Cicadas Begin Singing When Sun is Exactly 3.8 Degrees Below Horizon
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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. They call it photometric decision making.
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. One site was in a shrubland area with scattered grasses, whilst the other was in the different environment of a bamboo forest.
"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.”
“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.
Cicadas are small insects and have prominent eyes set wide apart, short antennae, and membranous front wings. They have an exceptionally loud song, produced in most species by the rapid buckling and unbuckling of drum-like tymbals.