Big Dinosaurs Heard Only Low-Pitch Sounds, Experts Suggest

Scott Norris for National Geographic News

June 8, 2007

Large dinosaurs' hearing was more sensitive to booms and thuds than squeaks and whistles, new research says. Dinos such as Brachiosaurus and Allosaurus probably could hear the deep-toned sounds of other dinosaurs' footfalls from miles away, researchers say. But these massive reptiles may have had little or no hearing at higher sound frequencies—suggesting that the squeals of a smaller animal facing a Tyrannosaurus rex may have literally fallen upon deaf ears. The theory of how dinosaurs may have perceived sound comes from studies of hearing in birds, which are widely believed to be dinosaurs' closest living relatives. (Related: "'Feathered' Dinosaur Was Bald, Not Bird Ancestor, Controversial Study Says" [June 1, 2007].) "We know a lot about hearing in birds," said acoustics researcher Robert Dooling of the University of Maryland, College Park. "Big birds hear better at low frequencies, and small birds hear better at high frequencies." Dooling's latest work, in collaboration with two German scientists, suggests the relationship between size and hearing extends from tiny songbirds to the 75-ton (68-metric-ton) Brachiosaurus. That's because the inner ear structure of all archosaurs—the group of related species that includes dinosaurs, birds, and crocodiles—is basically identical, the experts say. "What makes this whole thing work is that the ears of dinosaurs and birds are all scale models of one another," Dooling said. The scientist presented his team's findings this week at the annual meeting of the Acoustical Society of America in Salt Lake City, Utah. Close Correlations The size of an auditory organ called the basilar papilla in the inner ear of birds is closely correlated with birds' overall body size, Dooling said. Knowing this, the researchers gave different bird species hearing tests by training them to peck at a key for a food reward in response to sound. From this the team established the sound frequencies at which birds hear best, and the high-frequency limit to their hearing. Comparing more than 30 bird species, Dooling's team found a set of close correlations between basilar papilla length, frequency of optimal hearing, and the highest tone a species could detect. Based on these relationships, the hearing capacity of most bird species can be accurately predicted from body size alone, Dooling said. German collaborators Otto Gleich of the University of Regensburg and Geoffrey A. Manley at the Technical University of Munich added to Dooling's research. They used fossils to determine the average basilar papilla length of Archaeopteryx—a feathered reptile thought to be a prehistoric cousin to modern birds—along with those of two species of dinosaur. (Related news: "Dino-Era Bird Flew With Four Wings, Study Says" [September 28, 2006].) For all three ancient creatures, the relationship between the size of the hearing organ and overall body size fit the same pattern shown by modern birds. The researchers concluded that large dinosaurs may have had little or no hearing at sound frequencies to which the human ear is optimally attuned. "You wouldn't expect a dinosaur of yesterday to be able to hear a bird of today," lead author Dooling said. Lawrence Witmer, of Ohio University, has also studied the inner ear structure of dinosaurs. He called the new study "outstanding." "I think [Dooling's team] is exactly right," Witmer said. "My own data—not just from the inner ear but also other parts of the auditory system—fully support their contention that dinosaurs emphasized low-frequency hearing." "Infrasound" Knowing what dinosaurs could hear provides clues to their behavior, the experts say. No one knows what types of sounds dinosaurs made, but their vocal abilities may have evolved to fit their hearing. "Archosaurs in general are pretty vocal animals, and it's quite conceivable that dinosaurs vocalized in the low frequencies they could hear best," Dooling said. He also noted that low-frequency "infrasound," which is inaudible to humans, can travel great distances. Elephants are thought to hear infrasound vibrations produced by the strides of other elephants far away. (Read: "Elephants 'Hear' Warnings With Their Feet, Study Confirms" [February 16, 2006].) Larger dinosaurs could probably do the same, Dooling said, perhaps monitoring each other's movements over wide areas. In general, Ohio University's Witmer cautions experts against making too many nuanced assumptions about dino behavior, since many details are simply lost to the sands of time. "But careful studies like this one," he said, "reveal that we can indeed learn more than we had thought possible."