In a quiet, grassy field isolated by the tallest sea cliffs in the world, scientist Robin Tinghitella crouched in search of the Pacific field cricket.

What she found on Molokai’s remote Kalaupapa peninsula that night in May 2017 was a rare exhibition of real-time evolutionary change in the cricket’s courtship song.

Instead of the animal’s typical honeyed trill, the Kalaupapa crickets reverberated a sound that Tinghitella likens to a cat’s low, guttural purr.

Scientists have discovered a rapid evolutionary change in the sound of the Pacific field cricket’s long-distance call and courtship song. They liken the new evolved sound to a cat’s purr; others compare it to the clink of a typewriter.

E. Dale Broder

“I started looking around for a cat, and then I realized the sound was coming from the crickets,” said Tinghitella, who studies animal mating signals.

In a knock to Darwin’s theory of evolution as a slow, gradual process, the purring crickets are evidence of evolution as a rapid, adaptive response to environmental change. Capping a year of research, Tinghitella and her colleagues published their findings this month in the scientific journal The American Naturalist

“Evolution is not something that happened in the past,” said Tinghitella, a researcher and assistant biology professor at the University of Denver. “It’s happening now. It’s something that we can actually observe in real time and dig our teeth into and understand.”

Resonating From The Wing

Pacific field crickets are found on Oahu, Kauai, the Big Island and Molokai. Native to Australia, they are thought to have arrived in Hawaii around the same time the Polynesian voyagers made landfall. Possibly they were intentional cargo on the voyagers’ sailing canoes, Tinghitella said.  

The species’ wing structure dictates the sound it makes. Courting songs and long-distance calls generate through a file and scraper mechanism, similar to how a human might produce sound by dragging a thumb across the teeth of a comb.

The crickets’ file, or comb, is located on the underside of one of its wings. The other wing sports a hardened region. When those two body parts rub together, a harp-shaped structure on the wing resonates the sound, broadcasting it into the environment.

The defining difference between the purring cricket and the typical cricket is that these features are smaller.

What triggered the change is still unclear. It could be that the song preferences of female crickets in this isolated population have morphed, and the new courting song is an attempt by male crickets to develop a more appealing tune to lure mates.

Only about half of the female crickets in the Kalaupapa population respond to the new purring song, Tinghitella said. Those that don’t either can’t hear it or don’t like it.

Tinghitella said she expects the sound preferences of the females to continue to shape the song over time.

“There’s a lot of variation among the courtship songs of the different males in this group, from the sound of a cat purring to what some of my friends describe as Skeletor crickets, because the sound is like bones hitting together,” said Tinghitella, who has been studying the Pacific field cricket in Hawaii for 15 years. “Others sound like typewriters. So we think we caught this new sound in one of its earliest stages. Evolution hasn’t really had time yet to narrow in on what this sound is going to be like for the longer term.”

The Silent Treatment

There’s also evidence to suggest that the trigger could be less about the fickle tastes of female crickets and more about survival.

In addition to the purring crickets in Kalaupapa, there are some small populations of Pacific field crickets on Kauai and Oahu that have gone silent. No longer able to produce sound, these crickets have evolved in response to the parasitoid fly, which has learned the typical courtship song of the male cricket. The pregnant female fly will use the song to locate the cricket and then spray larvae on and around its body. The larvae burrow into the cricket’s body cavity, eating the cricket from the inside out — organs and all.

Scientists think the male cricket’s evolved purring song could be a response to changing sound preferences of female crickets.

E. Dale Broder

“This always results in death for the male cricket,” Tinghitella said. “So it could be that as another way of avoiding the fly, instead of going silent, these crickets have developed the new purring noise as their own private mode of communication where they can communicate with their own species but avoid being detected by this predatory fly.”

Natural examples of rapid evolution are rare, but they are becoming more common as plant and animal life responds to human-induced environmental factors like urbanization and climate change.

Another rapid evolution change Tinghitella is studying occurs in the threespine stickleback, a small, freshwater fish found across the Northern Hemisphere. A typical male threespine stickleback has a bright red throat. But in several populations in Washington state, males have shed the red coloration, which functions to attract a female mate.

Tinghitella is studying why the female fish are willing to mate with these males that have completely lost one of the species’ signature mating signals. She’s also trying to answer whether the fish will eventually split into two distinct species — one with a red throat, and another with a uniformly dark body.

The cause of the threespine stickleback’s evolutionary shift is still unknown.

“One of the things that climate change is doing, in addition to changing the climate itself, is leading changes in species distribution,” Tinghitella said. “And that means that species that wouldn’t have encountered one another previously are coming into contact and interacting with one another — and those interactions can cause rapid evolution.

“So it’s hopeful, in a sense, because it suggests that sometimes animals and plants can keep up with our rapidly changing environment.”

In recent months, Tinghitella has located more purring crickets in Hawaii. There’s a population of purring crickets in Wailua on Kauai and another on the campus of Brigham Young University-Hawaii.

“Are we going to end up with multiple, isolated groups of these Pacific field crickets that only mate with crickets that put out a certain mating signal — purring, silent or typical?” asked Tinghitella. “Because if we do, that’s what leads to speciation and the origins of biodiversity. Instead of a single species, we could end up with three.”

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