Hearing has evolved independently many times in the animal kingdom and is prominent in various insects and vertebrates for communication and predator detection. Among insects, katydid ears are unique, as they have evolved outer, middle, and inner ear components, analogous in their biophysical principles to the mammalian ear. In new research, paleontologists from the University of Lincoln and elsewhere reconstructed the geometries of the outer ear components and wings of Eomortoniellus handlirschi, an exceptionally well-preserved katydid fossilized in a piece of 44-million-year-old Baltic amber. They found that Eomortoniellus handlirschi was communicating at a peak frequency of 32 kHz and demonstrated that the ear was biophysically tuned to this signal and to providing hearing at higher-frequency ultrasounds (over 80 kHz), likely for enhanced predator detection. The results indicate that the evolution of the unique ear of the katydid, with its broadband ultrasonic sensitivity and analogous biophysical properties to the ears of mammals, emerged in the Eocene epoch.
Around 44 million years ago, an adult male of Eomortoniellus handlirschi became stuck in pine tree resin, which hardened encasing the bug.
The amber specimen was uncovered in 1936 in an area then known as East Prussia, Germany.
CT scans of the insect revealed that the sap had got into the katydid’s ear canal, located on the inside of its legs.
With the sap inside, the delicate structure of the insect’s ear was well-preserved.
“This discovery wouldn’t have been possible without such a well-preserved katydid, which highlights how important museum collections are in discovering specimens like these,” said Dr. Charlie Woodrow, a former Ph.D. student at the University of Lincoln who is now a researcher at Uppsala University.
“This katydid was frozen in time at a crucial moment in the arms race between echolocating predators and insects.”
“Shortly before this animal was fossilized, bats had developed the ability to echolocate, which may have driven the katydids to call at higher frequencies.”
“At the same time, their ears were adapting to listen out for bats trying to hunt them down.”
Evidence suggests that katydids developed the ability to send mating calls at increasingly higher frequencies to go undetected by their mammal predators, until they moved into ultrasonic sound.
These sounds would have gone undetected until the first bats evolved to use laryngeal echolocation around 52 million years ago.
“While katydids were likely already exploring high frequencies, both to avoid eavesdropping and to develop a greater diversity of signals, bats gave them a new impetus,” said University of Lincoln’s Professor Fernando Montealegre-Zapata.
“It might seem strange that katydids kept singing at these high pitches once they could be overheard, but ultrasound dissipates quickly in the environment.”
“This ensures that a distant bat won’t hear the singing katydid as the sound will break up before it can be heard.”
“The morphological and physiological characters that these insects evolved in response to mammalian predators, mainly bats, are the current main traits that define the entire bush cricket family (Tettigoniidae), which originated also in the Eocene.”
Using models of how sound travels and how living katydids produce sound, the authors were able to reconstruct the mating signal of Eomortoniellus handlirschi.
They calculated that the katydid was probably best at hearing sounds of around 30 kHz.
This suggests that the insect’s hearing evolved to be best at listening in to the calls of its species, giving the best chance of drawing couples together for mating.
The researchers also found two other peaks in the katydid’s hearing at around 60 and 90 kHz.
This was likely to help the insect tune in to any nearby echolocation calls of early bats, which were around 40-65 Hz.
The ability of the insects to listen to high frequencies would have been enhanced by the katydid’s pinnae, or earlobes in mammals.
While only partly developed in Eomortoniellus handlirschi, evolution in the years after it was trapped in amber has allowed its relatives to listen to calls of over 100 kHz.
“It’s now important to identify more fossils to track these changes,” Dr. Woodrow said.
“I think that more adults of this species, or its close relatives, will turn up as many are sold online to private amber collections as well as being held in public collections. It just takes the right people to notice it and study it.”
The team’s paper was published in the journal Current Biology.
Charlie Woodrow et al. An Eocene insect could hear conspecific ultrasounds and bat echolocation. Current Biology, published online November 13, 2023; doi: 10.1016/j.cub.2023.10.040