The activity of jaw and hyolingual muscles during the entire feeding sequence is examined in the lizard Agama stellio, with special focus on the intraoral transport and swallowing stages. Correlation of electromyography (EMG) data with kinematics shows that the kinematic phases (slow opening, SO; fast opening, FO; fast closing, FC; slow closing/power stroke, SC/PS) are characterised by distinct activities in the jaw and hyolingual muscles. The SO phase is clearly the result of tongue protraction (upon protraction, the tongue is pulled against the prey and consequently the lower jaw is pushed down), whereas the FO phase is caused by activity in the jaw opener and dorsal cervical muscles. Both the FC and SC/PS phases are characterised by pronounced activity in the jaw adductor muscles. Tongue retraction is produced by activity in the hyoid and tongue retractor muscles. A quantitative analysis of time-related EMG data shows that, in accordance with the kinematic analyses, three different stages can be recognised as components of the feeding cycle: prey capture, intraoral transport and swallowing. However, analysis of intensity-related data allowed a fourth stage, crushing, to be detected. Whereas there are indications that prey capture, intraoral transport and swallowing are controlled by different motor patterns, the differences between crushing and transport are likely to be caused by feedback mechanisms. Our results show the importance of including intensity-related data in quantitative analyses of EMG recordings in order to discriminate between feeding stages. Additionally, it is shown that both the jaw and the hyolingual muscles play crucial roles during feeding. During all stages, movements of the hyolingual apparatus are an essential part of the feeding cycle. Thus, when examining lizard feeding mechanisms, the activity patterns of the hyolingual muscles should not be neglected.
The kinematics of prey capture, intraoral transport and swallowing in lizards of the species Agama stellio (Agamidae) were investigated using cineradiography (50 frames s-1) and high-speed video recordings (500 frames s-1). Small metal markers were inserted into different parts of the upper and lower jaw and the tongue. Video and cineradiographic images were digitized, and displacements of the body, head, upper and lower jaw and the tongue were quantified. Twenty additional variables depicting displacements and timing of events were calculated. A factor analysis performed on the kinematic data separates prey capture and swallowing cycles from intraoral transport bites. However, the intraoral transport stage cannot be separated into chewing (reduction) and transport bites. The effect of prey type and size on the feeding kinematics of intraoral transport and swallowing cycles was investigated. During the intraoral transport stage, distinct aspects (e.g. durations, maximal excursions) of the gape and tongue cycle are modulated in response to both the size and type of the prey item. The results for A. stellio generally agree with a previous model, although it is the entire slow opening phase rather than solely the duration of the second part of this phase that is affected by the size of the prey. The intraoral transport cycles in A. stellio show the two synapomorphic characteristics of tetrapods (tongue-based terrestrial intraoral prey transport and the existence of a long preparatory period of prey compression). However, not all five characters of the feeding cycle previously proposed for amniotes are present in A. stellio. One major difference is that in A. stellio the recovery of the hyolingual apparatus does not take place during the slow opening phase but during the slow closing/powerstroke phase.