Acoustic Signals, Mate Choice And Mate Sampling Strategies in a Field Cricket
Acoustic communication in orthopterans and anurans provides a suitable model system for studying the evolutionary mechanisms of sexual selection mainly because males use acoustic signals to attract females over long distances for pair formation. Females use these signals not only to localize conspecific males but also to discriminate between potential mates. Investigations on the effect of sexual selection on acoustic signals requires an understanding of how female preferences for different features of the acoustic signal affect male mating success under ecological constraints in wild populations. The effect of female preferences on male mating success depends on the mate sampling strategy that females employ to search for potential mates. Despite its relevance, female mate sampling strategies based on male acoustic signals have rarely investigated in orthopterans and anurans, especially in the field. Considering the elaborate knowledge of the role of sensory physiology in female phonotaxis behaviour and characterization of the male acoustic signal, I used the field cricket species Plebeiogryllus guttiventris as a model system in this study. In this thesis, I first investigated the ecology of callers in wild populations. I then investigated female mate sampling strategies by incorporating relevant information on the ecology of signalers and the sensory physiology of receivers. Amount of calling activity is a strong determinant of male mating success in acoustically communicating species such as orthopterans and anurans. While many studies in crickets have investigated the determinants of calling effort, patterns of variability in male calling effort in natural choruses remain largely unexplored. I therefore investigated the spatio-temporal dynamics of acoustic chorusing behaviour in a wild population. I first studied the consistency of calling activity by quantifying variation in male calling effort across multiple nights of calling using repeatability analysis. Callers were inconsistent in their calling effort across nights and did not optimize nightly calling effort to increase their total number of nights spent calling. Next, I investigated calling site fidelity of males across multiple nights by quantifying movement of callers. Callers frequently changed their calling site across calling nights with substantial displacement but without any significant directionality. Finally, I investigated trade-offs between within-night calling effort and energetically expensive calling song features such as call intensity and chirp rate. Calling effort was not correlated with any of the calling song features, suggesting that energetically expensive song features do not constrain male calling effort. The two key features of signaling behaviour, calling effort and call intensity, which determine the duration and spatial coverage of the sexual signal, are uncorrelated and function independently Acoustic signal variation and female preference for different signal components constitute the prerequisite framework to study the mechanisms of sexual selection that shape acoustic communication. Despite several studies of acoustic communication in crickets, information on both male calling song variation in the field and female preference in the same system is lacking for most species. First, I quantified variation in the spectral, temporal and amplitudinal characteristics of the male calling song in a wild population, at two temporal scales, within and across nights, using repeatability analysis. Carrier frequency (CF) was the most repeatable call trait across nights, whereas chirp period (CP) had low repeatability. I further investigated female preferences based on song features with high and low repeatability (CF and CP respectively). Females showed no consistent preferences for CF but were more attracted towards calls with higher rates (shorter CP). I also examined the effect of signal intensity, which is known to play a critical role in female phonotaxis behaviour, on female preferences for faster calls. Females preferred louder calls over faster ones, implying a dominant role for signal intensity in female evaluation of potential mates based on acoustic signals. Call intensity was also the only signal feature that was positively correlated with male size. In the final chapter, I investigated female mate sampling strategies based on acoustic signals using both theoretical and empirical approaches. Analytical models of mate sampling have demonstrated significant differences in individual fitness returns for different sampling strategies. However these models have rarely incorporated relevant information on the ecology of signalers and the sensory physiology of receivers. I used simulation models to compare the costs and benefits of different mate sampling strategies by incorporating information on relative spacing of callers in natural choruses and the effect of signal intensity on female phonotaxis behaviour. The strategy of mating with males that were louder at the female position emerged as the optimal sampling rule in the simulations. When tested empirically in the field using callers in natural choruses, females seemed to follow the optimal strategy of mating with males that were perceived as louder at their position.
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