Scientists have made a surprising discovery: day-active moths have larger hearing organs than their nocturnal relatives, despite facing less threat from echolocating bats.
The international research team set out to test whether diurnal moths - less threatened by echolocating bats - would have reduced tympanal organs. These specialised hearing organs allow moths to detect the high frequency calls bats use to locate prey at night. The researchers compared day-flying moths to their closely related, night-flying counterparts, expecting reduction or loss of these hearing organs in the diurnal species. Surprisingly, the opposite was true.
"The results were unexpected and counterintuitive, as day-flying moths are not hunted by bats. We had therefore predicted that the tympanal organs would now be functionally redundant or reduced, but that wasn't the case," says Finnish Museum of Natural History visiting researcher and lead author of the study Pritha Dey of the National Centre for Biological Sciences in Bangalore, India.
Bats locate and catch insects using echolocation, emitting ultrasonic calls to locate prey in the dark. In response to this pressure, many moths have evolved sophisticated ears, called tympanal organs, to detect bat ultrasound. When a moth hears such signals, it often escapes by flying in a spiral manner or dropping down.
Interestingly, to avoid bats, some night-flying moths have shifted their activity to the daytime and evolved into day-active species. This behavioral shift offers researchers a unique opportunity to study the sensory evolution of tympanal organs: what happens to these structures when bat predation is no longer a selective force?
State-of-the-art micro-CT scanning and 3D geometric morphometrics
In the study species-geometrid moths-the tympanal organs (i.e., ears) are located at the base of the abdomen. These organs are tiny and fragile, making them notoriously difficult to examine using traditional morphological methods. To overcome this challenge, researchers employed non-invasive 3D imaging to study the internal anatomy of these delicate structures in 19 species of Geometridae moths with unprecedented precision. The specimens used in the study were museum-preserved and sourced from different continents.
The team conducted a phylogenetically informed comparative analysis using paired night- and day-flying species. They applied geometric morphometrics to 3D reconstructions of the hearing organs to compare tympanal organ volume and the shape of the ansa-a unique elongated structure found in geometrid moths. Contrary to expectations, the diurnal species had significantly larger tympanal organs than their nocturnal ancestors, and these differences were not clearly correlated with body size.
"In this study we demonstrate a new way to utilize old museum specimens in modern research," says Max Söderholm from the Finnish Museum of Natural History, University of Helsinki, who carried out scanning and 3D modeling of the tympanal organs.
According to the researchers, these findings suggest that diurnal moths may use their enlarged ears to detect lower-frequency sounds produced by birds or reptiles. This points to a possible new function for tympanal organs and suggests a broader role for hearing in predator detection across different environments.
"Micro-CT imaging and 3D morphometrics are opening new possibilities for studying functional morphology in an evolutionary context. Combined with the in-house imaging expertise at the Finnish Museum of Natural History, these tools are allowing us to explore previously unanswered evolutionary questions," explains senior author Pasi Sihvonen from the Finnish Museum of Natural History.
