In just about every dinosaur movie ever made, there's a scene in which Tyrannosaurus rex lunges into the frame, opens its tooth-studded jaws at a near-ninety-degree angle, and emits a deafening roar - perhaps toppling its human antagonists backward, perhaps only dislodging their hats. This gets a huge rise from the audience, every time, but the fact is that we know practically nothing about how T. rex and its ilk vocalized. It's not like there were any tape recorders 70 million years ago, during the late Cretaceous period, and sound waves don't tend to preserve well in the fossil record.
Before examining the evidence, it's amusing to go behind-the-scenes and explore how cinematic "roars" are produced. According to the book, "The Making of Jurassic Park," the roar of the movie's T. rex included a combination of the sounds made by elephants, alligators, and tigers. The Velociraptors in the film were vocalized by horses, tortoises, and geese. From the perspective of evolution, only two of those animals are anywhere near the ballpark of dinosaurs. Alligators evolved from the same archosaurs that spawned the dinosaurs during the late Triassic period. Geese can trace their lineage back to the small, feathered dinosaurs of the Mesozoic Era.
Did Dinosaurs Have Larynxes?
All mammals possess a larynx, a structure of cartilage and muscle that manipulates air emitted by the lungs and produces characteristic grunts, squeals, roars, and cocktail-party chatter. This organ also pops up (probably as the result of convergent evolution) in a confusing array of other animals, including turtles, crocodiles, and even salamanders. One lineage in which it's noticeably absent is birds. This presents a bit of a dilemma. Since it's known that birds are descended from dinosaurs, this would imply that dinosaurs (at least meat-eating dinosaurs, or theropods) did not possess larynxes, either.
What birds do have is a syrinx, an organ in the trachea that produces melodious sounds in most species (and harsher, mimicking noises in parrots) when vibrated. Unfortunately, there's every reason to believe that birds evolved syrinxes after they had already split off from their dinosaur ancestors, so it can't be concluded that dinosaurs were equipped with syrinxes, as well. That's probably a good thing; imagine a full-grown Spinosaurus opening its jaws wide and emitting a sonorous "cheep!"
There is a third alternative, proposed by researchers in July 2016: Perhaps dinosaurs indulged in "closed-mouth" vocalization, which presumably would require neither a larynx nor a syrinx. The resulting sound would be like the cooing of a pigeon, only presumably much louder.
Dinosaurs May Have Vocalized in Very Strange Ways
So does this leave history with 165 million years' worth of unnervingly silent dinosaurs? Not at all. The fact is that there are many ways in which animals can communicate with sound, not all of them involving larynxes or syrinxes. Ornithischian dinosaurs may have communicated by clicking their horny beaks, or sauropods by stomping on the ground or flicking their tails. Throw in the hisses of modern-day snakes, the rattles of modern-day rattlesnakes, the chirping of crickets (created when these insects rub their wings together), and the high-frequency signals emitted by bats. There's no reason to posit a Jurassic landscape that sounds like a Buster Keaton film.
In fact, there is hard evidence for one unusual way in which dinosaurs communicated. Many hadrosaurs, or duck-billed dinosaurs, were equipped with elaborate head crests. The function of these crests may have been exclusively visual in some species (say, recognizing a fellow herd member from afar), while in others it had a distinct auditory function. For example, researchers have performed simulations on the hollow head crest of Parasaurolophus, which show that it vibrated like a didgeridoo when funneled with blasts of air. The same principle may apply to the big-nosed ceratopsian Pachyrhinosaurus.
Did Dinosaurs Need to Vocalize at All?
All of this begs an important question: Just how essential was it for dinosaurs to communicate with one another via sound, rather than by other means? Let's consider birds again. The reason most small birds trill, cheep, and whistle is because they're very small, and would otherwise have a hard time locating each other in dense forests or even in the branches of a single tree. The same principle doesn't apply to dinosaurs. Even in thick underbrush, one presumes that the average Triceratops or Diplodocus would have no problem seeing another of its kind, so there would exist no selective pressure for the ability to vocalize.
A corollary to this, even if dinosaurs couldn't vocalize, they still had plenty of non-auditory ways to communicate with one another. It's possible, for instance, that the broad frills of ceratopsians or the dorsal plates of stegosaurs flushed pink in the presence of danger, or that some dinosaurs communicated by scent rather than sound. Perhaps a Brachiosaurus female in estrus emitted a smell that could be detected within a radius of 10 miles. Some dinosaurs may even have been hard-wired to detect vibrations in the ground. That would be a good way to avoid larger predators or catch up with a migrating herd.
How Loud Was the Tyrannosaurus Rex?
But let's get back to our original example. If you insist, despite all the evidence presented above, that T. rex roared, you have to ask yourself why modern animals roar? Despite what you've seen in movies, a lion won't roar while it's hunting; that would only scare away its prey. Rather, lions roar (as far as science can tell) in order to mark their territory and warn other lions away. As big and fierce as it was, did T. rex really need to emit 150-decibel roars to warn off others of its kind? Maybe, maybe not. But until science learns more about how dinosaurs communicated, that will have to remain a matter of speculation.
- Riede, Tobias, et al. “Coos, Booms, and Hoots: The Evolution of Closed-Mouth Vocal Behavior in Birds.” Evolution, vol. 70, no. 8, Dec. 2016, pp. 1734-1746., doi:10.1111/evo.12988.