Great green bush-cricket songs fail to reveal male strength in laboratory duels

When male great green bush-crickets gather to sing, the evening air fills with a loud, continuous trill that can be heard from remarkable distances. These choruses, however, are far from peaceful concerts. Males of Tettigonia viridissima, a large predatory katydid found across Europe, do more than broadcast courtship songs into the night. During choruses, they alter their song patterns when rivals come close, and they sometimes abandon sound altogether in favor of physical violence. Field observations have documented males attacking the body region between the pronotum and the wings, a physiologically crucial area for sound production, leaving behind visible injuries such as damaged pronota. Fighting, in other words, is a real and costly part of this species’ mating ecology.

Given the danger of such confrontations, evolutionary theory predicts that males should possess some mechanism for gauging a rival’s fighting ability before committing to a brawl. In many animal species, signals involved in male-male competition serve exactly this purpose: they allow competitors to assess each other indirectly, so that a weaker male can withdraw upon recognizing a stronger opponent, thereby avoiding serious injury or death. For acoustic animals such as katydids and crickets, the calling song is the obvious candidate for such an assessment signal. If the pitch or intensity of a rival’s song reliably indicates his body size and physical strength, then a male could simply listen, decide that he is outmatched, and fall silent, sparing both parties the wounds of combat.

This reasoning rests on a well-documented pattern in sound-producing insects. In several katydid species, larger species and larger individuals produce lower carrier frequencies than smaller ones, because the mechanics of stridulation and the morphology of the sound-producing apparatus scale with body size. Carrier frequency, the dominant pitch of the song, has therefore been proposed as a good indicator of acoustic strength in Orthoptera. A closely related species, Tettigonia cantans, provides suggestive behavioral evidence: when confronted with low-frequency playback sounds, males of that species reduce their singing and move away from the source, hinting that low pitch itself may function as a dominance cue. Whether such acoustic dominance behavior actually reflects physical strength, however, has rarely been tested empirically in bush-crickets.

A new study published in the Journal of Orthoptera Research set out to do precisely that. Working at Bielefeld University in Germany, the researcher tested two hypotheses in the great green bush-cricket. The first held that larger males exhibit greater acoustic competitiveness than smaller males. The second held that males producing lower mean carrier frequencies display higher acoustic competitiveness than those with higher-frequency calls. Acoustic dominance was defined as a behavioral advantage in acoustic competitiveness associated with differences in body size and song frequency between paired rivals. If both hypotheses held, the results would support the idea that males can assess each other’s physical strength through song alone, without any physical interaction.

The experimental design was meticulous. A total of 33 male final-instar nymphs were sampled from a meadow near Fachhochschule Bielefeld between May 20 and June 3, 2018, and reared individually in identical plastic cups at a constant temperature of 26 degrees Celsius. To minimize pre-experimental acoustic exposure among individuals, adults were moved immediately after molting to different levels of a bookshelf, where solid partitions created partial acoustic shadowing by blocking the direct sound propagation path. The photoperiod was reversed so that the insects, which naturally sing at night, would call during the day: the chamber was dark from 10 am to 10:30 pm and light from 10:30 pm to 10 am. This circadian manipulation allowed all recordings and behavioral observations to be conducted under controlled daytime conditions.

Individual calling songs were recorded between July 10 and 16, 2018, one month after the first adult molt, using an ultrasound microphone capable of capturing frequencies up to 120 kHz, connected to a recording device operating at a sampling rate of 300 kHz with 16-bit resolution. Each recording lasted roughly three minutes and took place in an anechoic chamber under dark conditions at 25 degrees Celsius. The calling song of T. viridissima is a continuous trill composed of disyllabic echemes, in which each syllable contains a low-intensity opening hemisyllable, produced by the opening stroke of the wings, followed by a very high-intensity closing hemisyllable, produced by the closing stroke. Only the closing hemisyllable was used for frequency analysis, for three reasons: females use it as the conspecific signal for mating, its intensity peaks above -10 dB ensuring robust automated measurement, and the opening hemisyllable’s maxima sit barely above the noise floor, preventing reliable extraction. The mean carrier frequency of each male was computed from every closing hemisyllable produced during the recording, yielding a per-individual representative pitch.

Acoustic competition experiments followed from July 27 to September 7, 2018, involving 19 randomly chosen individuals in 27 paired dyads. Two soft nylon meshed cages, each measuring 60 by 60 by 60 centimeters, were placed on a table in the anechoic chamber under dim red light, roughly two meters from the observer to minimize disturbance. Individuals from the preceding trial were not reused in the immediately subsequent trial, and insects reared on the same bookshelf level were never tested against each other, reducing the possibility of acoustic pre-detection. After a habituation phase of at least 10 minutes, each male’s behavior was scored using a dyad-based binary scheme: at any given moment, each male was classified as either calling or silent, with observations organized into one-minute bouts. If a male changed calling state during a bout, the states of both males were recorded at each transition. Singing activity, a proxy previously validated for sexual selection studies in this species, served as the measure of acoustic competitiveness: one point was awarded each time an individual sang, and the male with the higher total was classified as the acoustic winner.

Morphological traits were then measured to estimate body size and physical strength. Pronotum length and femur length were taken with a sliding digital caliper, and body weight was recorded on a precision balance. These parameters are widely used indicators of body size in Orthoptera. Statistical analysis using paired Wilcoxon signed-rank tests, chosen because Kolmogorov-Smirnov tests showed the residuals were not normally distributed, produced a striking series of null results. Mean carrier frequency did not differ significantly between acoustic winners and losers. Pronotum length showed no significant difference. Body weight had no significant influence on acoustic competitiveness. Femur length, too, failed to distinguish winners from losers. Across all four traits, the acoustic competitiveness of male T. viridissima could not be predicted by body size or song frequency under the conditions tested.

These null results point to a fascinating evolutionary asymmetry. Previous work by Arak and colleagues established that females of this species prefer males that sing more frequently, and that singing activity correlates with body weight, functioning as an honest signal for female choice. Yet the new findings demonstrate for the first time that the very same singing activity operates independently of physical attributes during male-male competition. The traits favored by intersexual selection, in other words, do not double as assessment cues in intrasexual rivalry. This dissociation may reflect the different physiological benefits each sex derives from fitness-related traits. Females can mate with multiple males and store sperm in their spermatheca, but sperm from only a single male is ultimately used for fertilization, a selective mechanism that may shape signaling behavior differently in each context. It is also possible that the cost of physical combat in this species is lower than expected, so that males simply do not bother using song as a strength indicator, with body size playing a role only during direct physical escalation.

The study’s authors are careful to note several caveats. The mean carrier frequencies observed in the laboratory, ranging from roughly 13 to 18 kHz, were higher than those reported from wild-caught individuals, which ranged from 10 to 12 kHz, and it remains unclear whether this discrepancy reflects biological differences between natural and laboratory populations or methodological differences in recording conditions. Captive environments may influence phenotypic expression in ways not commonly observed in nature, as suggested by the unexpectedly high proportion of brown individuals reared under laboratory conditions. The small sample size of 19 individuals and 27 dyads, with some males participating in multiple trials, may have introduced pseudoreplication, so p-values should be interpreted with caution. Future work should evaluate alternative acoustic parameters, such as the low-intensity opening hemisyllable, which other studies suggest may play a role in close-range male-male assessment even though females do not respond to it, as well as non-physical behavioral contexts, including territorial structures in which a resident male confronts an intruder, a situation known to trigger acoustic competition in the closely related T. cantans. Whether the great green bush-cricket’s song ultimately functions as a punch or an argument, the answer appears to depend on context, and the search for that context has only just begun.

Subject of Research: Acoustic dominance behavior and physical strength assessment in male great green bush-crickets

Article Title: “Punch or argument?” Is the acoustic dominance behavior of Tettigonia viridissima males reflected in their acoustical and physical strength?

Article References: Rhee, H. (2026). “Punch or argument?” Is the acoustic dominance behavior of Tettigonia viridissima males reflected in their acoustical and physical strength?. Journal of Orthoptera Research, 35(2), 329-336. https://doi.org/10.3897/jor.35.182513

Image Credits: AI Generated

DOI: 10.3897/jor.35.182513

Keywords: Tettigonia viridissima, bush-cricket, acoustic dominance, carrier frequency, male-male competition, singing activity, Orthoptera, sexual selection, stridulation, katydid, behavioral ecology, Journal of Orthoptera Research