Asymmetry in the song of crickets: Preferences of females and proximate mechanism of discrimination

Abstract

SUMMARYSubtle random deviations from perfect symmetry in bilateral traits are suggested to signal reduced phenotypic and genetic quality of a sender, but little is known about the related receiver mechanisms for discriminating symmetrical from asymmetrical traits. Here, we investigated these mechanisms in behavioural and neurophysiological experiments in the Mediterranean field cricket, Gryllus bimaculatus. A downward frequency modulation at the end of each syllable in the calling song has been suggested to indicate morphological asymmetry in sound radiating structures between left and right forewings. Even under ideal laboratory conditions on a trackball system, female crickets only discriminated between songs of symmetrical and asymmetrical males in two-choice experiments at carrier frequencies of 4.4 kHz and a large modulation depth of 600 and 800 Hz. Under these conditions they preferred the pure-tone calling songs over the modulated (asymmetrical) alternative, whereas no preference was observed at carrier frequencies of 4.9 and 5.2 kHz. These preferences correlate well with the responses of a pair of identified auditory interneurons (AN1), known for their importance in female phonotaxis. The AN1 interneuron is tuned to an average frequency of 4.9 kHz, and the roll-off towards lower and higher frequencies determines the magnitude of responses to pure-tone and frequency-modulated calling songs. The difference in response magnitude between the two neurons appears to drive the decision of females towards the song alternatives. We discuss the relevance of song differences based on asymmetry in the morphology of song-producing structures under natural conditions.

Selective phonotaxis of female crickets under natural outdoor conditions

“…However, their results are confounded by the fact that one cannot judge the discriminatory potential of receivers from preference functions obtained in no-choice trials, since females will track a male calling song for a large range of carrier frequency, given the SPL is well above the behavioural threshold for phonotaxis. Indeed, no-choice trials for carrier frequencies of 4.4, 4.9 and 5.2 kHz did not reveal any significant difference in the absolute amount of lateral steering on a trackball system (Hirtenlehner et al 2013 ). In a choice situation, however, the relative intensities of the two signals become highly relevant as a result of the underlying tuning of neuronal elements (Kostarakos et al 2008 ).…”

The potential influence of morphology on the evolutionary divergence of an acoustic signal

“…As both frequency (Libersat et al ., ; Hennig & Weber, ) and temporal structure (Stabel et al ., ; Balakrishnan & Pollack, ; Hennig & Weber, ) are important in both species recognition and female choice, such an effort might allow for more synthesis between biophysical and evolutionary approaches to cricket sexual signalling. In addition to the temporal patterning and dominant frequency of calls that we did measure, there are other potentially important call traits that we did not, for example frequency modulation (Simmons & Ritchie, ; Desutter‐Grandcolas, ; Hirtenlehner et al ., ). This is a particularly interesting avenue for future research in the light of evidence that females may be able to infer information about the fluctuating asymmetry of males' wings from the frequency modulation of their calls (Simmons & Ritchie, ; Hirtenlehner et al ., ).…”

“…In addition to the temporal patterning and dominant frequency of calls that we did measure, there are other potentially important call traits that we did not, for example frequency modulation (Simmons & Ritchie, ; Desutter‐Grandcolas, ; Hirtenlehner et al ., ). This is a particularly interesting avenue for future research in the light of evidence that females may be able to infer information about the fluctuating asymmetry of males' wings from the frequency modulation of their calls (Simmons & Ritchie, ; Hirtenlehner et al ., ).…”

Wing resonances in a new dead-leaf-mimic katydid (Tettigoniidae: Pterochrozinae) from the Andean cloud forests

“…FM 397 has been a widely reported phenomenon observed in the call of insects, mostly in crickets and 398 katydids. It was originally reported by Leroy, 1966 in crickets, and subsequently followed by 399 measurements of Morris and Pipher, 1967 the function of FM in the calling song of many species and the only existing neuro-406 ethological study in crickets suggests that females prefer non-modulated calls over modulated 407 ones (Hirtenlehner et al, 2013). 408…”