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Around 200 million years ago, katydids, an insect group related to grasshoppers and crickets, were already singing to communicate to attract mates and hearing long-distance calls, a new study has found.
A team of international scientists that studied fossils from China, Central Asia and Africa said katydids from the Mesozoic period were the earliest known animals to have evolved high-frequency musical calls, after analysing around 100 well-preserved specimens.
The fossils, which show the earliest insect ears and sound-producing system, are from the Late Triassic discovered in Kyrgyzstan and South Africa, as well as Middle Jurassic examples found in Inner Mongolia, China.
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“In ancient times, animals did not use sound as a way of communication. But as biodiversity grew, animals evolved more tactics for survival, just like an arms race,” said Xu Chunpeng, of the Nanjing Institute of Geology and Palaeontology under the Chinese Academy of Sciences.
“Katydids were the first to have acoustic abilities. When other animals were not able to use sounds, katydids had the upper hand, especially at night when vision is limited,” he said.
Xu is first author of a paper published on Monday by a team from China, Britain, France, Germany, Russia and the United States in the peer-reviewed American journal Proceedings of the National Academy of Sciences.
“Our analysis shows that katydids are the earliest known animals to have evolved complex acoustic communication, acoustic niche partitioning and high-frequency musical calls,” they wrote.
“Katydids evolved complex acoustic communication, including mating signals, inter-male communication and directional hearing, at least by the Middle Jurassic.”
Sending and receiving messages using sound is key to the survival and success of many animals, according to the team. Creatures make sounds as warning calls, to convey food source locations and for social learning and mating rituals.
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Katydids and crickets have tympanal ears on their front legs, unlike grasshoppers and locusts which have them on their abdomen, and katydids create noise by rubbing their wings together.
Extinct animals commonly had complex acoustic behaviour, but fossils reveal little about it. The lack of well-preserved ears and sound-producing systems in fossils makes studying them difficult.
To hunt for clues about the development of acoustic communication, Xu said the team of researchers had spent years scanning specimens around the world to find well-preserved fossils.
They identified 24 katydids with their ear structures exceptionally preserved from the Middle Jurassic period and 87 specimens of Mesozoic katydids which would allow the scientists to study the sound-producing structures in the forewings.
The team then reconstructed the singing frequencies and found that ancient katydids had a diverse song repertoire with frequencies between 4 kHz and 16 kHz, close to the upper limit of human adults.
Although high-frequency songs of Mesozoic katydids would weaken rapidly over distance, they were key to avoiding eavesdroppers being beyond the upper hearing limit of most Mesozoic animals, the team said.
The scientists also found that female katydids had ears but could not produce sounds while males could communicate among themselves to resolve territorial and aggressive behaviours.
“The sounds made by Jurassic male katydids were clearly used to advertise the singer’s location and to attract mute females,” they wrote. “Males may be selected by females based on the quality of their calls, such as the temporal patterns and loudness.”
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They also said some females were likely to have evolved directional hearing to detect sounds from afar.
“Some Daohugou katydids [in Inner Mongolia] produced a comparatively low-frequency song, which is a long-distance advertising signal,” they said, adding that such a system required directional sensitivity.
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