecopoda ‘Chirper’ and the implications of these for the evolution of acoustic synchrony is unknown. Furthermore, the advantage of leading chirps for males in the field, given trade-offs between the advantages of lead and intensity and the spacing of males has not been investigated in any bushcricket. In this thesis, I address these questions using a combination of behaviour, neurophysiology and computer simulations. Five distinct song types of the genus Mecopoda were found in Southern India. Four were morphologically indistinguishable. Some of them were both sympatric and had synchronous breeding seasons. The songs of these five song types ranged from simple short chirps to highly complex songs with multiple components. The temporal patterns of the songs were very distinctive, whereas their spectral features were similar. Component elements of the different songs were distinct despite overall similarity. The song types possibly represent sibling species. The mechanism underlying synchrony in the species Mecopoda ‘Chirper’ differs from previously reported mechanisms in that it involves both a change in the oscillator’s intrinsic rate and resetting on a chirp-by-chirp basis. The form of the phase response curve differs from those of previously reported firefly and bushcricket species including the closely related Malaysian species Mecopoda elongaa. Simulations exploring oscillator properties showed that the outcome of pairwise interactions was independent of initial phase and alternation was not possible. Solo intrinsic chirp period was a relatively good predictor of leading probability. Changing the intrinsic period during interactions could, however, enable males with longer periods to lead during acoustic interactions. In choruses in the field, chirp period and hence lead probability of males had low repeatability with no consistent ‘leaders’ and ‘followers’ across nights. During acoustic interactions, sixty percent of followers called oftener when leaders were not calling. Spacing enabled some softer males to gain areas where they were the loudest of all males in a chorus. Thus followers in choruses potentially have different strategies at their disposal by which they could offset their disadvantage in attracting mates. In neurophysiological experiments investigating selective attention, louder, leading chirps were preferentially represented in the auditory system but the representation of softer following chirps was not completely abolished. Following chirps that were 20 decibels louder than leading chirps were better represented than leading chirps. During acoustic interactions in response to playback, males synchronised with leading chirps even when the following chirps were 20 dB louder. Males did not restrict their attention to louder chirps during interactions but were affected by all chirps above a particular threshold intensity . In the field, males on average had only one or two neighbours whose calls were above this threshold. Selective attention in this species is thus achieved through spacing rather than neurophysiological filtering of softer signals. A simulation that combined these results to investigate the evolution of synchrony, revealed that the strategy of responding to one’s neighbours and synchronising with them was able to invade a population of males that did not respond to their neighbours only upto a point. The responder strategy was also not evolutionarily stable and could be invaded by the non-responder strategy. In both cases, the population stabilised at a point where the number of males with either strategy were approximately equal. This was true even if a different physiological resetting mechanism was assumed. The results also held true across a range of male aggregation patterns and different possible female preference values for time-intensity trade-off and lead windows. Thus it appears that though a responder strategy can spread in a population up to a proportion of approximately 0.5, it cannot completely take over a population, if the only selective advantage to synchronising males is due to female preference for leading chirps.