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Intraspecific variation and directional casque asymmetry in adult southern cassowaries (Casuarius casuarius)

The cranial casques of modern cassowaries (Casuarius) have long intrigued researchers; however, in‐depth studies regarding their morphological variation are scarce. Through visual inspection, it has been recognized that casque variability exists between conspecifics. Understanding casque variation h...

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Bibliographic Details
Published in:Journal of anatomy 2022-10, Vol.241 (4), p.951-965
Main Authors: Green, Todd L., Kay, David Ian, Gignac, Paul M.
Format: Article
Language:English
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Summary:The cranial casques of modern cassowaries (Casuarius) have long intrigued researchers; however, in‐depth studies regarding their morphological variation are scarce. Through visual inspection, it has been recognized that casque variability exists between conspecifics. Understanding casque variation has both evolutionary and ecological importance. Although hypothesized to be targeted by selection, intraspecific casque variation has not been quantified previously. Through a large sample of C. casuarius (n = 103), we compared casque shape (lateral and rostral views) between sexes and between individuals from non‐overlapping geographical regions using two‐dimensional (2D) geometric morphometrics. We found no statistically significant differences between the casque shape of females and males and few substantial shape differences between individuals from different geographic areas. Much of the intraspecific variation within C. casuarius is due to casque asymmetries (77.5% rightward deviating, 20.7% leftward deviating, and 1.8% non‐deviating from the midline; n = 111), which explain the high variability of southern cassowary casque shape, particularly from the rostral aspect. Finally, we discuss how our non‐significant findings implicate social selection theory, and we identify the benefits of quantifying such variation for further elucidating casque function(s) and the social biology of cassowaries. Cassowary casques are among the most iconic cranial ornaments among modern Aves. Geometric morphometric shape analysis of southern cassowary ornaments indicates no sexual dimorphism and few differences between regional populations. Instead, intraspecific casque shape variation is primarily due to directional, cranial asymmetries (illustrated as five typical casque orientations referenced to a single adult skull). These data from living cassowaries are crucial to our understanding of ornament evolution and functional morphology
ISSN:0021-8782
1469-7580
1469-7580
DOI:10.1111/joa.13733