Meeting Abstract
Terrestrial and amphibious tetrapods have developed a variety of means of regulating the patency of the external nares. Crocodilians have smooth muscles associated with the external nares and nasal vestibulum. There is a distinct narial dilator that attaches to the caudal surface of the narial vestibulum and nares. Though previous descriptions vary, there is a narial constrictor that, rather than encircling the nares, wraps around the dilator muscle. Narial closure is achieved when the relaxed narial dilator is protracted against the caudal surface of the nares and narial vestibulum by the compressive force of the narial constrictor. In this way, the narial dilator functions as its own antagonist, performing opposite roles depending on whether it is in an active or passive state. Anatomical analyses included dissection, histology, and a variety of imaging techniques (CT, MRI, and 3-D reconstruction of micro-CT imaging). We explored this unusual system using sub-adult (approximately 180 cm in total length) American alligators (Alligator mississippiensis). The animals were restrained without chemical sedation or anesthesia, in a way that did not impede normal ventilator movements. A Hall effect sensor was placed on the dorsolateral surface of the animal to document the movements of the ribs. A fluid-filled soft latex catheter was inserted into one nares and connected to a fluid pressure transducer. Bipolar EMG electrodes were inserted using hypodermic needles. There was no clear temporal pattern between ventilation and narial closure, and the force of the ventilatory airflow was highly variable. The nares were patent at rest. The dilator and constrictor muscles had distinct, non-overlapping activity pattern, the amplitude of which correlated with the strength of the ventilator airflow.
