Be it the blinking of fireflies, the chirping of crickets or the beat of human pop music: they all have a basic rhythm of around two beats per second. A study now suggests that this is a universal frequency that shapes the communication of humans and animals across species boundaries. This repetition rate corresponds exactly to the rhythm in which the nerve cells in the brain can optimally process signals.
During field research in Thailand in 2022, a team led by Guy Amichay from Northwestern University in Illinois made an astonishing observation: the researchers actually wanted to observe fireflies of the species Pteropyx malaccae, which are known for flashing synchronously. “While we were filming the fireflies, we noticed that nearby crickets seemed to be chirping in sync with the fireflies,” Amichay says. “We thought it was crazy that these two unrelated species should interact in this way.”
At the same pace
So the researchers followed up on this observation. When they analyzed their recordings more closely, they discovered that the crickets weren’t actually chirping at exactly the same time as the fireflies were blinking. Instead, they just repeated their chirping at the same rate as the fireflies blink, namely at 2.4 hertz, a little more than two repetitions per second. “Why should these two different species act at such a similar pace when they could, in principle, choose from a variety of options?” the researchers asked themselves.
To answer this question, Amichay and his colleagues combed through numerous published studies on communication between different animal species. They found: “There appear to be a large number of organisms that signal or communicate at a relatively narrow speed range,” reports Amichay. “They all seem to stay at around two or maybe three hertz.” In addition to fireflies and crickets, this also applied to crabs, frogs, birds, sea lions and monkeys, among others. “In principle, they could also communicate in other rhythms,” says Amichay. “Physically there is nothing stopping them from communicating at ten hertz, for example, but they don’t.”
Neural clock
This tempo is also widespread in human pop music. Most hits are around 120 beats per minute, or two beats per second. “This rhythm suits our bodies; it suits our limbs,” says Amichay. “We walk at about two steps per second, so it’s easy for us to dance to music at two hertz.” Animals, on the other hand, have completely different walking speeds depending on their size. So why does two hertz seem to be the most natural rhythm for them too?
The explanation probably lies in the brain. Our nerve cells, just like those of fireflies, birds or sea lions, need a certain amount of time before they can fire again after a signal. If the signals arrive too quickly one after the other, they have no additional effect. Processing in the brain works most effectively when the signals arrive about half a second apart – i.e. with the observed frequency of two hertz. “We suspect that having the carrier signal in the right speed range is crucial for efficient communication,” explains Amichay’s colleague Daniel Abrams. “It may not be that the tempo itself conveys information, but rather that it simply serves as a basis for attracting attention, with the actual content layered on top, much like notes following the beat of a song.”
From the researchers’ perspective, this finding could help to better interpret the communication and social behavior of animals across species boundaries. “It’s tempting to think that there’s a deeper connection here – that maybe we’re all on the same wavelength,” says Amichay. “But we’re still exploring what that might mean.”
Source: Guy Amichay (Northwestern University, Illinois, USA) et al., PLOS Biology, doi: 10.1371/journal.pbio.3003735