- Cicadas, a common insect variety, sing in chorus just before dawn every day. Individuals respond to the ambient sunlight, and then others join in the chorus.
- The evolutionary reasons for either individual or collective behaviour are unclear.
- The collective behaviour mirrors a similarity with particles in a magnet, but it’s not clear if the similarity says something fundamental about nature or is just a coincidence.
Rakesh Khanna’s pre-dawn search of unique bird sounds in the shrublands, grasslands, and bamboo forests in Hesaraghatta, northern Bengaluru, led him to a serendipitous discovery of the chorus of cicadas just before dawn. It was as if the whole grassland had woken up before the Sun. “It was mesmerising and unexpected — I hadn’t realised insects could create such a powerful soundscape at that hour,” he said.
Khanna didn’t know what he was hearing at first, but being an engineering professional, he began analysing the recordings. He calculated the characteristic frequency patterns and intensity of the chorus, which matched those of cicadas — a commonly found insect family generally active during the day. The sound of cicadas is well-known in various cultures. The males are also known to sing together.
For two weeks in April and May 2023, Khanna recorded the cicadas’ loud chorus. Back home, he was charting the pattern, and that made him curious. “When I plotted the data and saw the chorus rise at almost the same time every day, I was intrigued,” he said. That’s when he reached out to Raymond E. Goldstein, a professor at the University of Cambridge and went back to record the chorus of the cicadas in a different location across the Hesaraghatta lake. Over the next two years, the group, now comprising Khanna and researchers at the University of Cambridge and the Weizmann Institute of Science, showed that every day, the insects were triggered by the same level of sunlight just before dawn. What’s more, the cicadas coordinate with each other just like particles in a magnet. Their work was published in August in the Physical Review E journal.
Natural alarm clock
Khanna found it overwhelming when, day after day, he heard cicadas singing in unison at just about the same time. His curiosity and follow-up led him to confirm his hunch. Cicadas were, indeed, starting to sing together exactly at the same time every day — when the sun was just 3.8° below the horizon. On clear days, the chorus began at the same level of ambient sunlight. “We determined chorus onset times from the audio recordings and then used standard sunrise and twilight timings to calculate the solar elevation at those exact moments,” said Khanna.
The analysis also revealed that on cloudy days, the onset of the deafening chorus is delayed, indicating that it’s not the time of day that triggers the phenomenon, but rather the intensity of ambient sunlight.
“Cicada choruses begin later on cloudy days, indicating that individuals rely heavily on sensory information about how well illuminated their environment is,” said Swastika Issar, an ecologist at the National Centre for Biological Sciences (NCBS), which is located about 20 kilometres from the Hesaraghatta grasslands. Moth biologist Pritha Dey, also from NCBS, pointed out, from her field experiences, that this particular behaviour of male cicadas is not common across species, but limited to specific cicada species only. Neither was involved in the study. “Their internal clock tells them the general time to be active, while cues like dim light and warm temperatures trigger calling,” confirmed Saumya Gupta, another independent ecologist from the University of Washington.
Collective hum
Although cicadas are known to sing in unison, the study shows the striking behaviour of the cicadas interacting with each other — when a few cicadas sing, the others join. The amount of ambient light first triggers the call of individual cicadas. Then, each individual sings for about 20 minutes. But immediately after a few individuals start to sing, others join the chorus, which “takes 60 seconds to stabilise,” Khanna clarified.
The cicadas were distributed across large swathes of the grassland, so a large number of them must participate in the chorus to deliver the collective hum. The international team carried out further analysis to rule out the possibility that all individuals were triggered by ambient sunlight. To do so, they tested Khanna’s data from two individual sites in Hesaraghatta with a simple mathematical model of interacting individuals, much like how atoms interact collectively to create magnetism. The model predicted exactly how the intensity varied with time in the cicadas’ choruses.
“Despite local variations in vegetation and clouds, they all start singing at essentially the same light threshold, which suggests a collective and precise decision-making mechanism,” said Khanna.
Analogy or deeper significance?
Spin, a fundamental property of particles like electrons, aligns or misaligns in a solid to determine whether the solid behaves like a magnet or not. For example, iron behaves like a magnet below a certain temperature, while above it, it doesn’t. The cicadas exhibit the same behaviour. Before and after the sunlight reaches the threshold, individuals transition from a state of collective quiet to a collective chorus, triggered not only by sunlight but also by other cicadas’ calls.
“The similarity is not superficial or coincidental — it reflects a deeper, shared principle between physical and biological systems… how large-scale collective behaviour emerges from simple local interactions,” said Khanna. Nature, he said, employs the same organising principles across various systems, such as atoms in a magnet, neurons in the brain, or cicadas in a forest. When individuals interact and respond to shared stimuli, collective synchronization and abrupt transitions can emerge spontaneously, he said.
Deepak Dhar, a professor at the International Centre for Theoretical Sciences, situated less than 10 kilometres from the two sites where Khanna recorded the cicadas, disagreed with Khanna. He said that if there is indeed a deeper connection between the cicadas’ and magnets’ emergent behaviour, one can use either system to tell more about the other. The paper doesn’t.
Gupta though suggested that such studies reveal fundamental principles of how order arises without centralised control. “These insights extend beyond biology, inspiring applications in swarm intelligence and robotic systems,” she said.
Cicadas’ emergent behaviour
Why the cicadas trigger to the same intensity, however, remains unclear. The authors do not attempt to answer what causes the cicadas to respond to sunlight or why they chorus.
For birds, dawn choruses have been reported, with males from the same species collectively calling out. It has been hypothesised that the male birds call to mark their territories. Dey, the moth biologist, thinks that the male cicadas may also exhibit mating behaviour, where individuals attempt to attract females. Indeed, a 1995 study of cicadas in the United States hints this could be males competing against each other for mating, or a collective call to females.
Dey says that the collective behaviour, triggered by individual male cicadas responding to the calls of other male cicadas, serves as a survival mechanism against predators, as well as increasing the chances of mating. For example, bats — which may prey on the cicadas — would have a more difficult time locating individual cicadas if many were singing their mating calls together. “When an individual is calling, it is easier for the predator to detect. But if they are calling in a chorus, sometimes it is difficult for the predator to locate the prey exactly. So, the chances of getting eaten decrease, and at the same time, they can increase their mate attraction by calling in coordination,” she said.
Moreover, this way, they might be collectively avoiding predators that hunt at specific times, like bats much before dawn and birds later in the day. Dey also speculated that since calling out for mates is energetically expensive, doing so in a chorus might help amplify the message — ‘open to mating’ — much more easily than as individuals. Gupta also independently posited the same hypotheses.
Read more: Researchers describe a new cicada species, bicolor butterfly, from Meghalaya
Banner image: Cicadas begin calling when triggered by the first sunlight just before dawn. They even coordinate with each other just like particles in a magnet. Image by Rakesh Khanna.
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