While in Thailand’s mangrove forests filming Pteroptyx malaccae fireflies, known for synchronized flashing, researchers noticed a coincidence: nearby crickets appeared to chirp in time with the flashing insects. “I thought, whoa, could the two be interacting?” says Northwestern University biophysicist Guy Amichay, lead author of a new study on the phenomenon. “It also blew my mind because it’s light versus sound.”
A closer analysis showed that both produced signals at nearly the same tempo—about 2.4 hertz, or roughly 145 beats per minute.
While the tempos’ extreme closeness may indeed be coincidental, a survey of animal communication studies revealed that a broader range—around 0.5 to 4 hertz, or 30 to 240 beats per minute—appeared repeatedly across animals varying enormously in size, from fireflies and fiddler crabs to fish, birds, apes and humans.
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The researchers, who reported their findings in PLOS Biology, also screened 124 recordings from the vast wildlife sound archive xeno-canto and identified 50 species with regular rhythmic signals. These, too, clustered within the same tempo range, with a peak around 180 beats per minute.
“That there might be a common rhythm across a remarkably diverse set of species and communication systems is very intriguing indeed,” says University of Amsterdam music cognition researcher Henkjan Honing, who was not involved in the study. Most popular Western music fits within this range at around 120 beats per minute (2 hertz), he adds.
Many of these species are physically capable of signaling at tempos outside this range, too. So why don’t they? To answer that question, the researchers shifted their attention from signal senders to receivers—because all signals are ultimately processed by nervous systems that are made up of neurons.
Neurons don’t respond instantaneously to incoming signals; their responses can depend on input received over the previous few hundred milliseconds. The researchers wondered whether this processing time might make neural circuits especially responsive to certain tempos, much as the time a swing takes to move back and forth on a swing set determines when a push will have the greatest effect. A computer model suggested simple neural circuits built with an internal “tempo” of 120 beats per minute responded best to signals that were also around that range.
The findings echo decades of research on human rhythm perception, says Cornell University cognitive scientist Nori Jacoby, who was not involved in the study: although humans can synchronize across a wide range of rhythms, beat perception and production tend to peak within a preferred tempo range of roughly 85 to 120 beats per minute.
Honing cautions that the neural explanation is only one possibility. Other biological rhythms, such as movement or breathing, could also help set the pace of animal communication—leaving open how vastly different species arrive at similar tempos.