Jaw muscle architecture in the greater rhea (Rhea americana): Morphological patterns and postnatal ontogeny in an herbivorous bird

Abstract

The jaw muscle architecture is crucial for understanding how muscle morphology influences diet and feeding behavior in birds, yet ontogenetic scaling patterns and their characteristics in herbivorous species remain understudied. This study examines the physiological cross-sectional area (PCSA) and fiber length of the jaw muscles in Rhea americana, an herbivorous bird whose chicks are insectivorous during early stages. The main jaw muscles of 19 Rhea americana specimens across four ontogenetic stages were analyzed, including the depressor mandibulae (a jaw depressor) and the adductor mandibulae externus, pseudotemporalis, and pterygoideus lateralis (primarily jaw adductors). Both architectural parameters showed a negative allometric scaling and only significant differences in PCSA were found between immature ages and between these and adulthood in certain adductor muscles. These variations suggest increasing force demands in adductor muscles as chicks transition from an insectivorous to an herbivorous diet. The lower PCSA in early-staged chicks reflects the reduced force required for insectivory, while the increasing PCSA with age enables force generation needed for detaching plants in adults. In comparison to other avian diets, the herbivory of greater rhea seems to be linked to a lower force-generating capacity in the adductor muscle group. This study contributes to expanding the knowledge on avian jaw musculature by exploring postnatal ontogenetic changes and their potential relationship with dietary shifts while providing morphological data that can serve as a comparative basis for understanding the link between diet and morphology in birds.