Cicadas coordinate their early morning choruses with remarkable precision, timing their singing to a specific level of light during the pre-dawn hours.
In a study published in the journal Physical Review E, researchers have found that these insects begin their loud daily serenades when the sun is precisely 3.8 degrees below the horizon: a consistent marker of early morning light known as civil twilight.
The research, carried out by scientists from India, the UK and Israel, analysed several weeks of field recordings taken at two locations near Bangalore in India. Using tools from physics typically applied to the study of phase transitions in materials, the team uncovered a regularity in how cicadas respond to subtle changes in light.
"We've long known that animals respond to sunrise and seasonal light changes," said 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 crescendo of cicada song - familiar to anyone who has woken up early on a spring or summer morning - takes only about 60 seconds to build, the researchers found. Each day, the midpoint of that build-up occurs at nearly the same solar angle, regardless of the exact time of sunrise.
In practical terms, that means cicadas begin singing when the light on the ground has reached a specific threshold, varying by just 25% during that brief transition.
To explain this level of precision, the team developed a mathematical model inspired by magnetic materials, in which individual units, or spins, align with an external field and with each other. Similarly, their model proposes that cicadas make decisions based both on ambient light and the sounds of nearby insects, like individuals in an audience who start clapping when others do.
"This kind of collective decision-making shows how local interactions between individuals can produce surprisingly coordinated group behaviour," said co-author Professor Nir Gov from the Weizmann Institute, who is currently on sabbatical in Cambridge.
The field recordings were made by Bangalore-based engineer Rakesh Khanna, who carries out cicada research as a passion project. Khanna collaborated with Goldstein and Dr Adriana Pesci at Cambridge's Department of Applied Mathematics and Theoretical Physics.
"Rakesh's observations have paved the way to a quantitative understanding of this fascinating type of collective behaviour," said Goldstein. "There's still much to learn, but this study offers key insights into how groups make decisions based on shared environmental cues."
The study was partly supported by the Complex Systems Fund at the University of Cambridge. Raymond Goldstein is the Alan Turing Professor of Complex Physical Systems and a Fellow of Churchill College, Cambridge.
Reference:
Khanna, R.A., Goldstein, R.E., Pesci, A.I., & Gov, N.S. 'Photometric Decision-Making During the Dawn Choruses of Cicadas.' Physical Review E (2025). DOI: 10.1103/4y4d-p32q
