A new study has shown that fish and mammals chew their food differently. While fish use tongue muscles to thrust food backward, mammals use them to position food for grinding. According to the researchers from Brown University, 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.

They also concluded that evolutionary divergence likely occurred with the amphibians.

“It’s pretty clear that all of these animals chew, but the involvement of the tongue in chewing differs,” said Nicolai Konow, a postdoctoral researcher at Brown and the lead author on the study.

“And that brings up the question of what the muscles associated with the tongue and the jaw are doing,” he added.

Konow and his team studied how the muscles of three mammals alpacas, goats, and pigs acted during chewing. They outfitted each with electrodes planted in the jaw and tongue muscles to identify the activity of each set of muscles during chewing. The analysis revealed that the animals’ tongues thrust forward and upward, as they began to chew and then fell back or retracted, to their original position. It also emerged that with the animals facing left, the tongue traces an ellipse in a counter-clockwise direction for each cycle.

On the other hand, when the fish face left, the chewing cycle looks like an ellipse tilted at an angle, with the tongue moving in a clockwise direction.

Earlier studies have shown that during chewing, the fish fires the muscle, called the sternohyoid downward, retracting the tongue inward, before moving it forward again and upward, to its original position in the upper mouth. “We think the herbivore needs the bolus (the soft mass of chewed food) to be in a precise place between each chew. So the tongue may be constantly moving around to make sure the bolus is in the right place between chews, Konow explained. The study has been published in the journal of Integrative and Comparative Biology.

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