Cranial Musculoskeletal Description of Black-Throated Finch (Aves: Passeriformes: Estrildidae) with DiceCT.

K H T To,H D O’Brien,P M Gignac,M R Stocker

doi:10.1093/iob/obab007

Abstract

Synopsis Dietary requirements and acquisition strategies change throughout ontogeny across various clades of tetrapods, including birds. For example, birds hatch with combinations of various behavioral, physiological, and morphological factors that place them on an altricial–precocial spectrum. Passeriformes (=songbirds) in particular, a family constituting approximately more than half of known bird species, displays the most drastic difference between hatchling and adults in each of these aspects of their feeding biology. How the shift in dietary resource acquisition is managed during ontogeny alongside its relationship to the morphology of the feeding apparatus has been largely understudied within birds. Such efforts have been hampered partly due to the small size of many birds and the diminutive jaw musculature they employ. In this study, we used standard and diffusible iodine-based contrast-enhanced computed tomography in conjunction with digital dissection to quantify and describe the cranial musculature of the Black-throated Finch (Poephila cincta) at fledgling and adult stages. Our results reveal that in both the fledgling and the adult, cranial musculature shows clear and complex partitioning in the Musculus adductor mandibulae externus that is consistent with other families within Passeriformes. We quantified jaw-muscle sizes and found that the adult showed a decrease in muscle mass in comparison to the fledgling individual. We propose that this could be the result of low sample size or a physiological effect of parental care in Passeriformes. Our study shows that high-resolution visualization techniques are informative at revealing morphological discrepancies for studies that involve small specimens such as Passeriformes especially with careful specimen selection criteria.

Highlights

Throughout ontogeny, shifts in food resource acquisition are common across vertebrates (Duffield and Bull 1998; Whitfield and Donnelly 2006; Wang et al 2017)
The highly versatile keratinized rhamphotheca that covers the rostrum and mandible, along with the additional range of motion facilitated by cranial kinesis, is thought to have contributed to modern birds’ global ecological success, exemplified by Passeriformes because its members are exceptionally diverse in their ecologies, diets, and morphologies
We examine the utility of diffusible iodine-based contrast-enhanced computed tomography (diceCT) and digital dissection for small specimens by describing the jaw musculature of two growth stages, adult and fledgling, as well as qualitatively and quantitatively documenting morphology of the jaw adductor chamber and its components

Summary

Introduction

Throughout ontogeny, shifts in food resource acquisition are common across vertebrates (Duffield and Bull 1998; Whitfield and Donnelly 2006; Wang et al 2017). Such changes necessarily impact dietary opportunities and behaviors, which can be accommodated by subsequent functional–anatomical changes to the size (e.g., crocodiles; Gignac and Erickson 2015), morphology (e.g., caterpillars to butterflies), and physiology of the feeding system (e.g., birds; Starck 1993), or a combination of each. Passeriform skull osteology, with respect to the beak (Grant and Grant 1996; Abzhanov et al 2004; Podos et al 2004; Clayton et al 2005; Abzhanov et al 2006), has been the focus of many studies (Jollie 1958; Warter 1965; Rich et al 1985; Hernandez et al 1993; Thomas 2001; James 2004; Abzhanov et al 2006; Seijas and Trejo 2011; Turker 2012; Donatelli 2013; Previatto and Posso 2015a; Guzzi et al 2016; Ujhelyi 2016; Lima et al 2019), but only a small fraction of those studies documented the jaw musculature and other soft tissues that enable kinesis to function

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Conclusion