Bite force estimates and morphological correlations of jaw muscles in parrots

Parrots (Order: Psittaciformes) are renowned for their high bite force (BF) and morphological correlates thereof, including a highly mobile skull and two unique mandibular adductor muscles-m. pseudomasseter and m. ethmomandibularis, with the former being hypothesized to increase force output. High BF has been hypothesized to relate to their generally high seed and grain consumption. However, both in vivo and anatomical BF estimates for this group are limited, and the few empirical studies that exist found no relation between BF and diet in these birds. BF can be anatomically estimated via analysis of the physiological cross-sectional area (PCSA) of each masticatory muscle combined with biomechanical modeling of the skull. Simple single-joint models, based on mammalian mechanics, may not be adequate for application to the complex parrot skull. To this end, we performed sharp dissection and then used chemical digestion in nitric acid to collect masticatory muscle fiber data across a broad sample of 27 species. We then analyzed three biomechanical models of BF estimation as well as possible dietary correlates of the resulting estimates. Our results confirm that parrots do indeed have high BF abilities (up to 242 N in the largest species) and that muscle mass, PCSA, and BF scaled positively against body mass. As previously observed, dietary mechanical properties showed no significant correlation to BF. The two biomechanical models that reflect the more elaborate jaw movements possible in birds estimated greater BF than the simpler mammalian-like model and should be validated with future in vivo BF experiments.