In 2024, many parts of the United States experienced a rare cicada-geddon as two major broods of cicadas, Brood XIII and Brood XIX, emerged simultaneously for the first time in over 200 years. As morning broke, residents in many parts of the US woke up to a faint hum that quickly turned into a pulsating, deafening buzz, filling the air. It was the loud mating calls of millions of cicadas. These were the periodical cicadas that emerge after 13 or 17 years underground. Apart from the periodicals, the daily dawn chorus of their annually emerging cousins is a more regular phenomenon. Surprisingly, when dawn breaks, all these countless insects, spread across a wide area, decide to start singing at almost the exact same moment each morning. Researchers have been studying their behaviour to understand this synchronisation capability of the cicada.
New research from Bharat Electronics Limited, Ghaziabad, University of Cambridge, United Kingdom, and Weizmann Institute of Science, Israel, sheds light on this collective behaviour. Their study has revealed that cicadas don't just rely on the rising sun; they also listen to their neighbours, engaging in a form of group decision-making.
The researchers conducted a quantitative study of the dawn choruses of Platypleura capitata cicadas in their natural habitats, at two different sites near Bengaluru in April and May 2023. They used stereo recorders mounted on tree branches approximately one meter above the ground to capture acoustic signals for eight consecutive days at each location. The raw audio data was then processed using signal processing algorithms. They focused on frequencies between 4.3 kHz and 6.3 kHz, where the cicada chorus was most prominent and distinct from background noise.
Did you know, cicada choruses can be incredibly loud, sometimes reaching over 90-100 decibels, comparable to a lawnmower or a motorcycle? This deafening sound is produced by males using special vibrating membranes called tymbals on their abdomens to attract females. |
To connect the chorus onset to light, they used standard astronomical formulas to calculate the ground illumination based on the sun's elevation angle. They transformed the illumination data logarithmically, because visual systems often respond logarithmically to light, meaning we perceive relative changes in brightness rather than absolute ones, a principle known as the Weber-Fechner law. This allowed them to analyse the chorus's response to light in a way that better reflects how the cicadas might perceive it.
Finally, to model this collective behaviour, the team turned to a concept from statistical physics: the spin model or Ising model. Each cicada is modelled as a tiny spin that can be in one of two states: quiet or singing. These spins are influenced by the changing light intensity and also by their neighbours (other cicadas singing). Using a mathematical framework called Glauber formalism, which describes how these spins transition between states, they developed a model that accurately matched their experimental data. This model strongly suggested that the sharp, synchronised onset of the chorus is not just due to individual cicadas reacting to light, but also to their collective interactions, where hearing others sing encourages more to join in.
They discovered that the cicadas' collective singing was a precisely timed phenomenon. The choruses consistently begin when the sun reaches a specific elevation of -3.8° ± 0.2° below the horizon, regardless of day-to-day variations in sunrise times over many weeks. This indicates that light intensity plays a crucial role in triggering their activity. Interestingly, on cloudy days, the onset of the chorus was delayed, further confirming that it's the actual light level, not just the time of day, that matters. The scientists found that once the singing starts, the chorus amplitude, how loud and widespread the singing is, grows in an S-shaped curve, reaching its full volume within a characteristic time of about 60 seconds. This rapid escalation suggests a quick, coordinated response among the insects.
To understand this coordination, the team looked at how sensitive the cicada population was to changes in light. They defined a generalised susceptibility, which is a measure of how strongly the system, the cicada chorus in this case, responds to a small change in an external factor, like light intensity. They found that this susceptibility exhibited a sharp peak around the critical light level, where the chorus begins. This sharp peak, along with a similar peak in the variance (how much the singing activity fluctuates), strongly suggests that the cicadas are engaging in a collective decision-making process, meaning they are responding to other cicadas. The researchers propose that this sharpness in response is a key indicator of cooperative decision-making, likely driven by acoustic communication—the cicadas literally hear each other starting to sing and join in.
The authors note that further, longer-term field studies in different habitats and experiments comparing isolated individuals to groups would be beneficial to fully confirm the role of acoustic communication in this decision-making process. Nonetheless, the study might have finally answered how cicadas around the world can begin their dawn chorus in unison. The research offers more than just insights into insect behaviour. By understanding how groups of cicadas make collective decisions, we gain valuable insights into how populations, whether animal or human, respond to gradually changing environmental cues. These insights could inform fields ranging from robotics and artificial intelligence, where designing intelligent swarms is a goal. It's a testament to how studying the seemingly simple acts of nature can reveal profound truths about collective behaviours.
This article was written with the help of generative AI and edited by an editor at Research Matters.