Quantitative electromyography of the masticatory muscles of Pteropus giganteus (Megachiroptera)

Mastication has been studied by cinematography and quantitative electromyography while flying foxes, Pteropus giganteus, were freely feeding on standardized pieces of apple, soaked raisin, and banana. The primarily orthal mandibular movements are caused by mainly bilaterally symmetrical firing of all the masticatory muscles. Asymmetric activity in the superficial and deep masseter and medial pterygoid causes slight protrusion early in opening. Slight lateral deviations at the end of opening and at the start of closing are caused by asymmetric and asynchronous activity in the pterygoids and digastrics, and by asynchronous firing of the deep temporalis and zygomaticomandibularis. Food consistency affects movement characteristics as well as characteristics of muscular activity. In this study electromyograms were digitized and the number of spikes and mean amplitude per interval (set by the filming rate) recorded. Although a significant correlation exists between descriptors, the product thereof appears to be the best predictor of certain kinematic variables (cycle length and maximum excursion of the mandible). On the other hand, the changes in magnitude of muscular activity as a function of the position of a cycle in the reduction sequence and as a function of food consistency are more translated in a variation of the mean amplitude than in a variation of the number of spikes per interval. Observed variation differs among muscles studied. It is most apparent in the superficial and deep masseter and least in the temporalis and zygomaticomandibularis. Late cycles of apple and raisin mastication are long and exhibit large gapes but almost no anterior movement. The adductor activity frequently shows a synchronized, pulsatile pattern leading to an unfused tetanus.