Cicadas in southern India have been found to start their dawn chorus with remarkable accuracy, timing their song to a precise level of light during the pre-dawn hours.
Researchers discovered that the insects begin singing when the sun is exactly 3.8 degrees below the horizon – a moment known as civil twilight.
Recording cicada songs
The study, published in the journal Physical Review E, involved several weeks of field recordings from two sites near Bengaluru (formally Bangalore), the capital of India’s southern Karnataka state.
Site one was a shrubland area with scattered grasses and site two was a bamboo forest. The researchers focused their study on choruses produced by the species Platypleura capitata.
Using specialised tools, the team were able to reveal just how closely cicada singing is linked to light changes.
“We’ve long known that animals respond to sunrise and seasonal light changes,” says co-author professor Raymond Goldstein from Cambridge’s Department of Applied Mathematics and Theoretical Physics. “But this is the first time we’ve been able to quantify how precisely cicadas tune in to a very specific light intensity – and it’s astonishing.”
The researchers found that the insects’ loud chorus takes just 60 seconds to reach full volume each morning. The midpoint of that build-up happens at almost the same solar angle every day, regardless of when sunrise occurs. On the ground, the light at that moment varies by about 25%, underscoring the insects’ precision.
This is the first time we’ve been able to quantify how precisely cicadas tune in to a very specific light intensity – and it’s astonishing.
To understand more about this phenomenon, the team created a mathematical model inspired by magnetic materials, where tiny units align with both an external field and each other. In the cicadas’ case, individuals appear to base their decision to sing on both the light level and the sound of nearby insects.
“This kind of collective decision-making shows how local interactions between individuals can produce surprisingly coordinated group behaviour,” says co-author professor Nir Gov from the Weizmann Institute.
The recordings were made by Bengaluru-based engineer Rakesh Khanna, who studies cicadas as a personal passion. Khanna worked with Goldstein and Dr Adriana Pesci at Cambridge to turn his observations into a formal analysis.
“Rakesh’s observations have paved the way to a quantitative understanding of this fascinating type of collective behaviour,” says Goldstein. “There’s still much to learn, but this study offers key insights into how groups make decisions based on shared environmental cues.”
Top image: sunrise over Indian landscape (not the study site). Credit: Getty
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