A research group including former Graduate Student Jou Yoshigai (at the time of the research), Graduate Student Keisuke Nakaoka, and Associate Professor Masayuki Senzaki at the Graduate School of Environmental Science, Hokkaido University, has demonstrated that grasshoppers (Glyptobothrus maritimus) inhabiting habitats with higher traffic noise levels possess a higher density of file teeth on their stridulatory organs. The study also revealed that this change is highly likely not genetic but rather triggered by environmental stress experienced during the nymphal stage. This finding highlights the possibility that noise impacts not only animal behavior but also physical morphology. The research results were published in the journal Communications Biology on May 5.
Noise originating from human activities is known to disrupt acoustic communication in animals. In particular, to avoid acoustic overlap with low-frequency noise, it has been reported across various species groups, including birds, frogs, and insects, that animals raise the pitch (frequency) of the sounds they emit.
For instance, birds can flexibly and instantaneously adjust the pitch of their songs by altering the way they sing. On the other hand, in animals like insects that generate sound by rubbing body parts together, the physical morphology of the stridulatory apparatus dictates the properties of the sound. Therefore, behavioral adjustments alone cannot account for the mechanism of shifting the pitch. Consequently, a hypothesis had been proposed that morphological changes must be occurring in the stridulatory organs of these animals, but this had remained unverified.
The research group examined the hypothesis using the grasshopper (Glyptobothrus maritimus), a species that produces sounds and frequently inhabits noise-heavy urban areas.
The team focused on the acoustic emissions when male grasshoppers court females. They established survey sites across 20 locations spanning five regions in Hokkaido, encompassing grasslands with diverse acoustic environments ranging from busy roadsides to tranquil parks. Between July and August of 2023 and 2024, they measured noise levels at each site and collected grasshopper specimens.
The courtship sounds of the collected male grasshopper specimens (299 individuals in total) were recorded under laboratory conditions to measure peak frequency (the frequency with the maximum volume) as an indicator of pitch. Additionally, the male specimens were photographed under a microscope to measure body size and the density of the stridulatory organ (calculated as the number of file teeth divided by the organ's length). At the same time, the team also gathered localized data on cumulative temperature and cumulative precipitation during the period from May to July, which corresponds to the grasshopper's nymphal development stage. The data gathered was considered as non-noise environmental factors that influence grasshopper development.
In the statistical analyses, the researchers examined the relationships between the different morphological traits, the relationship between peak frequency and morphology, and the impact of habitat noise levels on both morphology and peak frequency.
The results revealed that individuals living in areas with higher noise levels possessed a higher density of stridulatory file teeth. In the noisiest habitats, file tooth density increased by as much as 13.4% compared to the quietest environments. Because this structural change fluctuated depending on the year, the researchers concluded that it is not a genetic trait but rather driven by environmental factors (stressors) experienced during the developmental nymphal stage.
On the other hand, no effect of noise levels on the pitch of the songs was found. This indicates that the file tooth density change does not provide an acoustic benefit for communication of the grasshoppers under noisy conditions. Instead, the morphological alteration is presumed to be a byproduct arising from environmental stress, including noise pollution. This mismatch between the effects of noise stress on morphology versus pitch suggests the possibility that the impacts of noise are species-specific and governed by complex mechanisms.
Nakaoka commented: "This study focused on the grasshopper (Glyptobothrus maritimus), a species commonly found in grasslands across Japan. The results remind us once again that even tiny insects living by the roadside are being impacted by human noise. While we could not definitively state that there was a difference in pitch relative to noise levels in this particular study, other insect species have been reported to sing at higher pitches in noisier environments. Investigating how the morphology of those specific species changes is an intriguing question, and I hope it will be explored in future research."
Journal Information
Publication: Communications Biology
Title: Male grasshoppers (Glyptobothrus maritimus) in roadside habitats have increased stridulatory sound-producing organs
DOI: 10.1038/s42003-026-10181-4
This article has been translated by JST with permission from The Science News Ltd. (https://sci-news.co.jp/). Unauthorized reproduction of the article and photographs is prohibited.