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Earliest insect ears reveal the soundscape of the dinosaur age
Katydids are the earliest known animals to evolve complex communication using sound.
Fossils reveal how these bush crickets likely dominated ancient soundscapes when dinosaurs were roaming the Earth and may have driven the co-evolution of improved hearing in insects and mammals.
Acoustic communication has played an essential role in the evolution of animals. From mating calls and warning signals to species recognition and social learning, these signals are widespread among insects and vertebrates.
As a result, the modern-day soundscape has become an incredibly diverse and complex array of different sounds.
Piecing together how our modern soundscape evolved is challenging. Sound-producing organs are rarely well-preserved in the fossil record, making it difficult to reconstruct ancient acoustic signals.
A new paper published in the Proceedings of the National Academy of Sciences carried out a detailed investigation of exceptionally well-preserved fossil katydids from the Mesozoic Era, also known as the age of the dinosaurs.
Researchers studied the earliest known insect ears and sound-producing organs in early katydids. They found that these animals may have evolved calls to avoid detection by predators and this could have helped drive the co-evolution of hearing in their mammalian predators.
Professor Edmund Jarzembowski, an associate scientist at the Museum and co-author of the study, says, 'We wanted to look at sound evolution of land animals, especially insects and tetrapods, after the Permo-Triassic turnovers and extinctions.'
'We discovered that bush crickets were the first on the scene, making musical calls using their wings as instruments and listening with ears on their legs. Moreover, the bones of early mammals show that their hearing had improved by the Jurassic, likely due to predatory eves-dropping on insects.'
The fork-tailed bush katydid is a living species of katydid found in north and central America. Image © Paul Reeves/Shutterstock
How do katydids communicate?
Over 100 million years ago, a long before the sound of vertebrates such as birds and frogs filled the air, ancient forests were dominated by the chirping of insects.
Insects were the first land animals to communicate by sending sound waves through the air, which enabled them to communicate over longer distances.
Katydids are grasshopper-like insects that use airborne soundwaves to communicate. In most species, the males produce sound by rubbing together modified front wings, called tegmina.
Each wing has a modified vein called a stridulatory file. This contains ridges which produces sound by rubbing against an area on the other wing called a scraper. The wings also have an area referred to as the mirror, which amplifies and radiates the sound produced. The size of the katydid, the spacing of ridges and the width of the scraper all determine the type of sound made by each species.
Other katydids hear these sounds using structures called tympanal ears located on their legs. They contain an eardrum similar to modern mammals but without ear bones.
The sound-producing stridulatory files (sf) on the wings of Triassic Katydids (Image A-C) and the tympanal ears (te) on the legs of Jurassic Katydids (Image D-E). Image © NIGPAS
Reconstructing ancient soundscapes
Katydids are an ideal subject to study acoustic evolution in insects because they have a rich fossil record dating back to the Triassic. Furthermore, their wings are amongst the best-preserved instruments in the natural world.
An international team of paleoentomologists carried out a detailed investigation of 24 fossil katydids found in China. They discovered tympanal ears dating to 160 million years ago, making them the earliest-known insect ears.
The researchers also studied the fossil wings and sound-producing apparatus on 87 fossils from China, South Africa and Kyrgyzstan dating from 150 to 240 million years ago during the Middle Triassic to the Middle Jurassic.
Reconstructing the sounds produced by katydids revealed they had evolved a high diversity of singing frequencies, including high-frequency calls, by at least the Late Triassic around 200 million years ago.
By at least the Middle Jurassic, they had evolved complex acoustic communication, including mating signals, communication between males and directional hearing.
The origins of some key acoustic evolutionary events according to fossil evidence. Image © NIGPAS
Did katydids drive the evolution of mammals?
The katydid's high-frequency, short-range musical calls allowed them to communicate over shorter distances. This would have been above the upper hearing limit of most Mesozoic animals and could have acted as a mechanism for avoiding predators listening in to their calls.
Concrete evidence of high-frequency communication was previously unknown in the Mesozoic, making katydids the earliest known animals to evolve this acoustic signal. This coincided with a hearing improvement in early mammals, supporting the theory that acoustic communication may have coevolved in mammals and katydids.
The high-frequency songs of Mesozoic katydids could have driven the evolution of the intricate hearing systems in early mammals. Conversely, mammals with progressive hearing ability could have exerted selective pressure on the evolution of katydids.
This study demonstrates the significance of insects in the Mesozoic soundscape, which has been largely unknown in the palaeontological record.
'This research has implications for sexual selection in crickets and shows adaptation to a landscape buffered with woody trees during the rise of the dinosaurs,' says Edmund.
'We realise that the Mesozoic soundscape was filled with chirruping and the real challenge now is to reconstruct the tunes played by these diverse classical musicians!'
