As cicadas begin to emerge throughout the South and Midwest in the U.S., Northwestern University’s Jorin Graham has noted how periodical broods synchronize their behaviours.
Physicist Graham studies complex networks and network science and has made careful note of the activities of these insects. Cicadas have prominent eyes set wide apart, plus short antennae, and membranous front wings. The insects are perhaps best known for their exceptionally loud song, which is produced in most species by the rapid buckling and unbuckling of drum-like tymbals.
One of the remarkable aspects of the behaviour of the insects is that each individual cicada counts the passing years based on annual cycles of food from trees. In almost all cases these behaviours are synchronised, with just a few cicadas (“stragglers”) miscounting and emerging too early or too late.
Graham, providing an overview of the insects’ activity, says: “Periodical cicadas spend years living underground before emerging together to briefly mate and die. Upon hatching, cicada nymphs return to the ground and repeat the process. This curious behaviour, in which billions of cicadas coordinate their behaviour, is an example of synchronization.”
So, why is synchronisation important and what is a physicist doing studying the biological world?
Graham explains: “The study of synchronization is itself rooted in physics, often considered to have originated with Dutch physicist Christiaan Huygens’ studies of pendulum clocks in the 17th century. Synchronization is now recognized as an important behaviour across many domains — from engineered systems like power grids, to biological systems such as neuron firing in the brain, to other ecological phenomena such as the coordinated production of fruit by oak trees.”
He adds: “Remarkably, such synchronized behaviour is commonly achieved without centralized planning or enforcement.”
In terms of what is happening, Graham summarises his observations: “So how do periodical cicadas coordinate their emergence? Each individual cicada counts the passing years based on annual cycles of xylem flows, a nutritious fluid in trees on which they feed.”
He had also noted: “Cicadas that are born the same year will emerge with the rest of their brood 13 or 17 years later. However, a few cicadas, known as stragglers, miscount and emerge too early or too late. If these cicadas reproduce, their offspring would also emerge out of cycle and, eventually, the periodic cicadas would lose synchronization.”
There are other interactions with the natural world, which Graham elaborates: “Synchronization is preserved by predators, such as squirrels, who quickly consume cicadas that emerge at the wrong time. During normal emergence years, so many cicadas come out that predators can’t eat all of them, allowing the remaining cicadas to reproduce, thus sowing the seed for the next coordinated emergence.”
