Owerkowicz, T.*; Brainerd, E. L.; Carrier, D. R.: Electromyographic pattern of the gular pump in monitor lizards.

Gular pumping has recently been shown to play an important role in lung ventilation of monitor lizards but its evolutionary origin has not yet been investigated. To compare this mechanism with the amphibian buccal pump, we studied the activity of throat muscles during gular pumping in savannah monitors (Varanus exanthematicus). We implanted bipolar electrodes in hyobranchial muscles, and recorded their firing patterns in synchrony with kinematic, airflow and pressure profiles of the gular pump. The monitor throat is supported by a highly mobile hyoid apparatus, the unfolding and folding of which produces large volume air flows into and out of the gular cavity. Mm. sternohyoideus and branchiohyoideus expand the gular cavity, whereas mm. constrictor colli, intermandibularis and mandibulohyoideus compress it. Closure of the choanae by sublingual plicae precedes gular compression, thus allowing positive pressure to be generated in the gular cavity and air to be forced into the lungs. The gular pump is found to exhibit a neuromotor pattern identical to the buccal pump of extant amphibians, and both mechanisms involve homologous muscles. This suggests that the gular pump of monitor lizards may derive from the buccal pump of basal tetrapods. Whether it does remains to be determined by mapping gular pumping as a character on the squamate phylogeny.