Solving the mystery of how and why fireflies flash in time can illuminate the physics of complex systems
In the still of the Tennessee night, my colleagues and I are watching thousands of dim little orbs of light, moving peacefully in the forest around us. We try to guess where the next flash will appear, but the movements seem erratic, even ephemeral.
This summer, as we set up our cameras and tents, I feel a crippling sense of dread. I had brought us all up here to the Great Smoky Mountains National Park, an unlikely group of computer scientists and physicists from my lab, in order to chase fireflies. We study firefly communication in hopes of unravelling the mystery of how and why they blink in unison with one another. This rare phenomenon is one of the most tantalising mysteries in complex systems science. If we could capture firefly synchronisation in an algorithm, it might help crack any number of riddles in cellular biology, animal communication and even swarm robotics. But there was no guarantee, and I worried about whether the experiment was going to work. It was a constant race against time, as these light shows last for only about 10 days per swarm. Though we’re lightyears away from the nearest star, I found myself glancing up at the distant constellations, which seemed predictable by contrast to the swarming sea of bioluminescence.
As it happens, I found my path to fireflies via the stars. In my late teens, I was obsessed with astronomy. I marvelled at the fact that I was such a tiny creature, surrounded by a vast Universe in which there was so much to explore. This discrepancy, between the scale of individual components and the entire global system, is prevalent in many of the things that physicists observe: from atoms crystallising into lattices, to soap-bubbles coalescing, to concrete bridges vibrating in resonance. What’s common to all these examples is the underlying physics of complex systems, where the microscopic interaction between individual building blocks determines the behaviour of the macroscopic whole.
I am forever in debt to the popular science books, movies and documentary TV shows that exposed me to such captivating natural phenomena, and gave me a healthy appetite for more formal education. As an undergraduate physics major, I went on to take a class on dynamical systems. We learned how electrons in superconductors synchronise their vibrations to allow for superconductivity, allowing electrical current to pass through it with minimal resistance. The concepts were heavy in mathematics and challenging to grasp. The major book that formed the spine of the course was called Nonlinear Dynamics and Chaos (1994), by Steven Strogatz – an applied mathematician who is not only renowned for his significant contributions to mathematical models of synchronisation, but also for being a superb science communicator.
To make the material more approachable, Strogatz discussed other relatable examples of synchronisation in nature, leaping back and forth from inanimate to animate systems. One of these examples was the flashing of fireflies: ‘In some parts of southeast Asia, thousands of male fireflies gather in trees at night and flash on and off in unison.’ To this day, I still remember discussing firefly synchronisation in that class, imagining them producing a light show influenced by the same physical laws that control electrons in a superconductor. And that was it: the problem of how and why was planted in my brain.
I have since become an interdisciplinary scientist, exploring natural history from the perspective of physics, and developing testable theories to bring a deeper understanding of the physics of living systems. These include the interacting proteins that control our cells, the way beetles navigate using celestial cues, and how honeybee clusters change shape to withstand stress and to regulate their temperature. Each of these examples is a complex system, where the individual building blocks (a protein, a cell, an insect) can sense its immediate ‘micro-environment’ and respond in a way that promotes fitness. Typically, the response then changes the wider ‘macro-environment’ that the individual is embedded in. This creates a coupling or causal loop that links the individuals, the group and the environment in a perpetual cycle of affecting and being affected.
hile fireflies have been found on every continent except Antarctica, synchronous species are rarer. Early scientists who investigated popular accounts of firefly synchrony often dismissed it as an illusion, a statistical accident, or an observational artefact caused by an observer blinking their eyelids or the fireflies’ light-producing organs being aligned by the wind. As synchronous displays are rare, not to mention complex and ‘messy’, scepticism persisted. Even after precise synchrony was first confirmed in Thailand in 1968, there was no record of the phenomenon in the western hemisphere until the 1990s. It was Lynn Faust – back then a firefly hobbyist, nowadays a world-renowned expert – who was the first to identify synchronous fireflies in the United States, in the backyard of her family’s cabin in Tennessee. Careful studies over the past 50 years have confirmed that synchronous fireflies are more common than originally thought. To date, three species of synchronous fireflies have been found in North America, and we might yet discover many more in the future.
When I started my own lab at the University of Colorado Boulder, I recalled Strogatz’s description of the fireflies on that distant tree in Asia. At first, I was surprised to find that there was little empirical data to relate the mathematical models of synchronisation to firefly behaviour. Admittedly, there are good reasons for this. Firefly synchronisation is rare, and the display is over almost as soon as it begins. The insects exist as flashing adults for only about 10 days out of the entire year, and then produce flashes just for a few hours a night. You really need to be at the right place at the right time to observe their display. Secondly, until not so long ago, you’d need very expensive and advanced recording devices to document the flashes. Nowadays, it is possible to use kit as simple as a GoPro or a smartphone.
So I decided to drive my lab across the country to record synchronous firefly flashes, and finally make a stronger connection between the mathematical models I grew up on and firefly behaviour in the wild. […]