Within-species skull shape variation of marsupial mammals is widely considered low and strongly size-dependent (allometric), possibly due to developmental constraints arising from the altricial birth of marsupials. However, species whose skulls are impacted by strong muscular stresses – particularly those produced through mastication of tough food items – may not display such intrinsic patterns very clearly because of the known plastic response of bone to muscle activity of the individual. In such cases, allometry may not dominate within-species shape variation, even if it is a driver of evolutionary shape divergence; ordination of shape in a geometric morphometric context through principal component analysis (PCA) should reveal main variation in areas under masticatory stress (incisor region/zygomatic arches/mandibular ramus); but this main variation should emerge from high individual variability and thus have low eigenvalues. We assessed the evidence for high individual variation through 3D geometric morphometric shape analysis of crania and mandibles of three species of grazing-specialized wombats, whose diet of tough grasses puts considerable strain on their masticatory system. As expected, we found little allometry and low Principal Component 1 (PC1) eigenvalues within crania and mandibles of all three species. Also as expected, the main variation was in the muzzle, zygomatic arches, and masticatory muscle attachments of the mandibular ramus. We then implemented a new test to ask if the landmark variation reflected on PC1 was reflected in individuals with opposite PC1 scores and with opposite shapes in Procrustes space. This showed that correspondence between individual and ordinated shape variation was limited, indicating high levels of individual variability in the masticatory apparatus. Our results are inconsistent with hypotheses that skull shape variation within marsupial species reflects a constraint pattern. Rather, they support suggestions that individual plasticity can be an important determinant of within-species shape variation in marsupials (and possibly other mammals) with high masticatory stresses, making it difficult to understand the degree to which intrinsic constraints act on shape variation at the within-species level. We conclude that studies that link micro- and macroevolutionary patterns of shape variation might benefit from a focus on species with low-impact mastication, such as carnivorous or frugivorous species.

中文翻译：

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咀嚼系统的个体差异在适应食草的有袋袋熊的 3D 头骨形状中占主导地位

有袋哺乳动物的种内颅骨形状变异被广泛认为是低且强烈的尺寸依赖性（异速生长），这可能是由于有袋动物早产引起的发育限制。然而，头骨受到强烈肌肉压力影响的物种——尤其是那些通过咀嚼坚硬食物而产生的物种——可能不会非常清楚地显示出这种内在模式，因为已知骨骼对个体肌肉活动的塑性反应。在这种情况下，异速生长可能不会主导物种内的形状变化，即使它是进化形状分化的驱动因素；通过主成分分析 (PCA) 在几何形态测量环境中对形状进行排序应该揭示咀嚼应力下区域（门牙区/颧弓/下颌支）的主要变化；但这种主要变异应该来自高个体变异性，因此具有低特征值。我们通过对三种专门放牧的袋熊的头盖骨和下颌骨进行 3D 几何形态测量形状分析来评估个体差异很大的证据，这些袋熊以坚韧的草为食，对其咀嚼系统造成了相当大的压力。正如预期的那样，我们在所有三个物种的颅骨和下颌骨中发现了很少的异速生长和低主成分 1 (PC1) 特征值。同样如预期的那样，主要变化在于下颌支的口吻、颧弓和咀嚼肌附着点。然后，我们实施了一项新测试，以询问 PC1 上反映的标志性变化是否反映在具有相反 PC1 分数和在 Procrustes 空间中具有相反形状的个体中。这表明个体和协调的形状变化之间的对应关系是有限的，表明咀嚼器中的个体变异性很高。我们的结果与假设有袋动物的头骨形状变化反映了一种约束模式是不一致的。相反，他们支持这样的建议，即个体可塑性可能是具有高咀嚼压力的有袋动物（可能还有其他哺乳动物）的种内形状变化的重要决定因素，这使得难以理解内在约束作用于内部形状变化的程度。 -物种水平。我们得出结论，将形状变化的微观和宏观进化模式联系起来的研究可能受益于对咀嚼影响低的物种的关注，例如食肉或食果物种。