Contribution of eye retraction to swallowing performance in the northern leopard frog, Rana pipiens
Levine, R.P. Monroy, J.A. and Brainerd, E.L.
Most frogs retract their eyes during swallowing. This observation has led to the suggestion that eye retraction may aid swallowing by helping to push food toward the esophagus. However, this hypothesis has never been tested. The goal of this study was to combine behavioral observations, cineradiography, electromyography, and nerve transection experiments to evaluate the contribution of eye retraction to swallowing in the northern leopard frog, Rana pipiens. Results from behavioral observations reveal a high degree of variability in eye retraction and swallowing, suggesting that swallowing behaviors may be modulated based on prey position or characteristics such as size. Cineradiography shows that during eye retraction associated with swallowing, the eyes move ventrally and caudally, come in contact with the prey and appear to force it towards the pharynx. Electromyographic recordings confirm that the retractor bulbi muscles are active during eye retraction. Although frogs maintain the ability to swallow following bilateral denervation of the retractor bulbi muscles, they show a 74% increase in the number of swallows required per cricket (from a mean of 2.3 to a mean of 4 swallows per cricket; ANOVA P<0.001). These results support the hypothesis that eye retraction is an accessory swallowing mechanism to the primary tongue-based mechanism in R. pipiens when feeding on medium sized crickets. (NSF IBN-9875245)
Ecology & Evolutionary Biology - Brown University