Cicadas Decide to Sing with a Little Help from Their Friends
One familiar sound of summer is the pulsating buzz from swarms of cicadas. This singing begins at the break of dawn, but it’s been unclear whether the chorus is triggered by light, by temperature, or by some other cue. Now a team of researchers has studied audio recordings from the wild, finding that cicada singing is synchronized with a particular position of the Sun in the sky [1]. The rapid rise in singing intensity suggests that each cicada decides to start singing in response to both the level of illumination and the behavior of its neighbors. The results could help researchers develop generic models of decision-making.
Male cicadas sing to attract females, creating sound with membrane structures on their abdomens. This serenade often involves thousands of insects singing simultaneously. “Their calls are not only extremely loud but also remarkably synchronized—starting and ending in a sharp, collective manner,” says Rakesh Khanna, an engineer and amateur scientist in Bengaluru (formerly Bangalore), India. Similar collective behavior is known in birds and other insects, but the mechanism that triggers the cicada chorus has not been studied previously.
Out of curiosity, Khanna began recording cicadas a few years ago using audio equipment that he set up in forest areas around Bengaluru. To help decode the recorded data, Khanna contacted Raymond Goldstein from the University of Cambridge in the UK and his colleagues, who had recently uncovered interactive behavior in the timing of the emergence of periodical cicadas [2]. Together, they analyzed the song recordings and developed a model of cicada behavior.
The team focused on recordings spanning two weeks in April and May of 2023 and found that the cicadas sing at seemingly random times, except at the start and end of the day. “The first thing that jumps out at you is that there’s clearly a dawn chorus, and there’s clearly a dusk chorus,” Goldstein says. The researchers found that—outside of a few cloudy mornings—the dawn chorus consistently started when the Sun was 3.8° below the horizon, suggesting that the cicadas decide to sing when the brightness of the predawn sky has reached a certain level.
But if that brightness measuring were the whole story, then one would expect a large variation in singing start times. “How much light an individual cicada sees is going to depend on many things, such as their neural processing and the amount of nearby tree cover,” Goldstein says. But the data show very little variation in start time. In fact, once a brave individual breaks the morning silence, all the other cicadas quickly join in within a minute or so.
To explain this behavior, the researchers devised a model in which a cicada is represented by a magnetic particle, or spin, inside a magnetic material. Each spin can point up or down, just as each cicada can either sing or not sing. Initially, the spins are all pointing down, but then an upward-pointing magnetic field is slowly turned on, representing the slowly brightening sky. The spins start to flip up, and one can calculate how long it takes for all the spins to align with the field—or equivalently, how long until all the cicadas are singing.
If the spins act independently, then their flip times should vary widely, and full alignment will take a relatively long time to occur. However, if the spins interact with each other, then alignment can be rapid. The researchers found that a spin-interacting model fit their data better than a spin-independent one. The implication is that the cicadas are reacting to the singing of their neighbors. “They make all this noise for a reason, so when they hear the chorus starting, they respond to it,” Goldstein says.
Spin-based models have been used before to study group decisions, such as audience members deciding when to clap at a performance. Each individual is aware of public information while being coupled to a set of neighbors who are also in the process of decision-making. Questions remain over cicada perception, but this physics-motivated model of cicada singing points the way toward a better understanding of the physiology of decision-making, Goldstein says.
As to why cicadas choose to sing together, one explanation is that it reduces the threat from predators, says ecologist Almo Farina from the University of Urbino in Italy. A solo-singing cicada would be a clear target for predation. “Being numerous is a major evolutionary advantage,” Farina says. He believes the model of Goldstein and colleagues could be applied to other organisms.
–Michael Schirber
Michael Schirber is a Corresponding Editor for Physics Magazine based in Lyon, France.
References
- R. Khanna A. et al., “Photometric decision making during the dawn choruses of cicadas,” Phys. Rev. E 112, 024401 (2025).
- R. E. Goldstein et al., “How do cicadas emerge together? Thermophysical aspects of their collective decision-making,” Phys. Rev. E 109, L022401 (2024).