Dinosaurs may have mumbled with their mouths shut

(Credit: iStockphoto)

Dinosaurs in the movies roar ferociously, but new research suggests they actually mumbled or cooed with their mouths closed.

The research, published online in the journal Evolution, examines the evolution of a specialized way birds emit sound—closed-mouth vocalization. The scientists are working to understand the origin and evolution of the unique vocal organ of birds and the large array of sounds it can produce. Because birds descended from dinosaurs, the research may also shed light on how dinosaurs made sound.

“This makes for a very different Jurassic world.”

Closed-mouth vocalizations are sounds that are emitted through the skin in the neck area while the beak is kept closed. To make them, birds typically push air that drives sound production into an esophageal pouch rather than exhale through the open beak. The coos of doves are an example of this behavior.

ring dove x-ray
X-ray image of a ring dove (Streptopelia risoria) producing cooing sounds with a closed mouth. (Credit: Tobias Riede)

Compared with sounds emitted through an open beak, closed-mouth vocalizations are often much quieter and lower in pitch. Birds making closed-mouth vocalizations usually do so only to attract mates or defend their territory. At other times, they emit sounds through an open mouth.

To understand when and how closed-mouth vocalization evolved, researchers used a statistical approach to analyze the distribution of this vocal ability among birds and other reptilian groups. In total, the researchers identified 52 out of 208 investigated bird species that use closed-mouth vocalization.

Bigger bodies

“Looking at the distribution of closed-mouth vocalization in birds that are alive today could tell us how dinosaurs vocalized,” says study coauthor Chad Eliason, a postdoctoral researcher at the University of Texas Jackson School of Geosciences.

“Our results show that closed-mouth vocalization has evolved at least 16 times in archosaurs, a group that includes birds, dinosaurs, and crocodiles. Interestingly, only animals with a relatively large body size (about the size of a dove or larger) use closed-mouth vocalization behavior.”

Tobias Riede, a physiology professor at Midwestern University in Arizona and the study’s first author, says the association with large bodies is a matter of physics.

“The inflation of an elastic cavity could present a size-dependent challenge,” Riede says. “The lung pressure required to inflate a cavity depends on the tension in the wall of the cavity, and this tension increases for smaller body sizes.”

Were these noises part of mating displays?

Researchers still are not certain about how the ancestors of modern archosaurs vocalized. But the occurrence of closed-mouth vocalization across birds and crocodiles—the two surviving groups of archosaurs—indicates that closed-mouth vocalization can emerge in diverse archosaur species depending on behavioral or environmental circumstances, Riede says.

“A cool thing about this work is the demonstration that closed-mouth behavior evolved many times,” Riede says. “That suggests it can emerge fairly easily and be incorporated into mating displays.”

Pigeons are smarter than you’d think

Because dinosaurs are members of the archosaur group, and many had large body sizes, it is likely that some dinosaurs made closed-mouthed vocalizations in a manner similar to birds today, perhaps during mating displays. However, at this point in time, no direct fossil evidence exists to reveal what dinosaurs sounded like.

Julia Clarke, a professor at the Jackson School of Geosciences and coauthor, says the study offers clues.

“To make any kind of sense of what nonavian dinosaurs sounded like, we need to understand how living birds vocalize,” she says. “This makes for a very different Jurassic world. Not only were dinosaurs feathered, but they may have had bulging necks and made booming, closed-mouth sounds.”

Future research by this collaboration will integrate information from fossils, experimental physiology, gene expression, and sound modeling to understand the sounds that extinct early avian species, and perhaps their dinosaur ancestors, produced.

Additional researchers from Memorial University of Newfoundland and the University of Utah contributed to the work, which the Gordon and Betty Moore Foundation funded.

Source: University of Texas at Austin