Comparative finite element analysis of the first thoracic vertebra in artiodactyls

Artiodactyls exhibit a striking diversity of the cervical vertebral column in terms of length and overall mobility. Using finite element analysis, this study explores the morphology at the cervico-thoracic boundary and its performance under loads in artiodactyls with different habitual neck postures and body sizes. The first thoracic vertebra of 36 species was loaded with (i) a compressive load on the vertebral body to model the weight of the head and neck exerted onto the trunk; and (ii) a tensile load at the spinous process to model the pull via the nuchal ligament. Additional focus was laid on the peculiar shape of the first thoracic vertebra in giraffes. We hypothesized that a habitually upright neck posture should be reflected in the greater ability to withstand compressive loads compared to tensile loads, whereas for species with a habitually suspended posture it should be the opposite. In comparison to species with a suspended posture, species with an upright posture exhibited lower stress (except Giraffidae). For compressive loads in larger species, stress surprisingly increased. Tensile loads in larger species resulted in decreased stress only in species with an intermediate or suspensory neck posture. High stress under tensile loads was mainly reflecting the relative length of the spinous process, while high stress under compressive loads was common in more "bell"-shaped vertebral bodies. The data supports a stability-mobility trade-off at the cervico-thoracic transition in giraffes. Performance under load at the cervico-thoracic boundary is indicative of habitual neck posture and is influenced by body size.