The elephant backbone: Morphological differences, dorsostability, and vertebral fusions

The vertebral column of elephants has several unique features that distinguish them from all other modern-day mammals. In this study, we examine various aspects of the functional morphology and intervertebral mobility of the elephant backbone, comparing it to that of other large herbivorous mammals, as well as to extinct Mammuthus primigenius, M. trogontherii, and Mammut americanum. The elephant vertebral column is characterized by a high degree of dorsostability. All three directions of intervertebral mobility (sagittal and lateral bending, and axial rotation), in all backbone modules, demonstrate the lowest amplitudes of motion known for mammals. In elephants, neck mobility is largely replaced by the mobility of the proboscis; the axial rotation in the thoracic region is not used for maneuvering, and the sagittal flexibility of the lumbar region is practically absent during locomotion. The mobility of the elephant cervical region in the sagittal and frontal planes is low; however, it is still responsible for movements that require significant force, such as tournament fights and breaking tree branches. The lumbosacral joint characterized by sagittal hypermobility in most terrestrial mammals is even less mobile than the intralumbar joints in elephantids. The thoracic-lumbo-sacral region of proboscideans is arch-shaped in lateral view, resulting from the ventral-ward tapering of the vertebral centra (the length along the spinal canal is longer than the length at the ventral side of the centrum). This tapering is most pronounced in the posterior thoracic and lumbar vertebrae. In contrast, the vertebral centra in the middle part of the trunk (T8-T14) are almost rectangular in lateral view. This arch-shaped structure provides static support to the vertebral column, preventing sagging. The absolute length of the spinous processes in proboscideans is comparable to those of the largest bovines and exceeds the lengths of extant rhinoceroses. However, relative to the height of the vertebral centrum, the spinous processes in the withers area of elephants are 2-4 times shorter than those in bovines and rhinoceroses. The profile and inclination angle of the spinous processes differ drastically among different taxa. Despite these differences, we found no significant variations in the sagittal flexibility of the backbone among the different elephantids. We hypothesize that the observed differences may relate to how the arch shape of the backbone is maintained in the posterior part of the thoracolumbar region. In modern-day elephants, dorsostability is primarily supported by a strong supraspinous ligament with short intersegmental portions. In mammoths, this is probably maintained by the linea alba and the abdominal muscles.