Fabre P-H, Herrel A, Fitriana Y, Meslin L, Hautier L
Institut des Sciences de l'Evolution (ISEM, UMR 5554 CNRS), Universite Montpellier II, Montpellier, France.
National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
J Anat. 2017 Sep;231(3):380-397. doi: 10.1111/joa.12639. Epub 2017 Jun 6.
Murines are well known for their generalist diet, but several of them display specializations towards a carnivorous diet such as the amphibious Indo-Pacific water-rats. Despite the fact that carnivory evolved repeatedly in this group, few studies have investigated associated changes in jaw muscle anatomy and biomechanics. Here, we describe the jaw muscles and cranial anatomy of a carnivorous water-rat, Hydromys chrysogaster. The architecture of the jaw musculature of six specimens captured both on Obi and Papua were studied and described using dissections. We identified the origin and insertions of the jaw muscles, and quantified muscle mass, fiber length, physiological cross-sectional area, and muscle vectors for each muscle. Using a biomechanical model, we estimated maximum incisor and molar bite force at different gape angles. Finally, we conducted a 2D geometric morphometric analyses to compare jaw shape, mechanical potential, and diversity in lever-arm ratios for a set of 238 specimens, representative of Australo-Papuan carnivorous and omnivorous murids. Our study reveals major changes in the muscle proportions among Hydromys and its omnivorous close relative, Melomys. Hydromys was found to have large superficial masseter and temporalis muscles as well as a reduced deep masseter and zygomatico-mandibularis, highlighting major functional divergence among omnivorous and carnivorous murines. Changes in these muscles are also accompanied by changes in jaw shape and the lines of action of the muscles. A more vertically oriented masseter, reduced masseteric muscles, as well as an elongated jaw with proodont lower incisors are key features indicative of a reduced propalinality in carnivorous Hydromys. Differences in the fiber length of the masseteric muscles were also detected between Hydromys and Melomys, which highlight potential adaptations to a wide gape in Hydromys, allowing it to prey on larger animals. Using a biomechanical model, we inferred a greater bite force in Hydromys than in Melomys, implying a functional shift between omnivory and carnivory. However, Melomys has an unexpected greater bite force at large gape compared with Hydromys. Compared with omnivorous Melomys, Hydromys have a very distinctive low mandible with a well-developed coronoid process, and a reduced angular process that projects posteriorly to the ascending rami. This jaw shape, along with our mechanical potential and jaw lever ratio estimates, suggests that Hydromys has a faster jaw closing at the incisor, with a higher bite force at the level of the molars. The narrowing of the Hydromys jaw explains this higher lever advantage at the molars, which constitutes a good compromise between a wide gape, a reduced anterior masseteric mass, and long fiber lengths. Lever arms of the superficial and deep masseter are less favourable to force output of the mandible in Hydromys but more favourable to speed. Compared with the small input lever arm defined between the condyle and the angular process, the relatively longer mandible of Hydromys increases the speed at the expense of the output force. This unique combination of morphological features of the masticatory apparatus possibly has permitted Hydromys to become a highly successful amphibious predator in the Indo-Pacific region.
鼠类以其多样化的饮食而闻名，但其中一些鼠类表现出对肉食性饮食的特化，比如两栖的印度-太平洋水鼠。尽管肉食性在这一群体中多次独立演化，但很少有研究调查其颌部肌肉解剖结构和生物力学的相关变化。在此，我们描述了一种肉食性水鼠——黄斑水鼠（Hydromys chrysogaster）的颌部肌肉和颅骨解剖结构。我们对在奥比岛和巴布亚捕获的6个标本的颌部肌肉组织架构进行了解剖研究和描述。我们确定了颌部肌肉的起点和止点，并对每块肌肉的肌肉质量、纤维长度、生理横截面积和肌肉向量进行了量化。使用生物力学模型，我们估计了不同张口角度下的最大门齿咬合力和臼齿咬合力。最后，我们进行了二维几何形态测量分析，以比较238个标本的颌部形状、机械潜力和杠杆臂比例的多样性，这些标本代表了澳大拉西亚-巴布亚的肉食性和杂食性鼠类。我们的研究揭示了黄斑水鼠与其杂食性近亲长吻袋貂（Melomys）之间肌肉比例的重大变化。研究发现，黄斑水鼠有较大的浅层咬肌和颞肌，而深层咬肌和颧下颌肌则有所减少，这突出了杂食性和肉食性鼠类之间的主要功能差异。这些肌肉的变化还伴随着颌部形状和肌肉作用线的改变。咬肌更垂直的方向、咬肌的减少以及下颌门齿前突的拉长颌部是表明肉食性黄斑水鼠推进式咀嚼减少的关键特征。在黄斑水鼠和长吻袋貂之间也检测到了咬肌纤维长度的差异，这突出了黄斑水鼠对大张口的潜在适应性，使其能够捕食更大的动物。使用生物力学模型，我们推断黄斑水鼠的咬合力比长吻袋貂更大，这意味着从杂食性到肉食性的功能转变。然而，与黄斑水鼠相比，长吻袋貂在大张口时却有着出人意料的更大咬合力。与杂食性的长吻袋貂相比，黄斑水鼠的下颌非常独特，较低且冠状突发达，角突向后延伸至升支。这种颌部形状，以及我们对机械潜力和颌部杠杆比例的估计表明，黄斑水鼠的门齿处颌部闭合更快，臼齿水平的咬合力更高。黄斑水鼠颌部的变窄解释了其在臼齿处更高的杠杆优势，这在大张口、前部咬肌质量减少和长纤维长度之间构成了良好的平衡。黄斑水鼠浅层和深层咬肌的杠杆臂对下颌的力输出不太有利，但对速度更有利。与髁突和角突之间定义的小输入杠杆臂相比，黄斑水鼠相对较长的下颌以输出力为代价提高了速度。咀嚼器官这种独特的形态特征组合可能使黄斑水鼠成为印度-太平洋地区非常成功的两栖捕食者。
