Adaptive plasticity of bushcricket acoustic signalling in socially heterogeneous choruses

Condition dependence and the paradox of missing plasticity costs

Phenotypic plasticity plays a key role in adaptation to changing environments. However, plasticity is neither perfect nor ubiquitous, implying that fitness costs may limit the evolution of phenotypic plasticity in nature. The measurement of such costs of plasticity has proved elusive; decades of experiments show that fitness costs of plasticity are often weak or nonexistent. Here, we show that this paradox could potentially be explained by condition dependence. We develop two models differing in their assumptions about how condition dependence arises; both models show that variation in condition can readily mask costs of plasticity even when such costs are substantial. This can be shown simply in a model where plasticity itself evolves condition dependence, which would be expected if costly. Yet similar effects emerge from an alternative model where trait expression itself is condition-dependent. In this more complex model, the average condition in each environment and genetic covariance in condition across environments both determine when costs of plasticity can be revealed. Analogous to the paradox of missing trade-offs between life history traits, our models show that variation in condition can mask costs of plasticity even when costs exist, and suggest this conclusion may be robust to the details of how condition affects trait expression. Our models suggest that condition dependence can also account for the often-observed pattern of elevated plasticity costs inferred in stressful environments, the maintenance of genetic variance in plasticity, and provides insight into experimental and biological scenarios ideal for revealing a cost of phenotypic plasticity.

Female preference for song frequency in the cicada Mogannia formosana Matsumura (Hemiptera: Cicadidae)

In chorusing species, sound frequency has been suggested as a decisive cue for male body size in female mate choice. However, few studies on the female choice of male song frequency have been conducted in cicadas, in which males of most species sing in chorusing groups to attract females for mating. In this study, we investigated female mate choice for song frequency and body size of males of a chorusing cicada, Mogannia formosana, by phonotaxis experiments using two-choice tests and field observation, respectively. The choice proportion of effective responses and the proximity to the stimulus were used to assess the phonotaxis preference. In the field, two types of males (copulating vs. random) were caught, and their body sizes were compared. In phonotaxis experiments, the results revealed that females showed a preference for low-frequency song by approaching the low-frequency stimuli to a significantly greater proximity. By comparing the body sizes of copulating vs. random males, no significant differences were found. However, the copulating males had significantly different body shapes, as expressed by pronotum width regression by body length, from those of random males. We concluded that the sound frequency of male songs and male body shape in M. formosana can be used as cues of mate quality during female mate choice within a chorus. Additionally, we suggest that females of this species might use multiple cues for mate choice in different ranges of communication and that body size might not be the sole criterion of mate quality.

Allometric scaling in two bushcricket species (Orthoptera: Tettigoniidae) suggests sexual selection on song-generating structures

In acoustically communicating bushcrickets (Orthoptera: Tettigoniidae), most signal properties are influenced by the dimensions of the stridulatory apparatus, which in turn reflects body size and condition of the signaller. Females can assess male quality based on acoustic signals, suggesting that male stridulatory structures may be under sexual selection. We investigated scaling relationships between stridulatory structures, body size and body mass in males of the bushcricket Poecilimon veluchianus veluchianus, in comparison to the congeneric Poecilimon ampliatus. Stridulatory structures in P. v. veluchianus exhibited strong left–right correlation and coupling with body size and mass, indicating stabilizing selection for functional integration. In addition, sound-generating (the width of stridulatory teeth) and sound-radiating (mirror area on the right tegmen) structures scaled hyperallometrically to tegmen area, suggesting that both are under sexual selection. Finally, interspecies comparison revealed a steeper slope in tegmen area and stridulatory file length in relation to body size in P. ampliatus than in P. v. veluchianus, implying stronger sexual selection in the former, smaller species. Our study emphasizes the significance of a comparative allometric approach in elucidating evolutionary patterns of sound-generating and -radiating structures.