The mysterious song of the dinosaurs

“I would describe the sound as otherworldly,” says Tom Williamson, one of those who worked on the dig and is now curator of palaeontology at the museum. “It sent chills through my spine, I remember.”

The closest analogues he can find in living animals today are the vibrating grunts of the southern cassowary, which lives in Australia. This flightless bird emits a series of deep bellows and growls that reverberate through the thick jungle where they live.

“It’s easy for me to imagine a misty Late Cretaceous rainforest setting with those eerie sounds thundering in the background,” says Williamson. “The sounds are of low frequencies – just what is necessary to penetrate the dense undergrowth.”

Williamson and his colleagues simulated the sound P. tubicen might have produced both with and without an assortment of vocal organs, such as the larynx found in mammals and modern reptiles. They found that even without a larynx or the equivalent voice box, the dinosaur may still have produced a noise due to the way air would have resonated inside the crest when the animal blew air through it, much like blowing over the opening of a jug.

“We didn’t have preserved soft tissues and we don’t know, for example, if these dinosaurs had sound-producing organs such as mammals and birds do,” says Williamson. “It became apparent that a sound-producing organ wasn’t necessary to get the crest to resonate because it is such a long structure.”

Other hadrosaurs had similar, if not so dramatic, musical crests on their skulls that are thought to have doubled as a visual display and an aid to vocalisation. Most would have produced low-frequency sounds, and the fossilised remains of these animals have even inspired some to create musical instruments based on hadrosaur skulls.

Not all dinosaurs were blessed with what amounted to a trumpet atop their heads. And we have no fossilised evidence of voice boxes from dinosaurs, leading some to speculate the animals may even have been mute.

“What we do have are fossil clues that can tell us about different parameters of the airways like its diameter and its length,” says Julia Clarke, a palaeontologist at the University of Texas at Austin. “We can compare those geometries to see how they relate to those dinosaurs that are living today – birds.”

But Clarke has another clue that has provided a further piece of the puzzle. In the mid-2000s, she and her colleagues conducted a detailed examination of the preserved skeleton of an early type of bird found over a decade earlier by Argentinian researchers on Vega Island, a tiny scrap of land on the tip of the Antarctic Peninsula. The fossil itself remains partially embedded in a piece of rock, but using advanced CT scanning techniques, Clarke and her team were able to detect bits of the fossil hidden from view. They then digitally reconstructed the fossil from the scans.

And there, nestled amongst the fossilised bone fragments, were the remnants of something astonishing – the mineralised rings of a syrinx, the sound producing organ found in birds, dating back to the time of the dinosaurs.

The primitive bird it belonged to – a goose-like creature called Vegavis iaai – would have coexisted with non-avian dinosaurs at the end of the Cretaceous period, 66-68 million years ago. At around this time, this part of modern Antarctica would have been covered in temperate forests and surrounded by shallow seas. The honking sounds of V. iaai were probably part of that landscape.

But for Clarke, the discovery reveals something else by its presence – that these sound-producing organs can fossilise, and their absence from most dinosaur fossils is telling. Birds, or avian dinosaurs to be more precise, evolved from theropod dinosaurs around 165-150 million years ago during the Jurassic period. If the syrinx from a bird living 66-68 million years ago could be preserved as a fossil, why have none been found among the remains of their extinct non-avian cousins, such as Tyrannosaurus rex?

It is a question that led Clarke to delve deeper into how modern birds produce sound. “There are around 10,000 living species of birds [some estimates put the number as high as 18,000], but there has been surprisingly little scientific research done on what sounds they actually make and how they do it,” she says. Her work has led her to a revelation that will shake the ground from under the feet of five-year-olds and movie goers around the world. Dinosaurs almost certainly didn’t roar. They probably cooed instead.

Or more accurately they may have produced sounds in ways similar to the way doves coo or ostriches boom. Many modern birds use what is known as closed-mouth vocalisation, where sound is made by inflating the throat rather than passing air through the syrinx. Crocodiles – another distant relative of the dinosaurs that split from a common ancestor around 240 million years ago – also use closed-mouth vocalisation to generate deep rumbles that can cause the water around them to “dance” around their bodies. Crocodiles, like other reptiles and mammals, have a larynx rather than a syrinx that produces the sound. But they bypass this when producing their mating bellows.

“The Jurassic Park films have got it wrong,” laughs Clarke. “A lot of the early reconstructions of dinosaurs have been influenced by what we associate with scary noises today from large mammalian predators like lions. In the Jurassic Park movies they did use some crocodilian vocalisations for the large dinosaurs, but on screen the dinosaurs have their mouths open like a lion roaring. They wouldn’t have done that, especially not just before attacking or eating their prey. Predators don’t do that – it would advertise to others nearby that you have got a meal, and it would warn their prey they are there.”

Instead Clarke believes that many non-avian dinosaurs may have produced sounds with their mouths closed by inflating the soft tissues of their throats, as part of some sort of mating display. But she says they could also have used open-mouth calls in other situations, such as moments of distress. “There’s going to be a lot of different kinds of sounds out there in the landscape of the Late Jurassic or Early Cretaceous,” she says.

It is a view supported by research on another part of dinosaur anatomy for which there is better evidence in the fossil record – their ears. Studies of dinosaur skulls have allowed palaeontologists to reconstruct what their inner ears were like. A few fossils have also revealed some of the delicate bones that helped dinosaur ears to function.

“Dinosaurs only had this single bone in their middle ear, the stapes – a key structure in translating vibrations in air, sound waves, to the inner ear that can then be processed by the brain,” says Phil Manning, professor of natural history at the University of Manchester. “Us mammals also possess the malleus (hammer) and the incus (anvil).”

Without these additional pieces of bony hearing apparatus, dinosaurs may only have been able to hear a much narrower range of frequencies compared to mammals, Manning says. And they were probably attuned to picking up low frequency sounds.

“The stapes in dinosaurs were often quite large, almost the size of a matchstick in T. rex, meaning it was well tuned to lower frequencies,” says Manning. “Small species of dinosaurs with smaller stapes would correlate with high-frequency sounds.”

The size of the cochlear ducts in the inner ears of dinosaur fossils offer other insights about their hearing abilities, and suggest they could have also been able to pick up high frequencies. “We know from living animals that the longer the cochlea, generally the greater range of sounds it can hear,” says Steve Brusatte, professor of palaeontology and evolution at the University of Edinburgh. “Mammal cochleas are coiled like a snake, to pack in a long length into a small region of skull. Dinosaur cochleas aren’t like this, but some of them are pretty long.”

One detailed study of a species of tyrannosaur – a horse-sized predator from the mid-Cretaceous called Timurlengia euotica, which prowled what is now the Kyzylkum Desert in modern-day Uzbekistan – has revealed that these animals had unusually long cochlear ducts in their inner ears. “That suggests that it could hear a wider range of sounds than many other dinosaurs,” says Brusatte who led the study. When we studied the CT scans of Timurlengia, we noticed that its cochlea was really, really long for a dinosaur.”

In fact dinosaurs might have developed these elongated cochleas fairly early on in their evolution, perhaps in the very early days of their branch of the evolutionary tree, known as the Archosauria, around 250 million years ago.

“The cochlear elongation denoting sensitivity to squeaky noises occurred near the origin of the archosaurian ‘ruling reptiles’, which includes birds and crocodiles,” says Bhart-Anjan Bhullar, associate curator of vertebrate paleontology at Peabody Museum of Natural History, Yale University, in New Haven, Connecticu. He has reconstructed the ear canals of several archosaurs using three-dimensional scans of their fossilised skulls. “We considered all sorts of possible drivers of this transformation and realised that the only one that was consistent with all evidence was the advent of a high level of parental care, and more specifically the use of chirping ‘location calls’ by the babies.”

So, could young dinosaurs have been tweeting in their nests to get their parents’ attention, like modern bird chicks and young crocodiles do today? Bhullar thinks they might have. “Given that baby birds and baby crocodiles chirp, it’s reasonable to infer that baby non-bird dinosaurs did as well, and that their parents listened to them and cared for them just as crocodile and bird parents do,” he says. “As far as what sensitivity to high-pitched sound means about the noises that adult non-bird dinosaurs made – it’s an open question. I would be entirely unsurprised if most dinosaurs, and especially those closely related to birds, made a variety of noises.”

The ability to hear a wide range of sounds could have been useful in many ways, such as detecting predators or other threats, or allowing them to scout out prey more effectively, says Brusatte. But it could have been used for communication with each other too – either to warn about danger, to attract mates, intimidate rivals or to help herds stick together.

“We know at least some tyrannosaurus travelled and maybe hunted in packs, so communication between individuals was probably important,” says Brusatte.

But with such large animals producing many of these sounds, how would they have sounded to our ears? Much of the booming calls of crocodiles and cassowaries is beyond the limits of human hearing in low frequencies known as infrasound (there are even reports of alligators living close to Cape Canaveral in Florida producing infrasound calls in response to the deep rumble of the rockets during launches of the Space Shuttle in the 1980s). Elephants are also known to communicate over long distances using infrasound and Sumatran rhinos use infrasound “whistles” that resemble humpback whale song to penetrate their thick forest habitat.

Low-frequency sounds and infrasound are especially good at travelling long distances, both in open environments and dense jungle habitats. In animals the size of the T. rex or giant sauropods like the Diplodocus, the sound could have been very low indeed.

“We know there is a fundamental scaling relationship between body size and frequency,” says Clarke. “Small animals produce higher frequency sounds in general because of the length of their vocal cords, unless they’ve got some weird modifications. Large animals produce lower frequency sounds. And so in dinosaurs, you have these animals that are the size of four elephants stacked on top of each other. They’re not producing sounds in the frequency range of human hearing.

“But you would probably feel them.”

Other research suggests that even if we could hear the biggest of the dinosaurs humming to one another, it would have sounded strange to our ears. Giants like the Supersaurus may not have had great control over their vocal abilities due to the relatively long delay for nerve signals to travel down the 28m (92ft) long necks from the brain. It would have meant any calls they produced may have seemed remarkably sluggish in relation to events around them.

Some paeolontologists, however, have proposed that giant sauropods like the Diplodocus and Supersaurus might have relied more upon tactile communication while moving in herds. It is perhaps the reason why they have such elongated tails, as they allowed them to stay in almost constant contact with their neighbours while they migrated.

It is evocative to imagine a Cretaceous alive with the squawks of smaller dinosaurs, chirps of newly hatched young and the menacing rumble of giants somewhere in the distance. Faced with such an assault on the ears and vibrating through your bones, it’s worth considering if you would stay to take a closer look, or simply turn and run.

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