The tongue and the muscles involved in chewing have undergone an alteration as animals transitioned from water to land.
- Mammalian tongues are used to position food in the mouth so the teeth can chew it.
- Fish tongues have less complicated motion and mainly function to move food from the front to the back of the mouth.
- Chewing has ancient evolutionary origins. It appears to have evolved before animals left the water.
Are you reading this over a snack or your lunch? Go ahead, take a bite and notice how you use your tongue.
Expertly, you nudge your food into the right place so your teeth come down right on top of the hunk of food, and with each chew, you reposition the bolus just a little to get all the right parts chewed up.
In a new study, researchers took a look at how different animals use their tongues and jaws together. Their findings suggest that ancient animals' move to land may have led to wider ranging tongue motion, and, not surprisingly, that animals' dietary habits have dictated how tongue use has evolved.
"Chewing really is an ancestral trait," said Nicolai Konow of Brown University, in Providence, RI, who led the new study published in Integrative and Comparative Biology. Indeed, the new evidence suggests that chewing originated before the ancestors of modern land vertebrates left the sea.
But then its purposes diverged, Konow explained. "For fish, they use the tongue as an active processing unit and they use the tongue to pull the food further into the mouth and hold prey when they open the jaw." They move the tongue upward and pull it back in the nose-to-tail direction, in a relatively simple, two-dimensional motion.
"A fish is swimming around in a viscous medium," he said. "It has no appendages. Whenever the fish has caught some food, every time it opens its mouth to chew, there's a risk of the fish losing the food." This means the tongue is primarily working to keep and get the food backward and into the gullet, according to Konow.
"When you move up to land, all of a sudden we're subject to gravity, but we also evolved appendages," he said.
This created new roles for the tongue. Unlike fish, mammals can move their jaws and tongues side-to-side as well as up and down. Teeth come together precisely to mash up food, but that means the food needs to be in exactly the right place, and it is the tongue that makes that happen.
The new study found these differences among fish and mammals by implanting electrodes in the jaws and tongue muscles of a number of animals and tracking the patterns of their movement. Konow highlighted the importance of looking at the combination of jaw and tongue for getting a complete picture.
Not only did Kolow's team find differences in how fish and mammals use their tongues, but also within the collection of mammals the researchers studied, including omnivores like pigs and ungulates like llamas and goats.
Ungulates obtain their sustenance from a poor quality source, he noted. "It's very hard to extract nutrients from grass," he said. "This means that the energetics of chewing are really important, so efficiency is key. The tongue has to reposition the food precisely between the teeth."
This may explain the team's observations that ungulates' chewing was not as rhythmic as they had expected, but also more coordinated than the pigs'.
"The tongue in the herbivores has to be moved in a very labile manner to catch and reposition and intercept parts of that chunk of grass and constantly do fine tuning in terms of putting the food in the right spot," Konow said. "It doesn't matter so much for the pig. It doesn't need the tongue to be so intrinsically engaged in feeding."
Now the researchers want to understand how other animals fit into the picture.
Previous work showed that some lizards move their tongues in ways that resemble mammals. The researchers plan to look at amphibians and other reptiles to try better to pinpoint at what stage land vertebrate's chewing patterns diverged from their aquatic cousins.
It's a relatively new notion that animals other than mammals can chew, said Robert Druzinsky of the College of Dentistry at the University of Illinois at Chicago. This paper takes that idea a step further, he said: "The roots of complex mammalian chewing are seen even in the fishes."
As for human tongues, Druzinsky agrees that we rarely appreciate our own dexterity. "It's one of those things that you don't realize how important it is until you lose it," he said, noting that those who suffer strokes and other conditions can end up having problems with their tongue and lips.
So savor your snack -- and the contribution of your unsung tongue.