Etienne Cyril, Houssaye Alexandra, Hutchinson John R
UMR 7179 Mécanismes adaptatifs et évolution (MECADEV), Centre National de la Recherche Scientifique, Muséum National d'Histoire Naturelle, Paris, France.
Structure and Motion Laboratory, Royal Veterinary College, Hatfield, United Kingdom.
PeerJ. 2021 May 11;9:e11314. doi: 10.7717/peerj.11314. eCollection 2021.
Land mammals support and move their body using their musculoskeletal system. Their musculature usually presents varying adaptations with body mass or mode of locomotion. Rhinocerotidae is an interesting clade in this regard, as they are heavy animals potentially reaching three tons but are still capable of adopting a galloping gait. However, their musculature has been poorly studied. Here we report the dissection of both forelimb and hindlimb of one neonate and one adult each for two species of rhinoceroses, the Indian rhinoceros () and the white rhinoceros (). We show that their muscular organisation is similar to that of their relatives, equids and tapirs, and that few evolutionary convergences with other heavy mammals (e.g. elephants and hippopotamuses) are present. Nevertheless, they show clear adaptations to their large body mass, such as more distal insertions for the protractor and adductor muscles of the limbs, giving them longer lever arms. The quantitative architecture of rhino muscles is again reminiscent of that of horses and tapirs, although contrary to horses, the forelimb is much stronger than the hindlimb, which is likely due to its great role in body mass support. Muscles involved mainly in counteracting gravity (e.g. , , , ) are usually highly pennate with short fascicles facilitating strong joint extension. Muscles involved in propulsion (e.g. , , ) seem to represent a compromise between a high maximal isometric force and long fascicles, allowing a reasonably fast and wide working range. Neonates present higher normalized maximal isometric force than the adults for almost every muscle, except sometimes for the extensor and propulsor muscles, which presumably acquire their great force-generating capacity during the growth of the animal. Our study clarifies the way the muscles of animals of cursorial ancestry can adapt to support a greater body mass and calls for further investigations in other clades of large body mass.
陆生哺乳动物利用其肌肉骨骼系统支撑身体并进行移动。它们的肌肉组织通常会随着体重或运动方式的不同而呈现出不同的适应性变化。在这方面，犀科是一个有趣的类群，因为它们体型庞大，体重可能达到三吨，但仍能采用疾驰步态。然而，它们的肌肉组织却鲜有研究。在此，我们报告了对两种犀牛（印度犀（）和白犀（））的一只新生犀牛和一只成年犀牛的前肢和后肢进行的解剖。我们发现，它们的肌肉组织与它们的近亲马科动物和貘科动物相似，与其他大型哺乳动物（如大象和河马）几乎没有进化趋同现象。尽管如此，它们仍表现出对其庞大体重的明显适应性，例如四肢的前伸肌和内收肌的附着点更靠近肢体远端，从而使它们具有更长的杠杆臂。犀牛肌肉的定量结构再次让人联想到马和貘的肌肉结构，不过与马不同的是，犀牛的前肢比后肢强壮得多，这可能是因为前肢在支撑体重方面发挥着重要作用。主要参与对抗重力的肌肉（如 、 、 、 ）通常具有高度羽状结构且肌束较短，有利于实现强大的关节伸展。参与推进的肌肉（如 、 、 ）似乎是在高最大等长力和长肌束之间取得了平衡，从而实现合理快速且广泛的工作范围。除了有时伸肌和推进肌外，几乎每块肌肉的新生犀牛的标准化最大等长力都高于成年犀牛，推测这些肌肉在动物生长过程中获得了强大的力量产生能力。我们的研究阐明了具有奔跑祖先的动物的肌肉如何适应支撑更大体重的方式，并呼吁对其他大型类群进行进一步研究。
