Tiny bone string solves sabertooths' purr-roar mystery

“Evolutionarily speaking, sabertooths split off the cat family tree before these other modern groups did,” said Adam Hartstone-Rose, professor of biological sciences at NC State and corresponding author of the research. “This means that lions are more closely related to housecats than either are to sabertooths.

“That’s important because the debate over the kind of vocalization a sabertooth tiger would have made relies upon analyzing the anatomy of a handful of tiny bones located in the throat,” Hartstone-Rose says. “And the size, shape and number of those bones differ between modern roaring and purring cats.”

Scientists were able to identify one key difference between roaring and purring cats: the bones that held the larynx in place (the hyoid bones) were different for each kind of animal. Most notably: they were of different size and number.

“While humans have only one hyoid bone, purring cats have nine bones linked together in a chain and roaring cats have seven,” said Ashley Deutsch, a Ph.D. student at NC State and lead author of the research. “The missing bones are located toward the top of the hyoid structure near where it connects to the skull.”

A complicated matter

“Because sabertooth tigers only have seven bones in their hyoid structure, the argument has been that of course they roared,” Hartstone-Rose said. “But when we looked at the anatomy of modern cats, we realized that there isn’t really hard evidence to support this idea, since the bones themselves aren’t responsible for the vocalization. That relationship between the number of bones and the sound produced hasn’t ever really been proven.”

By comparing the hyoid structures of today’s cats and 105 of the same bones from the sabertooth tiger Smilodon fatalis, they were able to determine the noises the animal would have made when it roamed the earth. Although the final answer remains unclear, they have come up with two potential scenarios based on current cat noises and even a third option.

“You can argue that since the sabertooths only have seven bones they roared, but that’s not the whole story,” Hartstone-Rose said. “The anatomy is weird. They’re missing extra bones that purring cats have, but the shape and size of the hyoid bones are distinct. Some of them are shaped more like those of purring cats, but much bigger.”

“If vocalization is about the number of bones in the hyoid structure, then sabertooths roared. If it’s about shape, they might have purred. Due to the fact that the sabertooths have things in common with both groups, there could even be a completely different vocalization,” Hartstone-Rose added. 

The study is published on the Journal of Morphology.

Study abstract:

Animal vocalization is broadly recognized as ecologically and evolutionarily important. In mammals, hyoid elements may influence vocalization repertoires because the hyoid apparatus anchors vocal tissues, and its morphology can be associated with variation in surrounding soft-tissue vocal anatomy. Thus, fossil hyoid morphology has the potential to shed light on vocalizations in extinct taxa. Yet, we know little about the hyoid morphology of extinct species because hyoid elements are rare in the fossil record. An exception is found in the Rancho La Brea tar pits in Los Angeles, California, where enough hyoids have been preserved to allow for quantitative analyses. The La Brea Tar Pits and Museum houses one of the largest and most diverse collections of carnivore fossils, including hyoid elements from the extinct felids Smilodon fatalis and Panthera atrox. Here, we found that extant members of Felinae (purring cats) and Panthera (roaring cats) showed characteristic differences in hyoid size and shape that suggest possible functional relationships with vocalization. The two extinct taxa had larger and more robust hyoids than extant felids, potentially reflecting the ability to produce lower frequency vocalizations as well as more substantial muscles associated with swallowing and respiration. Based on the shape of the hyoid elements, P. atrox resembled roaring cats, while S. fatalis was quite variable and, contrary to suggestions from previous research, more similar overall to purring felids. Thus P. atrox may have roared and S. fatalis may have produced vocalizations similar to extant purring cats but at a lower frequency. Due to the confounding of vocalization repertoire and phylogenetic history in extant Felidae, we cannot distinguish between morphological signals related to vocalization behavior and those related to shared evolutionary history unrelated to vocalization.