How will billions of cicadas synchronize their swarms? Researchers finally unlock mystery

CAMBRIDGE, United Kingdom — A remarkable natural spectacle happens every 13 or 17 years in the eastern United States: billions of cicadas emerge from the ground, filling the air with their unmistakable chorus. Why they simultaneously do this has left scientists stumped for years, especially given the varying temperatures across small geographical areas that could potentially disrupt their synchronized appearance. However, a seminal new study by researchers from the University of Cambridge has shed light on this mystery, revealing the intricate communication system cicadas may use to coordinate their mass emergence.

Researchers have introduced a mathematical model that captures the essence of cicada nymphs’ decision-making process in the face of temperature variations. It suggests that cicadas communicate with each other underground to reach a consensus on the local average temperature, which then triggers their collective emergence.

The molted exoskeletons of Brood X cicadas
The molted exoskeletons of Brood X cicadas. (credit: Ian Hutchinson/Unsplash)

This phenomenon occurs across the eastern and southeastern U.S., with 17 distinct cicada broods appearing in staggered years. In the spring of 2024, an unusual event is expected as two different broods, one on a 13-year cycle and another on a 17-year cycle, will emerge simultaneously, creating a cacophony and spectacle unparalleled in recent years.

Past studies have pinpointed 64 degrees Fahrenheit (18 degrees Celsius) as the critical soil temperature for cicada emergence. However, factors like sun exposure, foliage cover, and humidity can cause temperature variations even within a small area, potentially complicating the synchronization of cicada swarms. The Cambridge researchers’ model illustrates how communication among nymphs can overcome these microclimatic differences, ensuring the swarm emerges in unison.

Drawing parallels to “avalanches” in decision-making observed among stock market traders, which can lead to market crashes, the model employs concepts from physics to explain biological behavior. Each cicada nymph is represented by a “spin,” similar to the magnetic spins, with two states indicating the decision to “remain underground” or “emerge.” These decisions, influenced by the local temperature (akin to a magnetic field) and communication between nymphs (interaction between spins), lead to a cohesive decision-making process across the population.

Scientists suggest that acoustical signaling might be the mode of communication among nymphs. This theory aligns with the loud sounds cicadas make upon surfacing, possibly indicating a continuation of underground communication patterns.

“As an applied mathematician, there is nothing more interesting than finding a model capable of explaining the behavior of living beings, even in the simplest of cases,” says Dr. Adriana Pesci, a senior research associate at Cambridge’s Department of Applied Mathematics and Theoretical Physics (DAMTP), in a university release.

Cicada emerging Brood in Maryland
Cicada emerging Brood in Maryland.(Photo by Bill Nino on Unsplash)

“If our conjecture that communication between nymphs plays a role in swarm emergence is confirmed, it would provide a striking example of how Darwinian evolution can act for the benefit of the group, not just the individual,” explains Raymond Goldstein, the Alan Turing Professor of Complex Physical Systems in DAMTP.

This study received support from the Complex Physical Systems Fund and is published in the journal Physical Review E.


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