Comparative morphology of the spinal cord and associated vasculature in shallow versus deep diving cetaceans

The cetacean vertebral canal houses the spinal cord and arterial supply to and venous drainage from the entire central nervous system (CNS). Thus, unlike terrestrial mammals, the cetacean spinal cord lies within a highly vascularized space. We compared spinal cord size and vascular volumes within the vertebral canal across a sample of shallow and deep diving odontocetes. We predicted that the (a) spinal cord, a metabolically expensive tissue, would be relatively small, while (b) volumes of vascular structures would be relatively large, in deep versus shallow divers. Our sample included the shallow diving Tursiops truncatus (n = 2) and Delphinus delphis (n = 3), and deep diving Kogia breviceps (n = 2), Mesoplodon europaeus (n = 2), and Ziphius cavirostris (n = 1). Whole, frozen vertebral columns were cross-sectioned at each intervertebral disc, scaled photographs of vertebral canal contents acquired, and cross-sectional areas of structures digitally measured. Areas were multiplied by vertebral body lengths and summed to calculated volumes of neural and vascular structures. Allometric analyses revealed that the spinal cord scaled with negative allometry (b = 0.51 ± 0.13) with total body mass (TBM), and at a rate significantly lower than that of terrestrial mammals. As predicted, the spinal cord represented a smaller percentage of the total vertebral canal volume in the deep divers relative to shallow divers studied, as low as 2.8% in Z. cavirostris. Vascular volume scaled with positive allometry (b = 1.2 ± 0.22) with TBM and represented up to 96.1% (Z. cavirostris) of the total vertebral canal volume. The extreme deep diving beaked whales possessed 22-35 times more vascular volume than spinal cord volume within the vertebral canal, compared with the 6-10 ratio in the shallow diving delphinids. These data offer new insights into morphological specializations of neural and vascular structures that may contribute to differential diving capabilities across odontocete cetaceans.