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

Abstract

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.

Keywords: cranial structure; geometric morphometrics; paleognathous birds; phenotypic variation; shape analysis; social selection; theropod; visual display.

FIGURE 1

Illustrations depicting the phenotypic casque variation within Casuarius casuarius (a−b). Individuals are based on female representatives. Artwork by J. A. Campbell‐Smith.

FIGURE 2

Cranial line illustrations of Casuarius casuarius (casques in pastel green) in lateral (a) and rostral views (b), the two anatomical aspects from which morphometric photographs were collected. Crosshairs were centered vertically and horizontally on the eye/orbit for lateral samples; alignment of the horizontal center of the eye/orbit and rostralmost boundary of the casque with the cranial midline was used for rostral samples (indicated by transparent gray dashed lines). Photograph illustrating casque asymmetry (c), a common anatomical feature in cassowaries. Casque asymmetries in C. casuarius manifest as either rightward sinusoidal (d), leftward sinusoidal (e), non‐deviated (f), rightward (g), and leftward deviations (h) from the midline. Photo by T. L. G.

FIGURE 3

Map of Australasian Casuarius casuarius range with regional subdivisions based on historical and physical boundaries indicated on museum voucher tags (western islands near New Guinea = dark orange; Indonesian Papua = green; Northern Papua New Guinea = black; Southern Papua New Guinea = blue; Australia = dark yellow). Redrawn from BirdLife International (2019) geographical range data.

FIGURE 4

Methods for determining degrees of deviation categories (none–minimal, 0°–5° leftward/rightward = gray; slight–moderate rightward, 6°–30° = dark gold); slight–moderate leftward, 6°–30° = dark purple; severe–radical rightward, 31°–60° = light gold), and (5) severe–radical leftward, 31°–60°; = light purple) for Casuarius casuarius casques in rostral view (pastel green).

FIGURE 5

Output of the PCA comparing lateral casque outlines between sexes (female = pink datapoints and polygon; male = blue datapoints and polygon) of Casuarius casuarius (pastel green casque icon at lower left). Female and male convex hulls illustrate substantial overlap in lateral casque morphospace between the sexes. Theoretical casque shape based on the principal component axes shown as gray outlines.

FIGURE 6

Output of the PCA comparing rostral casque outlines between sexes (female = pink datapoints and polygon; male = blue datapoints and polygon) of Casuarius casuarius (pastel green casque icon at lower left). Female and male convex hulls illustrate substantial overlap in rostral casque morphospace between the sexes. Theoretical casque shape based on the principal component axes shown as gray outlines.

FIGURE 7

Output of the PCA comparing lateral Casuarius casuarius (pastel green casque icon at lower left) casque outlines between geographical regions: Australia (AUS = dark yellow), Indonesian Papua (INDP = green), Southern Papua New Guinea (SPNG = blue), and Western Islands near New Guinea (WIS = dark orange). Geographic convex hulls illustrate substantial overlap in lateral casque morphospace between the regions. Theoretical casque shape based on the principal component axes shown as gray outlines.

FIGURE 8

Output of the PCA comparing rostral Casuarius casuarius (pastel green casque icon at upper left) casque outlines between geographical regions: Australia (AUS = dark yellow), Indonesian Papua (INDP = green), Southern Papua New Guinea (SPNG = blue), and Western Islands near New Guinea (WIS = dark orange). Geographic convex hulls illustrate substantial overlap in rostral casque morphospace between the regions. Theoretical casque shape based on the principal component axes shown as gray outlines.

FIGURE 9

Figure summarizing the degree of deviation results (by percentage, rounded to the nearest tenth) for the Casuarius casuarius casque sample, which exhibited a right directional asymmetry. Micro‐CT rendering of cassowary skull (green) pictured from rostral view, and various casque asymmetry positions superimposed. Deviation categories included: (1) none–minimal (0°–5° leftward/rightward; gray), (2) slight–moderate rightward (6°–30°; dark gold), (3) slight–moderate leftward (6°–30°; dark purple), (4) severe–radical rightward (31°–60°; light gold), and (5) severe–radical leftward (31°–60°; light purple).

FIGURE 10

Pie charts summarizing degree of casque deviation results of known‐sex Casuarius casuarius samples (left, female = pink symbol; right, male = blue symbol). Leftward and rightward deviation categories were combined into the following categories: (NM) none–minimal (0°–5° leftward/rightward; lightest gray), (SM) slight–moderate leftward/rightward (6°–30°; middle gray), (SR) severe–radical leftward/rightward (31°–60°; darkest gray). Superimposed cassowary icons (rostral views) represent degrees of casque asymmetry deviations (pastel green). Percentages are rounded to the nearest tenth and sample sizes are indicated (n) under each sex symbol.