Morphological adaptations of cervical vertebrae in plunge-diving sulids.

Foraging strategy frequently drives morphological diversification in birds. The plunge-diving behavior of certain species makes the functional and ecological trade-offs of variation in cervical vertebrae an especially intriguing question. This study compares cervical vertebrae across five plunge-diving species and two non-plunge-diving species, using morphometric analysis of micro-CT scans to test whether diving birds share specialized cervical vertebrae morphologies distinct from non-divers. Plunge-divers repeatedly experience extreme impact forces during headfirst water entry, requiring the neck to function as a shock-absorbing spring rather than a flexible manipulative system. To withstand these forces, plunge-divers exhibit reinforced cervical vertebrae-short, wide vertebrae with expanded joints and thickened arches. These features help distribute compressive and bending loads and reduce stress on the spinal column during impact. Gannets and boobies, the most extreme plunge divers, have an additional articulating surface on the dorsal arch of their anterior cervical vertebrae which limits flexion in the beginning of the neck further reinforcing against high loads. Additionally, we see that all plunge-diving sulids experience similar water impact durations and that the ratio between the radius of the head and the neck scales with impact velocity.