Vertebral pneumaticity is correlated with serial variation in vertebral shape in storks

Birds and their ornithodiran ancestors are unique among vertebrates in
exhibiting air-filled sinuses in their postcranial bones, a phenomenon
called postcranial skeletal pneumaticity. The factors that account for
serial and interspecific variation in postcranial skeletal pneumaticity
are poorly understood, although body size, ecology, and bone biomechanics
have all been implicated as influencing the extent to which pneumatizing
epithelia invade the skeleton and induce bone resorption. Here, I use
high-resolution computed-tomography to holistically quantify vertebral
pneumaticity in members of the neognath family Ciconiidae (storks), with
pneumaticity measured as the relative volume of internal air space. These
data are used to describe serial variation in extent of pneumaticity and
to assess whether and how pneumaticity varies with the size and shape of a
vertebra. Pneumaticity increases dramatically from the middle of the neck
onwards, contrary to previous predictions that cervical pneumaticity
should decrease towards the thorax to maintain structural integrity as the
mass and bending moments of the neck increase. Although the largest
vertebrae sampled are also the most pneumatic, vertebral size cannot on
its own account for serial or interspecific variation in extent of
pneumaticity. Vertebral shape, as quantified by three-dimensional
geometric morphometrics, is found to be significantly correlated with
extent of pneumaticity, with elongate vertebrae being less pneumatic than
craniocaudally short and dorsoventrally tall vertebrae. Considered
together, the results of this study are consistent with the hypothesis
that shape- and position-specific biomechanics influence the amount of
bone loss that can be safely tolerated. These results have potentially
important implications for the evolution of vertebral morphology in birds
and their extinct relatives.