School of Archaeology and Anthropology, The Australian National University, Canberra, Australian Capital Territory, Australia.
Department of Anatomy and Developmental Biology, Biomedicine Discovery Institute, Monash University, Clayton, Victoria, Australia.
J Anat. 2024 Jun;244(6):995-1006. doi: 10.1111/joa.14018. Epub 2024 Feb 3.
Primate mandibular morphology is often associated with jaw functionality of the masticatory complex in the context of variation in diets. Recent research into the disparities between the diet and jaw functionality in male and female hominoids is inconclusive and suggests that sexual dimorphism in the mandible may be influenced by external factors such as temporalis and masseter muscle morphology, which in turn may be influenced by sexual selection. As the muscles associated with mastication (i.e., the type of chewing exhibited by primates and other mammals) encompass the mandible as well as the neurocranium, including the sagittal crest among some individuals, this study investigates sex-specific associations between regions of the mandibular ramus and neurocranium associated with mastication in a dentally mature sample of Gorilla and Pongo. A total of four cranial and mandibular variables were measured in two Gorilla taxa (Gorilla gorilla gorilla and Gorilla beringei graueri) and one Pongo taxon (Pongo pygmaeus pygmaeus) (n = 220). For all three taxa, we investigate (a) whether the degree of sexual dimorphism in cranial regions associated with sagittal cresting (sagittal crest size (SCS) and temporalis muscle attachment area (TMAA)) is proportional to the degree of mandibular ramus area (MRA) and coronoid process height (CPH) sexual dimorphism, (b) whether there are sex differences in scaling relationships between TMAA and MRA, and (c) whether there are sex differences in the strength of association between TMAA and CPH. We show that for G. g. gorilla, variables associated with sagittal cresting show higher sexual dimorphism values than our two mandibular ramus variables, which is not the case for G. b. graueri or for P. p. pygmaeus. All three taxa show similar sex-specific scaling relationships between TMAA and MRA, where for males this relationship does not diverge from isometry, and for females there is a negative allometric relationship. Our findings also show intraspecific sex differences in allometric slopes between MRA and TMAA for all three taxa. Only G. g. gorilla shows a significant association between TMAA and CPH, which is observed in both sexes. Although there are some statistical associations between the cranial and mandibular regions associated with mastication, our results show that among male gorillas and orangutans, patterns of variation in the sagittal crest, TMAA, mandibular ramus and the coronoid process cannot be explained by the muscle attachment hypothesis alone. These findings have implications surrounding the associations between social behaviour and the morphology of the craniofacial complex.
灵长类动物的下颌形态通常与咀嚼复合体的咀嚼功能有关，而咀嚼功能又与饮食的变化有关。最近的研究表明，雄性和雌性人科动物的饮食和咀嚼功能之间存在差异，但结果并不确定，这表明下颌的性二态性可能受到颞肌和咬肌形态等外部因素的影响，而这些因素又可能受到性选择的影响。由于与咀嚼有关的肌肉（即灵长类动物和其他哺乳动物表现出的咀嚼类型）不仅包括下颌，还包括颅腔，包括一些个体的矢状嵴，因此本研究调查了大猩猩和猩猩中下颌支和与咀嚼有关的颅腔区域的性别特异性关联在一个牙成熟的样本中。在两个大猩猩种（大猩猩 gorilla gorilla 和大猩猩 beringei graueri）和一个猩猩种（猩猩 pygmaeus pygmaeus）（n = 220）中测量了总共四个颅面和下颌变量。对于所有三个种，我们研究了：（a）与矢状嵴有关的颅区的性二态性程度（矢状嵴大小（SCS）和颞肌附着区（TMAA））是否与下颌支区（MRA）和冠状突高度（CPH）的性二态性程度成正比，（b）TMAA 和 MRA 之间的比例关系是否存在性别差异，以及（c）TMAA 和 CPH 之间的关联强度是否存在性别差异。我们表明，对于 G. g. gorilla，与矢状嵴有关的变量显示出比我们的两个下颌支变量更高的性二态性值，而 G. b. graueri 或 P. p. pygmaeus 则不是这样。所有三个种都显示出 TMAA 和 MRA 之间相似的性别特异性比例关系，其中对于雄性，这种关系没有偏离等距，而对于雌性，则存在负异速关系。我们的研究结果还表明，在所有三个种中，MRA 和 TMAA 之间的比例斜率存在种内性别差异。只有 G. g. gorilla 显示出 TMAA 和 CPH 之间存在显著的关联，这种关联在两性中都存在。尽管与咀嚼有关的颅面区域之间存在一些统计关联，但我们的结果表明，在雄性大猩猩和猩猩中，矢状嵴、TMAA、下颌支和冠状突的变异模式不能仅用肌肉附着假说来解释。这些发现围绕着社会行为与颅面复合体形态之间的关联产生了影响。
