Assessing the microstructure of simple rhamphothecae in chickens (Aves: Galliformes: <i>Gallus gallus domesticus</i> )

Corneous sheaths on the jaws of beaked tetrapods, or rhamphothecae, are classified as simple or compound based on their external morphology. In birds, simple rhamphothecae possess single corneous sheaths covering the upper jaws and the mandibles, respectively, rather than the several corneous plates of compound rhamphothecae. More than 60% of modern birds have simple rhamphothecae, but not all birds with simple rhamphothecae use their beaks for the same feeding style or dietary preferences. Grossly similar simple rhamphothecae may possess unreported microanatomical differences in response to different functional demands, but this requires first exploring whether the microanatomy of simple rhamphothecae is homogenous. We hypothesized based on foraging biomechanics that the rostral portion of the rhamphotheca is more mechanically resistant than the caudal portion, and the microanatomy will reflect this. We examine the microanatomy and biomechanical properties of rhamphothecae using chickens (Gallus gallus domesticus). We map the microstructure and relative hardness of corneous materials throughout the rhamphotheca using plastic-embedded histology and microindentation. Histological sections reveal eight corneous regions, more numerous on the rostral portion of the beak, making up the seemingly continuous corneous sheath of the upper jaws and mandibles. Microindentation of individual regions reveals hardness differences, with the rostral rhamphothecal tomia (cutting edge) having the highest hardness, and layers comprising the caudal portion of the rhamphotheca having the lowest. This configuration shows that more resistant corneous layers are found rostrally to accommodate the mechanical interactions happening at the tip. Our study shows that unreported complexity within simple beaks will allow us to determine simple rhamphotheca diversity and address the homoplasy of simple rhamphothecae in modern birds.