Efficient communication to coordinate the actions of up to a million specialised nestmates is fundamental to the success of social insects, especially ants, which represent some of the most sophisticated animal societies.
Luca Pietro Casacci
Communication in social insects has fascinated scientists for centuries. One of the most famous examples is the dance language of honeybees, a breakthrough for which Karl von Frisch was granted the Nobel Prize in 1973. Various modalities of signalling have been identified in ants, including the predominant release of chemical substances, visual behavioural displays, tactile interactions and the emission of sounds and vibrations. Vibratory messages can be generated by using unspecialised morphological features, but at least five ant subfamilies have evolved a specialised stridulatory organ made of a ‘plectrum’ rasping across a ‘file’ (‘pars stridens’).
Until now, it was thought that acoustic cues constitute a minor part in ant communication, representing simple signals conveying alarm or used for orientation, but it has recently become clear that they can also contain additional and significant information. Furthermore, recent studies have demonstrated the ability of several social parasites to imitate ants’ language to live for long periods in close contact with their host ants.
During the project, we characterised the variation in ant acoustic signals and emission organs, disentangling the factors that have shaped the evolution of acoustic communication in ant societies. Ants were recorded by using a device that amplifies their sounds, and stridulatory organs were visualised using a Scanning Electron Microscope which allows pictures of very tiny structures to be obtained.
First, we verified if sounds vary at multiple scales, i.e. among colonies and populations, and between species or genera. We evaluated the entity of variation at colony and population levels along a latitudinal gradient (from northern to southern Europe) using Myrmica scabrinodis, a common red ant, as the model. We found that sounds are slightly different between colonies belonging to the same population but mostly differ among populations for certain sound characteristics. Second, to evaluate the acoustic differences among species and genera, we recorded and compared the sounds of 40 European ant species.
We found an enormous variability of sounds that was not explained by the weaker variability in stridulatory organs, suggesting that ants can ‘play’ very diverse signals using an ‘instrument’ which is constant in its structure across species.
In addition, the ants’ acoustic patterns do not seem to have evolved across species and genera following phylogenetic trajectories. Interestingly, our data suggested that some acoustical signal characteristics could be explained by the substrate used to build ant nests (e.g. soil, wood) or the temperature and humidity of the environments they colonise.
Finally, we investigated how vibroacoustic signals have evolved in socially parasitic ants using two model systems, the inquiline ant Myrmica karavajevi and its host ant M. scabrinodis, and the slave-making Myrmoxenus ravouxi and its host ants belonging to the genus Temnothorax. We discovered that the parasite sound has evolved towards an imitation of the host signals produced by the host queen and workers. This imitation strategy was confirmed in playback trials where the host workers reacted in a similar way when we reproduced the sound of the parasite and host queens.
Our results have shown that an enormous variability of acoustic signals exists across ant colonies, populations and species, overtaking the concept that sounds are little used in ants. Thus, the project VIBRANT has brought to light the idea that there are still many aspects to be discovered in this insect communication channel.
How did you benefit from the POLONEZ fellowship?
The POLONEZ fellowship allowed me to implement a project, conceived over years, that I particularly cared about. I had the opportunity to deepen knowledge on ant taxonomy and ecology and learn new genetic methodologies. Also, the participation in several international congresses allowed me to establish numerous collaborations with researchers to strengthen some aspects of the project VIBRANT. Finally, the countless amount of data collected during the project has paved the way for carrying out new studies and explore further perspectives on the bioacoustics of social insects.
Dr Luca Pietro Casacci obtained his PhD in Evolutionary Biology and Biodiversity Conservation at the University of Turin. Currently, he is a researcher at the Museum and Institute of Zoology in Warsaw, Polish Academy of Sciences, where he pursues several pieces of research on the investigation of vibroacoustic and chemical signals involved in ant communication. His research activities are also focused on the study of biology, ecology and conservation of diurnal Lepidoptera, with particular reference to the butterflies of the genus Maculinea, on which he studies as co-investigator in an OPUS project.