An electromyographic study of the function of the jaw adducting muscles in Varanus exanthematicus (varanidae)

The function of major features of the skull of Varanus exanthematicus during feeding was examined using cineradiography and electromyography. During the initial stages of feeding, Varanus grabs and orients a prey item in the mouth with no mastication, tearing of the prey, or killing bite. Ingestion is through a highly stereotyped movement, inertial feeding. The tongue plays no role in food transport. Once the prey is in the pharyngeal region, the hyoid apparatus squeezes the prey into the esophagus and stomach. Activity of jaw adducting muscles during prey orientation and inertial feeding is strikingly different. In prey orientation, the adductor musculature is active over long periods, and intermuscular differentiation and unilateral activity are common. During these phases the musculature is producing force against the resistance of the prey item held between the teeth. In inertial feeding, the jaw musculature functions to close the jaws rapidly against little resistance. A consistent pattern of intramuscular differentiation is present, with some portions of the musculature being active during both jaw opening and closing. Activity of the Mm. adductor mandibulae externus and pterygoideus is indistinguishable. Neither meso- nor metakinetic movement was observed during inertial feeding; resolution of interacranial movement was less certain during power phases. The quadrate moved during jaw opening and closing in inertial feeding. However, its movement was not linked with that of the palatomaxillary segment. These data are discussed in three contexts: cranial kinesis, intramuscular differentiation, and the mechanics of whole muscles.