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A surprising number of creatures across a wide swath of sizes, species, and communication methods all seem to time their signals to the same basic beat.
According to an analysis of communication signals across the animal kingdom, from bird mating dances to frog songs to human music to firefly flashes, the tempo of these signals clusters around 2 beats per second.
Given how long all these species have been evolving independently of one another, this common signature could tell us something about the origins of communication.
"There seems to be an abundance of organisms signaling or communicating at a relatively narrow band of tempos. They all seem to stay around 2 or maybe 3 hertz. In principle, they could communicate at other rhythms," says mathematician Guy Amichay of Northwestern University in the US.
"Physically, there is nothing preventing them from communicating at, say, 10 hertz, yet they do not. To explain this phenomenon, we propose that this tempo of 2 hertz might be easier to understand because it resonates with your brain. It resonates with the human brain, firefly brain, sea lion brain, frog brain, and so on."
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The work started in Thailand. Amichay has been studying how animals use synchrony in communication, and one animal known for its breathtaking, synchronized mating displays is the firefly. While in the field, he and his colleagues noticed that the chirping of crickets seemed to sync up with the fireflies' pulsing light.
"At some point, I thought that the flashing of the fireflies and the chirping of the nearby crickets were in sync with each other," Amichay says.
It wasn't until they were reanalyzing the recordings that the researchers realized that the animals weren't synchronizing with each other at all. Each species was blithely engaged in its own mating ritual. It just so happened that they had similar tempos.
That seemed like a wacky coincidence, so the scientists did what scientists do: They got busy investigating. They turned to published studies on faunal communication, sampling two dozen species across six groups – insects, amphibians, birds, fish, crustaceans, and mammals.
They also randomly selected 50 signals from the xeno-canto database, 10 from each of the five animal groups into which the database is divided – birds, bats, frogs, grasshoppers, and land mammals.
The range of the signal types included firefly flashes, cricket chirps, frog calls, birds' mating displays, sound and light pulses from fish, and vocals and gestures from mammals.
From there, it was a matter of determining the tempo of each communication signal, then plotting them all into a graph. And this is where the research transitioned from "Huh, that's interesting" to "This is really something".
Across eight orders of magnitude in body weight, and across land, air, and sea, most species tend to communicate at a basic "carrier frequency" of 0.5 to 4 hertz – 0.5 to 4 beats a second. And yes, that includes humans; as the researchers note, a great many rock and pop songs are written at 120 beats per minute – which is two beats per second.
"That rhythm fits our body; it fits our limbs," Amichay explains.
"We walk roughly at 2 hertz, so it's easy for us to dance to music that's 2 hertz. Of course, more experimental music can have drastically different beats. But if you turn on the radio and hear Taylor Swift – that's often 2 hertz."
We know that humans and other animals are capable of signaling outside that range. Biophysicist Vijay Balasubramanian of the University of Pennsylvania supplied the clue. Neurons need time to process information before firing again – and the optimal timing for that seems to be about, you guessed it, half a second.
So the team conducted an exploratory experiment to determine if this could be the reason for the clustering. They built a computer model of a neural circuit and observed how it responded to pulsed signals with different periods.
The circuit had the strongest response to the 2-hertz signal.
"We suspect that getting the 'carrier' signal in the right tempo range is key to communicating efficiently," says engineer Daniel Abrams of Northwestern University.
"It might not be that the tempo itself conveys any information, but it just serves as a baseline for getting attention, with actual content sent on top of it like musical notes following along with the beat in a song."
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There are some limitations to the study. Our planet contains millions of animal species; 74 communication types constitute just a drop in the ocean, and there may be a selection bias at play due to our tendency to pay more attention to signals at that frequency.
Nevertheless, the discovery is a surprising one that warrants further study.
"It's tempting to think there's a deeper connection here – that maybe we're all on the same shared wavelength," Amichay says.
"But we're still exploring what this might mean."
The research has been published in PLOS Biology.
English (US)