ST. LOUIS — Sure, it’s hot out, but you don’t let that spoil a chance at mingling with the opposite sex. Besides, it sounds like she’s into you. Your songs are her type, and she’s answering your calls.
So goes the mating routine of treehoppers — insects that suck sap from stems and communicate with potential mates by sending vibrational signals through plants. Males initiate with an “advertising signal” and wait for females to answer with a call of their own. Without a response, they won’t be able to find a mate.
Although their signals and duets are inaudible to humans, they make quite a sound when picked up by research equipment, producing a series of clicks and unusual tones.
“Just the sheer beauty of it is really compelling,” said Kasey Fowler-Finn, an assistant professor of biology at St. Louis University who studies the insects. “Once you’re no longer constrained by having to sing through the air, you can produce really cool sounds … . I think of them as little miniature whales.”
But as insects, treehoppers rely on a specific temperature window to successfully perform those mating calls and reproduce — a quality they share with other cold-blooded species, and a seemingly risky trait in a warming world.
At higher temperatures, for instance, male treehoppers send out higher-pitched calls which may be mistaken for those from other species, or may reach a pitch that females simply may not prefer. A team of SLU researchers led by Fowler-Finn examined whether that could prove problematic for the insects with climate change and greater temperature swings in mind.
But their findings published last month show that, even in hotter conditions, treehoppers show signs of adjusting their mating rituals to avoid disruptions. The results provide a glimmer of hope about ecological resilience — at least for treehoppers — “at a time when we are increasingly concerned about how global warming will influence animals,” according to Fowler-Finn.
The study looked at insects captured from four populations of treehoppers in Missouri and Illinois. In Fowler-Finn’s lab at SLU, the individuals were exposed to a range of temperatures, while special equipment was used to listen to — and initiate — the vibrational calls and responses from males and females.
“Basically, we chat with her,” Fowler-Finn said, describing how the research team would send out recordings of male signals and wait to see if females answered at different temperatures, and which type of signal they liked the most. “(For) males, play them a male-female duet just to get them in the mood, and they respond right back.”
The experiment found that while male signals were altered by temperature, female preferences also shifted enough to attune for the change in pitch. Fowler-Finn added that an ongoing study has shown that treehoppers raised in hot environments still signal at temperatures of 115 degrees.
So should the findings inspire broader optimism about how species might adjust to climate change? Not so fast, says Fowler-Finn.
“Signals and preferences is just one part of the puzzle,” she said, noting that climate effects on food and day-to-day survival are obviously important, too. And when it comes to signaling and reproduction, she adds that more research is needed for more species before scientists can make generalizations about heat-related resilience — especially with more than 90% of acoustic insects using vibrational communication, along with other animals.
“It’s the most common form of acoustic communication in the animal kingdom,” said Fowler-Finn.
But others in the field say studies like Fowler-Finn’s mark key steps in moving toward a general understanding of how those species cope.
“We’re inching that way,” said Laurel Symes, assistant director of the Bioacoustics Research Program at Cornell University, and an author of similar research on how temperature affects tree cricket signaling. “Some pieces will be incredibly resilient and others won’t … It’s important to understand which species will be part of which, and which contexts will be part of which.”
But Symes suggested that “there is room for optimism for resilience,” and that it wasn’t too surprising to see it exhibited in treehoppers from Missouri and Illinois — where species have evolved to withstand both extreme cold and extreme heat over the course of a year. By comparison, she said it’s possible that species might not fare so well in tropical environments, where “there’s been no evolutionary history of dealing with extremes.”
Others agreed that the research, while valuable and encouraging, needs to be expanded to more types of animals.
“It does suggest that some animals have adaptation to temperature, at least for breeding,” said Damian Elias, a professor at the University of California, Berkeley, who has done similar work with two kinds of spiders. “That’s basically something we need to look at across all arthropod systems, or all cold-blooded systems.”
But at least for treehoppers, he said it clearly bodes well.
“This is hopeful that treehoppers will continue to have sex at hotter temperatures.”
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