The Hawaiian archipelago is home to Hawaiian honeycreepers, an adaptive radiation of 54 finches known for its diverse ecomorphology. Speciation was driven by changes in food availability to colonizing lineages and was transformative to anatomical regions that accommodate feeding, which include the skull, tongue muscles, and tongue bones (hyoid apparatus). The hyoid functions as a bony platform for tongue attachment, and its morphology is highly variable in Drepanidinae. Incorporating both novel and existing research about musculature, skull type, feeding behavior, and historical ecology, we used a comparative biological approach to interpret the functional significance of hyoidal features with the goal of obtaining a better understanding of what happened to the collective feeding anatomy of lineages during the adaptive radiation event. To visualize skeletons, we applied X-ray microcomputed tomography to 14 species (whole, preserved specimens), 4 of which were treated with iodine to reveal musculature (diffusible iodine-based contrast enhanced computed tomography). We sourced musculature for an additional 4 species from a pre-existing monograph. Information about feeding behavior and relevant historical ecological findings informed our hypotheses linking form to function..
There are three highly discernable ecomorphotypes (nectarivorous, insectivorous, and seed-eating) where anatomy and diet coincide. Three sets of hyoid functionality hypotheses emerged. Seed-eater anatomy is predictable with a conical beak, stocky hyoid, and anatomy well suited for intraoral handling of food. Highly nectarivorous species have deep (tall) beaks and hyoids that are long in most aspects and should accommodate torsion of the foretonge. The condition has evolved at least twice and typification excludes beak length and curvature. In all instances where insectivory predominates, species have narrow beaks and hyoids that are extremely developed anteriorly, short in the midsegment, and as long or longer than the nectarivorous species in its posterior segment. a condition common among passerines. Insectivore hyoids should cater to a well-anchored, flat tongue that is powered by muscles that are concentrated relative to the two other aforementioned, generalized types. This tongue may require less energy to extend, allowing for an optimal number of tongue flicks per unit of energy expenditure. Alternatively, this is a relatively restive tongue that plays a smaller role in feeding than in seed-eaters and nectarivorous species. Muscles appear to have transformed, with two instances of muscles lost.