Small-sample animal study finds species and geographic differences in Maltese harvester ant stridulation

A small-sample animal study found that distress-signal vibrations differed significantly among three harvester ant species in the Maltese archipelago and among sampled populations classified as Messor ibericus. The study recorded stridulation from M. capitatus, M. bouvieri and M. ibericus, with major workers of M. ibericus compared across Malta, Gozo, Sicily, Crete and Bulgaria.

The researchers collected ants by hand across Malta and Gozo in November 2017, April 2024 and March 2025 to assess species occurrence and examine specimens morphologically. Ants used for acoustic recordings were collected in April 2024 in Malta and Gozo, September 2023 in Crete, May 2024 in Bulgaria and May 2025 in Sicily. They were kept alive until indoor processing. Most analyses included three major workers per species or population. The only M. ibericus queen, collected in Gozo, was measured but excluded from statistical comparisons.

Signals were recorded with a DIY piezoelectric contact microphone and a Zoom F3 digital recorder, then processed in GNU Audacity. Measurements included chirp rate, total chirp duration, forward- and backward-part durations, frequency peaks for both parts, and the forward-to-backward amplitude ratio. Because the apparatus could not provide calibrated absolute amplitudes, the study used relative amplitudes measured under constant conditions. Analyses also used scanning electron microscopy, principal component analysis, non-parametric permutational multivariate analysis of variance, permutation tests for homogeneity of multivariate dispersions, and stepwise discriminant analysis.

When restrained with a pincer, the ants produced stridulatory signals through dorsoventral abdominal movements. A passage of the plectrum across the stridulatory field generated a disyllabic chirp with forward and backward parts, while repeated passages produced trains of chirps. The stridulatory apparatus was positioned on the dorsal postpetiole and first metasomal segment. Its pars stridens was distinctly oval, with transverse ridges that became denser posteriorly. Well-developed tegumentary “pillars” beneath the pretergite margin were visible in M. ibericus; previous work has proposed that such structures may amplify sound, but this study did not test that function.

The three Maltese species differed significantly in their measured acoustic characteristics, with PERMANOVA yielding F = 15.96 and p(MC) < 0.001. Post-hoc tests distinguished every species pair. The first two principal-component axes explained 84.7% of the variation. M. bouvieri had the longest mean chirps at 428.22 milliseconds and the slowest mean rate at 2.31 chirps per second. M. capitatus had the highest mean frequency peaks—3777.89 Hz for the forward part and 3723.44 Hz for the backward part—whereas M. ibericus had the fastest mean rate at 2.98 chirps per second and the shortest mean chirp duration at 295 milliseconds.

Stepwise discriminant analysis identified chirp duration, chirp rate, forward-to-backward amplitude ratio and backward-part duration as the most influential variables separating the Maltese species. Cross-validation yielded 100% classification accuracy, but the authors cautioned that the small samples may not capture the full range of population-level acoustic variation. M. bouvieri produced the longest, slowest and most stereotyped signals across most parameters. M. capitatus showed greater variation in the durations of the two chirp parts, while M. ibericus had the lowest variability in chirp rate and a longer forward than backward part.

Acoustic characteristics of M. ibericus major workers also varied significantly among the five localities, with F = 60.76 and p(MC) < 0.001. Pairwise tests distinguished every population pair except Malta and Bulgaria. Sicily had the highest mean chirp rate at 6.33 chirps per second, followed by Crete at 5.41, while Malta, Gozo and Bulgaria were near 3 per second. Mean total chirp durations were 160.33 milliseconds in Sicily, 169.89 in Crete, 312.67 in Malta, 276.44 in Gozo and 302.78 in Bulgaria. Gozo had the highest mean frequency peaks at 3646.22 and 3779.33 Hz for the forward and backward parts, respectively, while Bulgaria had the lowest at 3105.67 and 3032.56 Hz.

The occurrence records clarify the status of M. ibericus in Malta and Gozo, where populations had previously been attributed to M. structor before the 2018 taxonomic revision of that species group. The new Maltese localities are among the southernmost documented for M. ibericus. At the sampled sites, nests occurred in a dry valley in Malta, xerothermous grassland in Sicily, an urban setting in central Plovdiv, a garden in Crete and crevices in sealed urban surfaces in Ta’ Pinu, Gozo. Except at the urban sites, nests were beneath stones. In Gozo, workers and queens were also observed outside their nest at night.

Although the study was neither genomic nor taxonomic, it used the established name M. ibericus for workers, noting recent evidence that these workers are hybrids derived from M. ibericus queens and M. structor males. Overall, the limited samples support differences in recorded stridulation among the three Maltese species and among sampled M. ibericus populations. The functional role of the signals remains unresolved, as does the origin of the geographic variation. The authors noted that chirp rate and duration can depend on factors such as temperature and physiological state, but those factors were not assessed directly in these ants.

Subject of Research: Comparative analysis of stridulation signals in Maltese and geographically separated Messor harvester ant populations

Article Title: Vibroacoustic signalling of the Maltese Messor ants ( Hymenoptera , Formicidae ), with geographical insights into M. ibericus

Article References: Lapeva-Gjonova, A., Gjonov, I., Kostova, R., & Mifsud, D. (2025). Vibroacoustic signalling of the Maltese Messor ants (Hymenoptera, Formicidae), with geographical insights into M. ibericus. Journal of Hymenoptera Research, 98, 1087-1106. https://doi.org/10.3897/jhr.98.170415

Image Credits: AI Generated

DOI: 10.3897/jhr.98.170415

Keywords: Messor ibericus, harvester ants, vibroacoustics, stridulation, Maltese archipelago, acoustic signals, animal behavior, geographic variation, Formicidae, Vibroacoustic, signalling, Maltese