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

Prolonged use of a soft diet during early growth and development alters feeding behavior and chewing kinematics in a young animal model

In infants and children with feeding and swallowing issues, modifying solid foods to form a liquid or puree is used to ensure adequate growth and nutrition. However, the behavioral and neurophysiological effects of prolonged use of this intervention during critical periods of postnatal oral skill development have not been systematically examined, although substantial anecdotal evidence suggests that it negatively impacts downstream feeding motor and coordination skills, possibly due to immature sensorimotor development. Using an established animal model for infant and juvenile feeding physiology, we leverage X-ray reconstruction of moving morphology to compare feeding behavior and kinematics between 12-week-old pigs reared on solid chow (control) and an age- and sex-matched cohort raised on the same chow softened to a liquid. When feeding on two novel foods, almond and apple, maintenance on a soft diet decreases gape cycle duration, resulting in a higher chewing frequency. When feeding on almonds, pigs in this group spent less time ingesting foods compared to controls, and chewing cycles were characterized by less jaw rotation about a dorsoventral axis (yaw) necessary for food reduction. There was also a reduced tendency to alternate chewing side with every chew during almond chewing, a behavioral pattern typical of pigs. These more pronounced impacts on behavior and kinematics during feeding on almonds, a tougher and stiffer food than apples, suggest that food properties mediate the behavioral and physiological impacts of early texture modification and that the ability to adapt to different food properties may be underdeveloped. In contrast, the limited effects of food texture modification on apple chewing indicate that such intervention/treatment does not alter feeding behavior of less challenging foods. Observed differences cannot be attributed to morphology because texture modification over the treatment period had limited impact on craniodental growth. Short-term impacts of soft-texture modification during postweaning development on feeding dynamics should be considered as potential negative outcomes of this treatment strategy.

Mechanical properties of food and masticatory behavior in llamas, Llama glama

Mammals typically process food items more extensively in their oral cavities than do other vertebrates. Dental morphology, jaw-muscle activity patterns, mandibular movements, and tongue manipulation work to facilitate oral fragmentation of dietary items. While processing mechanically challenging foods, mammals modulate mandibular movements and bite forces via recruitment of greater jaw-adductor muscle forces and protracted biting or chewing. Because jaw-loading patterns are influenced by magnitude; frequency; and duration of muscular, bite, and reaction forces during routine feeding behaviors, relatively larger jaws are thought to be more characteristic of mammals that experience higher masticatory loads due to the processing of mechanically challenging foods. The ease of food fracture during post-canine biting and chewing is mainly determined by food stiffness and toughness. Such foods have been associated with increased loading magnitude and/or greater amounts of cyclical loading (i.e., chewing duration). Dietary properties are thought to modulate cyclical loading through changes in chewing frequency and chewing investment. On the other hand, chewing frequency has been found to be independent of dietary properties in rabbits and primates; however, little evidence exists regarding the influence of dietary properties on these parameters in a broader range of mammals. Here, we assessed chewing behavior in seven adult llamas (Llama glama) processing foods with a wide range of mechanical properties (grain, hay, carrots, and dried corn). Each subject was filmed at 60 frames/s, with video slowed for frame-by-frame computer analysis to obtain length of feeding bout and number of chewing cycles for each food type. These parameters were used to calculate chewing frequency (chews/s), chewing investment (chews/g), and chewing duration (s/g). Chewing frequency was not significantly related to mechanical properties of food, but chewing investment and chewing duration were significantly related to dietary stiffness and toughness. Therefore, cyclical loading is positively influenced by stiff and tough foods. This suggests that variation in jaw morphology in extinct and extant mammals is positively related to dietary stiffness and toughness, which requires greater chewing investment and increased chewing duration.

Jaw kinematics and tongue protraction-retraction during chewing and drinking in the pig

Mastication and drinking are rhythmic and cyclic oral behaviors that require interactions between the tongue, jaw and a food or liquid bolus, respectively. During mastication, the tongue transports and positions the bolus for breakdown between the teeth. During drinking, the tongue aids in ingestion and then transports the bolus to the oropharynx. The objective of this study was to compare jaw and tongue kinematics during chewing and drinking in pigs. We hypothesized there would be differences in jaw gape cycle dynamics and tongue protraction-retraction between behaviors. Mastication cycles had an extended slow-close phase, reflecting tooth-food-tooth contact, whereas drinking cycles had an extended slow-open phase, corresponding to tongue protrusion into the liquid. Compared with chewing, drinking jaw movements were of lower magnitude for all degrees of freedom examined (jaw protraction, yaw and pitch), and were bilaterally symmetrical with virtually no yaw. The magnitude of tongue protraction-retraction (Txt), relative to a mandibular coordinate system, was greater during mastication than during drinking, but there were minimal differences in the timing of maximum and minimum Txt relative to the jaw gape cycle between behaviors. However, during drinking, the tongue tip is often located outside the oral cavity for the entire cycle, leading to differences between behaviors in the timing of anterior marker maximum Txt. This demonstrates that there is variation in tongue-jaw coordination between behaviors. These results show that jaw and tongue movements vary significantly between mastication and drinking, which hints at differences in the central control of these behaviors.

Unilateral lingual nerve transection alters jaw-tongue coordination during mastication in pigs

During chewing, movements and deformations of the tongue are coordinated with jaw movements to manage and manipulate the bolus and avoid injury. Individuals with injuries to the lingual nerve report both tongue injuries due to biting and difficulties in chewing, primarily because of impaired bolus management, suggesting that jaw-tongue coordination relies on intact lingual afferents. Here, we investigate how unilateral lingual nerve (LN) transection affects jaw-tongue coordination in an animal model (pig, Sus scrofa). Temporal coordination between jaw pitch (opening-closing) and 1) anteroposterior tongue position (i.e., protraction-retraction), 2) anteroposterior tongue length, and 3) mediolateral tongue width was compared between pre- and post-LN transection using cross-correlation analyses. Overall, following LN transection, the lag between jaw pitch and the majority of tongue kinematics decreased significantly, demonstrating that sensory loss from the tongue alters jaw-tongue coordination. In addition, decrease in jaw-tongue lag suggests that, following LN transection, tongue movements and deformations occur earlier in the gape cycle than when the lingual sensory afferents are intact. If the velocity of tongue movements and deformations remains constant, earlier occurrence can reflect less pronounced movements, possibly to avoid injuries. The results of this study demonstrate that lingual afferents participate in chewing by assisting with coordinating the timing of jaw and tongue movements. The observed changes may affect bolus management performance and/or may represent protective strategies because of altered somatosensory awareness of the tongue.NEW & NOTEWORTHY Chewing requires coordination between tongue and jaw movements. We compared the coordination of tongue movements and deformation relative to jaw opening-closing movements pre- and post-lingual nerve transection during chewing in pigs. These experiments reveal that the timing of jaw-tongue coordination is altered following unilateral disruption of sensory information from the tongue. Therefore, maintenance of jaw-tongue coordination requires bilateral sensory information from the tongue.

Effects of food properties on chewing in pigs: Flexibility and stereotypy of jaw movements in a mammalian omnivore

Chewing is a rhythmic oral behavior that requires constant modifications of jaw movements in response to changes in food properties. The food-specific kinematic response is dependent on the potential for kinematic flexibility allowed by morphology and modulation of motor control. This study investigates the effects of food toughness and stiffness on the amplitude and variability of jaw movements during chewing in a typical omnivorous mammalian model (pigs). Jaw movements were reconstructed using X-ray Reconstruction Of Moving Morphology (XROMM) and kinematic data associated with the amplitude of jaw pitch (opening-closing) and jaw yaw (mediolateral rotation) were extracted for each cycle. Between-food differences were tested for the amplitude of jaw movements during each phase of the gape cycle, as well as in their respective within-food variability, or stereotypy, as indicated by coefficients of variation. With increasing toughness, jaw pitch amplitude is decreased during fast close, larger and more stereotyped during slow close, smaller but more variable during slow open, and more variable during fast open. In addition, when chewing on tougher foods, the amplitude of jaw yaw during slow close only increases in a subset of individuals, but all become less variable (i.e., more stereotyped). In contrast, increasing food stiffness has no effect on the amplitude or the variability of jaw pitch, whereas jaw yaw increases significantly in the majority of individuals studied. Our data demonstrate that food stiffness and toughness both play a role in modulating gape cycle dynamics by altering the trajectory of jaw movements, especially during the slow-close phase and tooth-food-tooth contact, albeit differently. This highlights how a generalist oral morphology such as that of pigs (e.g., bunodont teeth lacking precise occlusion, permissive temporomandibular joint allowing extensive condylar displacements in 3 dimensions) enables organisms to not only adjust chewing movements in their amplitude, but also in their variability.

The effect of unilateral lingual nerve injury on the kinematics of mastication in pigs

OBJECTIVE

This study evaluates the effect of unilateral lingual sensory loss on the spatial and temporal dynamics of jaw movements during pig chewing.

DESIGN

X-ray Reconstruction of Moving Morphology (XROMM) was used to reconstruct the 3-dimensional jaw movements of 6 pigs during chewing before and after complete unilateral lingual nerve transection. The effect of the transection were evaluated at the temporal and spatial level using Multiple Analysis of Variance. Temporal variables include gape cycle and phase durations, and the corresponding relative phase durations. Spatial variables include the amplitude of jaw opening, jaw yaw, and mandibular retraction-protraction.

RESULTS

The temporal and spatial dynamics of jaw movements did not differ when chewing ipsilateral versus contralateral to the transection. When compared to pre-transection data, 4 of the 6 animals showed significant changes in temporal characteristics of the gape cycle following the transection, irrespective of chewing side, but the specific response to the lesion was highly dependent on the animal. On the other hand, in affected individuals the amplitude of jaw movements was altered similarly in all 3 dimensions: jaw opening and protraction-retraction increased whereas jaw yaw decreased.

CONCLUSION

The variable impact of this injury in this animal model suggests that individuals use different compensatory strategies to adjust or maintain the temporal dynamics of the gape cycle. Because the amplitude of jaw movements are more adversely affected than their timing, results suggest that maintaining the tongue-jaw coordination is critical and this can come at the expense of bolus handling and masticatory performance.