Meeting Abstract
Cranial kinesis, the movement of cranial bones in relation to the neurocranium, requires the coordination of muscles and ligaments to finely move the bill and mandible. These soft tissues are often extremely small, making studying their growth and development in many groups difficult, especially for taxa with convoluted musculature and small body size such as Passeriformes. We used micro-computed tomography (μCT), diffusible iodine-based contrast-enhanced CT imaging, and digital dissection to study and quantify the ontogeny of mandibular and kinetic musculature in the passerine jaw (black-throated finch, Poephila cincta) for comparison to a common non-passerine model (domestic chicken, Gallus gallus domesticus). Notably the altricial finch had more muscle partitioning than the precocial chicken, particularly among Musculus adductor mandibulae. This was apparent throughout ontogeny, indicating that the passerine jaw adductor and protractor bauplan is set relatively early in development. Such partitioning, seen in several species of finch, is a hallmark of passerine jaw anatomy. Ontogenetic shifts in muscle mass were unexpected in our dataset, however. For example, absolute jaw muscle mass was reduced in the adult finch as compared to the fledgling individual. This may be from low sample size, or it may represent the peculiarities of avian life-history patterns (e.g. displaying males often show declining body mass with courtship, and females demonstrate similar declines associated with parental care). Thus, selecting specimens from outside of the breeding season is likely crucial for obtaining high-quality, comparative data that includes quantifying functional anatomical features in birds.
