Calling song maturity in two-spotted cricket, Gryllus bimaculatus: its patterns and a possible physical explanation

Males of Gryllus bimaculatus (two-spotted cricket) emit acoustic signals by stridulating two forewings. One of their songs, calling song, plays a role in attracting females to mate, yet the significance of each song trait in attracting conspecific females remains unclear. Among such traits, the relevance of frequency component (i.e. song pitch) has been underestimated, as orthopterans had long been believed incapable of song pitch recognition. However, recent literatures suggested that ears of orthopteran species are capable of frequency recognition as mammalians do. My previous report demonstrated that their calling song recorded from mature adults has constant and pure peak frequency component around 5.7 kHz, further supporting a biological significance of the frequency component. In this study, I tracked its change over time in sexual maturity phase (i.e. from early adult phase). 300 calling songs were recorded over time from a pool of 122 adult crickets, as it required large number of animals because the crickets rarely sing at very early adult stage. A maturation process of calling song was observed, where the peak frequency distributed lower and more variable frequency in early adult phase (e.g. mean peak value was 4.9 kHz on day 3), then it gradually increased and converged to 5.8 kHz with two weeks. The coefficient of variance also decreased over the process, showing minimum around day 20. Also, I found that the young crickets (supposed to emit immature song), emit perfectly tuned calling song with 5.8 kHz peak in helium-substituted (80% Helium and 20% Oxygen) environment. These findings suggest that the robustly regulated frequency of the calling song is acquired during the early-to-mid adult stage, and it may be associated with sexual maturity of males. Also, the helium-substitution experiment suggests that physical resistance from surrounding gas molecules negatively impacts stability of calling songs of young males, implying that a muscle development and/or forewing hardening may help song maturation. This study highlights a biological significance of the frequency component, such that females may adaptively select sexually mature males based on the song trait.