Primates chew their food on one side of their mouth. Try it with a sandwich: take a bite, and immediately the piece of food will be ushered to the left or right cheek teeth. This action occurs when the jaws open up and happens because of one mysterious and versatile structure – the tongue. Whilst it is easy to see the action of the upper and lower jaw when we eat, it is far harder to know how the motion of the tongue simultaneously pushes food around, as it's encased by the flesh and bone of our heads. So far, some have theorised what might be happening, but no one has yet been able to see the motion of the tongue in three dimensions.
Kara Feilich and Callum Ross from the University of Chicago, USA, wanted to know how primates use their tongue during eating. Together with their colleagues Jeffrey Laurence-Chasen (University of Chicago, USA), Courteny Orsbon (University of Vermont Medical Center, USA) and Nicholas Gidmark (Knox College, USA), Feilich and Ross set out to shed light on how the tongue moves during the ingestion of food. They trained four rhesus macaques to sit and eat grapes whilst being filmed using a three-dimensional x-ray system. This allowed the researchers to see inside the mouth whilst the macaques ate their grapes, to figure out how the tongue makes its twists and turns.
Feilich found that the bending of the tongue was in synchrony with the opening–closing motion of the jaws. After biting down on the grapes, the jaws opened whilst the body of the tongue pushed the food sideways by rolling to one side, feeding it between the upper and lower cheek teeth – all ready to be chewed. You'll be aware of doing the same thing, perhaps even more so when it goes wrong. If you've ever bitten your tongue during eating, chances are you've over-enthusiastically squashed your own tongue into your gnashing teeth.
Analysing the tongue's manoeuvres, Feilich and colleagues realised that the tongue bending came about by asymmetrical changes along its length. Whilst the tongue rolled towards the chewing side, it simultaneously bent itself away, so that the tip pointed to the opposite side – perhaps to avoid the pain of being chewed on. Feilich pondered how the tongue contorts itself into these complex shapes. Assuming that the tongue keeps a constant volume, asymmetrical muscle contractions could produce the tongue-twisting shapes demonstrated by the macaques.
Primates’ lingual dexterity assists in other functions too, such as vocalisation and grooming. But despite this varied repertoire, Feilich and colleagues found that tongue motion during eating was remarkably consistent, which hints that this pattern is important for primates in general, you and I included. Irrespective of the occasional masticatory mistake, Feilich and colleagues revealed the tongue's twisting role in shifting food, which can be further studied by medical colleagues to help patients recovering from tongue damage and neuromuscular disorders.