2D Geometric Morphometric Shape Analysis of Casuarius casuarius(Aves: Paleognathae) Cranial Casques

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Abstract

Many vertebrates (e.g., chameleons, bovids, hornbills) possess cranial elaborations; however, studies investigating the morphological variation of these structures are rare. For example, the conspicuous cranial casques of modern cassowaries have long been hypothesized to perform a number of functions (e.g., physical ramming, vocalization, thermoregulation, visual display) despite an absence of quantitative methodologies to determine whether ornament shape variation within the group aligns with proposed biological roles. In order to fill this data gap, we compared casque shape between sexes and native geographic regions in a large sample of adult southern cassowaries (Casuarius casuarius; n = 103). We quantified casque shape in lateral and rostral views using elliptical Fourier analysis, ordinated the shape data, and used multivariate analysis of variance to address specific comparisons. We hypothesized that casques would be sexually dimorphic and regionally specific, consistent with proposed display functions. However, we found no significant differences in casque shape between female and male C. casuariusand few significant shape differences between individuals from specific geographical areas. Much of the intraspecific casque shape variation, particularly from the rostral aspect, is due to asymmetries across the midline. In our sample, casques were 77.5% rightward deviating, 20.7% leftward deviating, and 1.8% non-deviating. Although it is unclear if this directional casque asymmetry provides a functional advantage, future studies may elucidate this feature. Altogether, two-dimensional shape analysis does not support a role for the casque as a de factodiscriminator between sexes nor regional sub-populations in C. casuarius. These findings stand in contrast to those of other ornamented birds (e.g., Numida meleagris) and potentially point towards alternative biological functions.

Original languageEnglish
JournalFASEB Journal
Volume36
DOIs
StatePublished - 1 May 2022

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