“Purring crickets: The evolution of a novel sexual signal”

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Robin M. Tinghitella, E. Dale Broder, Gabrielle A. Gurule-Small, Claudia J. Hallagan, and Jacob D. Wilson (Dec 2018)

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Male crickets evolve novel ‘purring’ sound that attracts female mates. Are they crick-cats?

How do new conversations begin?

A purring male from Molokaʻi.
(Credit: E. Dale Broder)

Like us, many organisms communicate with one another using signals that can take the form of sounds, smells, colors, or behaviors. Communication between males and females of the same species is often required for reproduction, so when mating signals change dramatically there are important implications for speciation. Like any question about evolutionary change, when we study animal communication we usually do that by comparing species or other groups that have differed in signals for quite some time. But we cannot test definitively for a role of mating signals in creating and maintaining diversity unless we find a signal that has just evolved so that we can measure the signal and the receiver’s response to that signal at “time 0”, and then track the changes over time.

Three of the researchers enjoying the fact that cricket field work happens at night (from left to right: Robin Tinghitella, Jacob Wilson, and E. Dale Broder).
(Credit: E. Dale Broder)

Robin Tinghitella, Dale Broder, and colleagues recently discovered just such a situation when they visited the Kalaupapa National Historical Site on the Hawaiian island of Molokaʻi. Tinghitella has studied field crickets in Hawai‘i for 15 years and has been interested in the songs males produce by rubbing their wings together to attract females. Last May, just after dusk, she collected crickets on a lawn in Kalaupapa for the first time. She took them back to her room where she noticed a quiet purring sound. She instantly wondered whether she had a feline visitor. But the sound was coming from her box of crickets. In this unique place where there was no background anthropogenic noise, she was able to hear a totally novel cricket song for the first time! The Tinghitella lab group is calling the males who make this novel sound purring crickets—or crick-cats! Back in the lab, they began recording and characterizing the new song. It is much quieter, more broadband, and has a higher average peak frequency than a typical song from a male of this species. They also wondered if females could hear the new sound and use it to locate males. They played recordings to females from a speaker 1 meter away and found that about half of them could locate the speaker. This is such an exciting discovery—males are singing a brand new song and females can hear it and use it! In future work, the team will track this system over time to see whether and how the purring song coevolves with female receivers in real time—evolution in action! This one of very few discoveries of a novel sexual signal, and they will use it to better understand how new conversations begin and what they mean for boundaries between species.


A purring male singing to attract mates.
(Credit: E. Dale Broder)

Abstract

Opportunities to observe contemporary signal change are incredibly rare, but critical for understanding how diversity is created and maintained. We discovered a population of the Pacific field cricket (Teleogryllus oceanicus) with a newly evolved song (“purring”), different from any known cricket. Male crickets use song to attract females from afar and to court females once near. Teleogryllus oceanicus is well-known for sexual signal evolution, as exemplified by a recent signal loss. In this study, we characterized the new purring sound and investigated the role of the purr in long distance and short distance communication. The purring sound differed from typical ancestral calls in peak frequency, amplitude, and bandwidth. Further, the long-distance purring song facilitated mate location, though the role of courtship purring song is less clear. Our discovery of purring male crickets is an unprecedented opportunity to watch the emergence of a newly evolved sexual signal unfold in real time, and has potential to illuminate the mechanisms by which evolutionary novelties arise and coevolve between the sexes.