Posted on behalf of Dan Jones.
The fossilized remains of long-extinct animals provide clear evidence of their size, stature and gait, but can we ever know what they sounded like? A reconstruction of the song sung by a fossilized katydid that lived 165 million years ago shows how it can be done. The results are published in the Proceedings of the National Academy of Sciences (PNAS).
Like other crickets, male katydids (also known as bush crickets) sing to females through a technique called stridulation, in which noise is generated by rubbing a thick, ridged vein (called a file) on one wing against a ‘scraper’ on another. The length of the file, and the speed with which it is dragged over the scraper, determines the frequency of the noise it generates. The fossil katydid in the new study, dubbed Archaboilus musicus and described in the PNAS paper for first time, is exceptionally well preserved for such an old specimen, allowing for detailed measurements of the sound-making file.
The shape and structure of the file determines whether a katydid creates a pure (musical) tone of a single frequency, or ‘elaborate noise’ that ranges over a broad bandwidth of frequencies. The A. musicus fossil suggests it produced a pure, musical tone.
To predict the frequency of the tone A. musicus emitted, the team first plotted the lengths of files from nearly 60 living species of katydid against the frequency of sound they produce, showing that shorter files tend to create a lower-frequency sound. Next, this model was validated by showing that it could accurately predict the songs known to be sung by two living katydid species most closely related to A. musicus. Then it was simply a matter of seeing where A. musicus fell on this graph, and estimating the frequency of its song.
This analysis suggests that A. musicus produced a relatively low and pure tone of around 6.4 kHz (you can hear a reconstruction of the song in the video above). The team says this low tone would have been well-suited to communicating over long distances and close to the ground in the sparse vegetation of Jurassic forests. In particular, females could have picked out this pure tone above the rabble created by other creatures in the area, says Fernando Montealegre Zapata of Bristol University, UK, who is a co-author on the paper. The next challenge, he says, is to work out why the low, pure tones of early katydids evolved into the variety of higher-frequency songs and non-pure tones that are heard among katydid species living today.