The feeding apparatus of Rhea americana (Aves, Palaeognathae): Jaw myology and ontogenetic allometry

Abstract

In birds, the jaw musculature is a crucial adaptive feature involved in feeding. The morphological traits and postnatal growth patterns of jaw muscles constitute a useful proxy to interpret feeding function and ecology. This study aims to describe the jaw muscles of Rhea americana and explore their postnatal growth pattern. A total of 20 specimens of R. americana representing four ontogenetic stages were studied. Jaw muscles were described, weighed and their proportions with respect to body mass were calculated. Linear regression analysis was used to characterize ontogenetic scaling patterns. The morphological patterns of jaw muscles were characterized by their simplicity: bellies with few or no subdivisions and similar to those described for other flightless paleognathous birds. In all stages, the muscles pterygoideus lateralis, depressor mandibulae, and pseudotemporalis had the greatest mass values. The proportion of total jaw muscle mass decreased with age from 0.22% in 1‐month‐old chicks to 0.05% in adults. Linear regression analysis showed that all muscles scaled with negative allometry with respect to body mass. The progressive decrease of jaw muscle mass relative to body mass in adults could be related to the generation of less force, which is in accordance with the herbivorous diet of adults. In contrast, the diet of rhea chicks includes a large proportion of insects thus, this greater muscle proportion could be associated with the ability to generate more force, thus providing better abilities to grasp and hold more mobile prey.

Differential growth of the adductor muscles, eyeball, and brain in the chick Gallus gallus with comments on the fossil record of stem‐group birds

“…Furthermore, other phasianid galliforms have jaws with high mechanical advantage, which is associated with high bite force (Navalón et al, 2019). Furthermore, the jaw muscles of the ratite Rhea americana are reduced relative to body mass in mature individuals, which are herbivorous, compared with birds in their first few months of life, which are omnivorous and feed on harder-bodied food items (Picasso et al, 2023). If anything, these studies suggest that the jaw muscles in Gallus may be more developed, not less, than their ancestors on the galliform or avian stems.…”

“…Although we cannot easily assess developmental dynamics in extinct species except in extraordinary cases of fossilization, we can observe these dynamics in their extant relatives. There has been interest in the homology (e.g., Holliday & Witmer, 2007), size and complexity (e.g., Kalyakin, 2015), and allometry of the jaw‐adductor apparatus across bird species (e.g., Navalón et al, 2019), but few ontogenetic series of avian jaw muscles are described outside of Psittaciformes (Tokita, 2004) and a recent study on Rhea (Picasso et al, 2023).…”