A peculiar mechanism of bite-force enhancement in lungless salamanders revealed by a new geometric method for modeling muscle moments.

Desmognathine salamanders possess unusual morphological features for lungless salamanders that have been proposed to aid in burrowing and biting, including well-ossified jaws and skull and a pair of robust ligaments connecting the atlas to the mandible. We evaluated the function of these and other peculiar desmognathine cranial features in biting by examining the morphology, mechanics and in vivo biting performance of the large Desmognathus quadramaculatus We estimated theoretical biting force using a novel geometric method that we describe. Results provide quantitative evidence to bolster earlier conclusions that the unusual atlanto-mandibular ligaments couple ventral head flexion, a unique desmognathine behavior, with biting performance. Our analysis also reveals that the ligaments not only transmit, but also amplify the force of head flexion when acting together with the unusual stalked occipital condyles, enlarged atlas and massive quadratopectoralis muscles. The geometric model predicts that this mechanism contributes five times the biting force of the three jaw levator muscles combined and predicts that maximum biting force in D. quadramaculatus matches or exceeds forces reported for similarly sized lizards. The in vivo biting performance we measured was several times greater in D. quadramaculatus than another plethodontid salamander, Pseudotriton ruber, which lacks the unusual morphology and mechanism of desmognathines. The effective biting mechanism of D. quadramaculatus we describe is an emergent property of many of the distinguishing morphological features of desmognathine salamanders and likely plays an important role in their natural history given that desmognathines use biting in feeding, defense and even courtship.