The tongue makes all the difference

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It may come as a surprise to learn that fish chew differently from mammals and the difference is all in the tongue.

A study by scientists from Brown University has found that that while fish use tongue muscles to funnel food backward for processing like on an assembly line, mammals use their tongue to position food for grinding.

The difference in chewing shows that animals have changed the way they chew and digest their food and that evolution must have played a role.

Lead author Nicolai Konow said: "It's pretty clear that all of these animals chew, but the involvement of the tongue in chewing differs, and that brings up the question of what the muscles associated with the tongue and the jaw are doing."

A previous study by Konow and his colleagues using bowfin, pike and salmon, found that chewing in fish begins with the tongue in the upper mouth and is then retracted first in and then forward and up to its original position in an elliptical pattern.  

Earlier research had already established the same pattern in bichir and lungfish - an important transition from water to land.

In this paper, Konow and his team established that mammal tongues thrust forward, and upward, as they began to chew and then back to their original position in the opposite direction ellipse to fish.

The authors think that the difference in chewing is because fish need to transport the food quickly through the mouth often to an extra set of jaws to grind the food.

In addition, the tongue moves oxygenated water in through the mouth to the gills. Mammals, on the other hand, use their tongues to set the food in the right spot in the mouth to maximise chewing.

It is thought that the evolutionary divergence happened with the amphibians as other studies have found that lizards move their tongues in the same way as mammals, so Konow plans to study amphibians next.

"They're still locked to the water for reproduction," he said. "But you have some that become all terrestrial. And that's the next step on the evolutionary ladder."

For more information see: N. Konow, A. Herrel, C. F. Ross, S. H. Williams, R. Z. German, C. P. J. Sanford, C. Gintof. Evolution of Muscle Activity Patterns Driving Motions of the Jaw and Hyoid during Chewing in Gnathostomes. Integrative and Comparative Biology, 2011; DOI: 10.1093/icb/icr040

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