Monroy, J.A.*; Nishikawa, K.C.: Aiming during prey capture in microhylid frogs

Microhylid frogs have the unusual ability to aim their tongues independently of the head during prey capture. The tongue is elongated hydrostatically, which requires a novel protractor muscle (m. genioglossus) morphology. This novel morphology and demand for fine motor control raised the question: Are there unusual aspects to the innervation of the tongue muscles in microhylids? Retrograde labeling of the hypoglossal nerve revealed an exceptionally large number of motor neurons distributed more caudally in the brainstem when compared to other anurans. In addition, denervation and EMG experiments have demonstrated that the m. genioglossus is the primary muscle responsible for aiming the tongue within the Microhylidae. Thus, an increase in motor neurons may have resulted from an increase in the innervation of the m. genioglossus required to produce hydrostatic elongation and aim the tongue. The speed of tongue protraction is highly variable between species (30 � 200 cm/s). Preliminary results suggest that microhylids with slower protraction speeds will have a larger number of motor neurons innervating the m. genioglossus, which allows more control over the tongue than microhylids that protract their tongues at faster speeds. This hypothesis suggests a trade-off between the speed of protraction and the degree of control over the tongue in three dimensions. This trade-off may also reflect a frog’s feeding strategy and ecological niche.