Abstract
ABSTRACTThe masticatory apparatus amongst closely related carnivoran species raises intriguing questions about the interplay between allometry, function, and phylogeny in defining interspecific variations of cranial morphology. Here we describe the gross structure of the jaw adductor muscles of several species of canid, and then examine how the muscles are scaled across the range of body sizes, phylogenies, and trophic groups. We also consider how the muscles are accommodated on the skull, and how this is influenced by differences of endocranial size. Data were collected for a suite of morphological metrics, including body mass, endocranial volume, and muscle masses and we used geometric morphometric shape analysis to reveal associated form changes. We find that all jaw adductor muscles scale isometrically against body mass, regardless of phylogeny or trophic group, but that endocranial volume scales with negative allometry against body mass. These findings suggest that head shape is partly influenced by the need to house isometrically scaling muscles on a neurocranium scaling with negative allometry. Principal component analysis suggests that skull shape changes, such as the relatively wide zygomatic arches and large sagittal crests seen in species with higher body masses, allow the skull to accommodate a relative enlargement of the jaw adductors compared with the endocranium. Anat Rec, 299:951–966, 2016. © 2016 The Authors The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology Published by Wiley Periodicals, Inc.
Highlights
We evaluate the hypothesis that species with a high bite force and large body mass, such as the hypercarnivores (Wroe et al, 2005; Christiansen and Wroe, 2007), have absolutely and relatively larger muscles, and we speculate that the gross morphology of the masticatory musculature of hypercarnivorous canid species differs from those of generalists and small prey specialists and deviates significantly from simple predictive patterns of size scaling
This was reflected in the scaling of endocranial volume surface area (EVSA) to body mass, which has a slope of 0.43 from an expected slope of 0.66 (t test P value 0.0001)
We reported a couple of differences of muscle subdivision scaling between the different dietary groups, non were statistically significant and morphological variance was minimal and much less than we expected—that is, hypercarnivorous species, which might be expected to have a have relatively larger muscles to generate greater bite force, have the same ratio of muscle masses to body mass as those with assumed weaker bites, the generalists and small prey hunters
Summary
SpecimensSpecimens from 8 of the 13 genera that make up the canidae family were obtained from either euthanased zoo stock or vermin control (Table 1). The data set is not inclusive of all canid species, it covers a broad range of head shapes, body sizes and phylogenetic groups, and it includes all four of the hypercarnivorous species (Van Valkenburgh, 2007). For the purposes of this study species were identified as being from one of the three trophic groups as described by Slater et al (2009) (Table 1). Some degree of sexual dimorphism has been documented in many canid species, but the literature concurs that it is modest and that overall body size is the greatest differential factor. Where species have been shown to exhibit dental dimorphism, males typically have 8–15% longer canines but this is thought to relate to behavioral displays and is not correlated to body mass or skull length (Gittleman and Van Valkenburgh, 1997; Kim et al, 2012). All specimens were dissected at near occlusal bite, that is, with minimal gape (Fig. 1a)
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