Flexibility of feeding movements in pigs: effects of changes in food toughness and stiffness on the timing of jaw movements

In mammals, chewing movements can be modified, or flexible, in response to changes in food properties. Variability between and within food in the temporal characteristics of chewing movements can impact chewing frequency and rhythmicity, which in turn may affect food breakdown, energy expenditure and tooth wear. Here, we compared total chewing cycle duration and intra-cycle phase durations in pigs chewing on three foods varying in toughness and stiffness: apples (low toughness, low stiffness), carrots (high toughness, low stiffness), and almonds (high toughness, high stiffness). We also determined whether within-food variability in timing parameters is modified in response to changes in food properties. X-ray Reconstruction Of Moving Morphology (XROMM) demonstrates that the timing of jaw movements are flexible in response to changes in food properties. Within each food, pigs also exhibited flexibility in their ability to vary cycle parameters. The timing of jaw movements during processing of high-toughness foods is more variable, potentially decreasing chewing rhythmicity. In contrast, low-toughness foods result in jaw movements that are more stereotyped in their timing parameters. In addition, the duration of tooth-food-tooth contact is more variable during the processing of low-stiffness foods compared with tough or stiff foods. Increased toughness is suggested to alter the timing of the movements impacting food fracture whereas increased stiffness may require a more cautious control of jaw movements. This study emphasizes that flexibility in biological movements in response to changes in conditions may not only be observed in timing but also in the variability of their timing within each condition.