The effect of food consistency upon jaw movement in the macaque: a cineradiographic study

Variation in the form of masticatory cycles in individuals is often assumed to be limited. The contrary hypothesis, that jaw cycles vary widely but systematically with food consistency, was tested in macaques fed similarsized pieces of monkey chow, apple, and banana. With the animals under general anesthesia, radiopaque markers were inserted into the jaw, tongue, and hyoid. Oral movements were recorded by cineradiography at 100 frames/sec in lateral projection synchronously with frontal view cinephotography (50 frames/sec). The films were examined for the events that subdivide each jaw movement cycle into its constituent phases (fast closing, slow closing, intercuspal, slow or early opening, final opening). The frame numbers at which these events occurred were used to define phase durations. The numbers of cycles preceding a swallow increased with the hardness of the ingested food item. Regardless of the test food, every feeding sequence (initial ingestion to final clearance of mouth) contained multiple swallows, each of which defined the end of a sub-sequences when the animals were feeding on chow, the sub-sequences were initially long (20 cycles or more), but when they were feeding on banana, the sub-sequences were short (10 cycles or fewer). Although the form of individual cycles (defined by phase durations) was often unrelated to that of neighboring cycles, the general cycle characteristics in a sub-sequence typified a particular food. Chow feeding cycles were characterized by slow-closing (SC) phases of long duration with slow-opening (SO) phases of short duration; the characteristics of banana feeding cycles were the reverse. SC duration correlated directly and SO duration correlated inversely with food hardness (p < 0.001). The evidence supports the view that the centrally generated pattern of movement is highly dependent upon intra-oral sensory feedback.