The natural world is always brimming with sound, whether it’s the chirping of birds, the howl of canids, or the calls of primates. These lush sounds are by no means random. Although they don’t use language, in the sense of a symbol-based system, virtually all animals employ vocalization as a form of communication, an ability that is evolutionarily advantageous. But if evolution fosters vocalization, why is it that some species are completely silent? That’s the thing, they likely aren’t.

In a new study, researchers led by evolutionary biologist Gabriel Jorgewich Cohen from the University of Zurich showed that 53 species previously thought to be mute do in fact produce vocalizations — and the researchers have the recordings to prove it. This evidence fills in the blanks in the deep-time evolutionary origin of acoustic communication, helping biologists trace the origin of acoustic communication to the last ancestor of all vertebrates possessing internal nostrils, which lived about 407 million years ago.

Not so silent after all

Gabriel Jorgewich Cohen was reading about the amazing sounds made by an Amazonian turtle when a thought popped into his mind: could his pet tortoises swimming inside his home aquarium make sounds too? He soon came in contact with a former colleague from Brazil who developed a hydrophone, a special microphone that can record sounds underwater.

When Cohen tested the device inside his home aquarium, he was amazed to hear his tortoises actually making noises. It was the first time in his life that he had heard them. But this got him thinking: how many other animals must be fooling us that their lips are sealed?

Over the years, he recruited the help of other biologists from over nine institutions in five countries, and the team got to work recording turtles, caecilians (snake-like amphibians), tuataras (rare reptiles found only in New Zealand, and the only survivors of an order of reptiles that thrived in the age of the dinosaurs), and lungfish (air-breathing fish).

Remarkably, all recorded species were found to possess a varied acoustic repertoire comprising a number of different sounds. Additionally, the researchers scoured the literature and were again surprised to find they weren’t the only scientists reporting similar results. A total of 106 species, including 54 turtles, which were considered to lack acoustic communication are, in fact, engaging in vocalization.

Now, don’t imagine these critters have impressive vocalizations. After all, there’s a reason why their voice has been so elusive. Their rare vocalization sounds like squeaks, hisses, knocks, and screeches, with some examples including the red side-necked turtle (Rhinemys rufipes), the tuatara (Sphenodon punctatus), and the saw-shelled turtle (Myuchelys latisternum). Have a listen to their short soundbites below.

The researchers combined the data they gathered with other evidence across all major tetrapod clades totaling in excess of 1,800 species to map vocalizations onto the evolutionary tree. Their conclusion is that sound production and acoustic communication are traits shared by all vertebrates that breathe through their nostrils, with the earliest common ancestor dating from the Palaeozoic, some 407 million years ago.

“Sometimes it’s surprising how much we still don’t know about things that aren’t necessarily uncommon but live alongside us,” Neil Kelley, a paleontologist at Vanderbilt University, told NPR, adding that “it’s very hard to trace that in the fossil record, because sounds obviously don’t fossilize and most vocal equipment is soft tissue-based,” he notes.

The fact that all these creatures were, in fact, ‘talkative’ all this time is quite astonishing. However, just because an animal makes sounds with its mouth and nostrils doesn’t necessarily mean it is actually communicating in the same way that, for instance, a cat hisses when it feels threatened or purrs when it feels comfortable. Those are two different things, which is why the next step is figuring out what these animals are trying to ‘say’. For instance, new research could employ cameras to try to correlate the sounds these creatures make with changes in their behavior.

The findings appeared in the journal Nature Communications.