Chew On This: Muscles Used for Munching Underwent Significant Evolutionary Shift
Researcher Nikolai Konow has been studying the mechanics of chewing. Photo via Brown University.
Fish do it. Lizards do it. Cows do it. Get your heads out of the gutter, readers. We’re talking about chewing.
Three years ago, scientists reported that all vertebrate animals with a jaw, even the least expected — (read: fish) — chew their food before swallowing. Last week, a team led by Nikolai Konow, a postdoctoral researcher at Brown University, released a study showing that the muscles involved in chewing appear to have undergone a sharp reorganization as animals transitioned from water to land. The study was published June 24 in the journal, Integrative and Comparative Biology.
Chewing is a highly complex system that requires precise coordination between the jaws and the tongue, and a lot of neural feedback.
The team studied the mechanics of chewing in 25 different animal species, and focused on the muscles that move the jaw, along with a bone crucial to chewing and common in all vertebrate animals: the hyoid bone.
The hyoid bone is a horseshoe-shaped bone, the size of a U.S. dollar coin and located in the front of the throat in humans. If you looked at a human from the side, it would be sitting just below the jawline. And it appears to play an active role in chewing among all vertebrates.
The human hyoid is a strange, free-floating bone — the only bone in the body that’s connected only to ligaments and not other bones. Fish, on the other hand, “have really robust bony connections from the hyoid back to cranium, and back to the skull,” Konow said.
The team implanted fine wire electrodes into four muscles in the mouths of seven fish and three mammal species. By studying tiny electrical currents that activate as muscles are fired, scientists could determine how long the muscles were active during chewing. At issue was how the hyoid bone acts relative to the jaw among different species. They also compiled the literature on similar tests of other vertebrate species.
Among their findings, they saw sharp changes and what appears to be an evolutionary shift in chewing between water and land animals.
Unlike mammals, fish use the hyoid to reduce their food to smaller, more manageable pieces. In fish, the hyoid bone first thrusts back through the oral cavity, raking through and tearing apart the food. “Fish do actually chew and they chew like cows,” Konow said. “They move their jaw up and down, up and down.” The tongue acts to keep the food inside the mouth and pull it closer to the stomach, functioning as both a food processing and a food retaining device, he said.
Among land critters though, chewing occurs at the the jaw, not the tongue. Land vertebrates instead use the tongue to position and stabilize the food for more rigorous munching and grinding. As the mouth closes around the food, the hyoid and tongue start by moving forward. The tongue sticks to the food, and then the tongue and hyoid together transfer the food backward into the oral cavity, where the jaw and teeth do the chewing.
Of course, land vertebrates don’t all chew the same. Lizards, for example, chew much less extensively than humans. Cows chew rhythmically, staying on one side for a period, then switching to the other side.
But with no exceptions, scientists observed the first activity pattern in all fish studied and the second in all land vertebrates.
“There’s a dramatic change,” said Anthony Herrel, Belgian biologist and a co-author on the paper. “In fish, the bone moves backward. In terrestrial animals, all of a sudden, that same bone moves forward. So you get this really drastic reorganization from what these bones are doing through evolution.”
All living land animals appear to use the tongue and hyoid this way, Herrel added, to pull the food backward — to transport it.
Still unknown though is exactly where the split occurred as animals evolved. Konow suspects it was probably early amphibians.
The scientists also studied chewing patterns in alpacas and goats, both cud chewers, and pigs, omnivores, and found that, while these species are closely related, the muscle patterns changed, possibly according to food type. Even though pigs had a more flexible diet than the herbivores, their tongue activity pattern was much more predictable than the activity pattern in herbivores, Konow said, an unexpected finding.
Dr. Jeffrey Palmer, leader of the Swallowing Disorders Program at the Johns Hopkins School of Medicine, said the inclusion of both the jaw and hyoid muscles in the research was important. “Most papers, in both human and animal literature, focus on one and not the other,” he said.
Studies in this area could contribute to the treatment of dysphagia, or trouble swallowing, a disorder suffered by roughly half of stroke patients, Palmer said.
An important element still missing in the research, he added, are studies that look at the combination of muscle activity and motion. “They didn’t say anything about what were the movements being caused by those contractions,” Palmer said. “You can’t tell if it’s a shortening or a lengthening contraction. And those can have quite different implications for function.”
The ability to chew also has relevance beyond evolution and medicine, Konow says, but applies to something much more basic: survival.
“Chewing your food is amongst one, two or three crucial survival tasks in life,” Konow said. “Animals need to be able to get away from predators, reproduce and obtain nutrition. If you think hard, you can boil life down to that aspect … And if there’s any constraint on our survival, it’s the ability to be able to ingest nutrition.”
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