Functional morphology of lingual protrusion in marine toads (Bufo marinus)

Bufo marinus catches its prey by stiffening the intrinsic muscles of the tongue, rapidly flipping the tongue out of the mouth. High-speed cinematography synchronized with computer-analyzed electromyograms (EMGs)shows that during the flip the tongue is supported by the M. genioglossus medialis and that this muscle stiffens into a rod when stimulated. Coincident stiffening of the transversely arranged M. genioglossus basalis provides a wedge under the anterior tip of this rod. Stiffening of the M. submentalis depresses the mandibular symphysis and brings the dentary tips together. The M. submentalis also acts on the wedge of the basalis to raise and rotate the rigid rod of the medialis over the symphysial attachment. The tip of this lingual rod carries along the pad and soft tissues of the tongue. The lingual pad, positioned the posterodorsal portion of the resting tongue, rotates during eversion so that its dorsal surface impacts onto the prey object. Retraction starts by contraction of the elongate, parallel fibers of the M. hyoglossus; this retracts the medical sulcus of the pad and holds the prey by a suction cup-like effect. The extensibility of the buccal membranes allows the pad to be retracted first; it reaches the posterior portion of the buccal cavity before the still-rigid, backward rotating M. genioglossus has reached the level of the symphysis. Protraction of the hyoid facilitates the extension of the M. hyoglossus. The M. sternohyoideus only retracts the hyoid and stabilizes it when the tongue starts to pull posteriorly; it does not assist tongue protrusion. The Mm. petrohyoideus and omohyoideus show only incidental activity, and the M. depressor mandibulae participate in mouth opening but is not otherwise involved in the flip. Previous hypotheses of the flipping mechanism are reviewed and evaluated.