Light postcranial skeleton vs heavy skull: a possible deep diving adaptation in beaked whales (Cetacea: Ziphiidae)

A pachyosteosclerotic rostrum is a synapomorphic feature observed in several beaked whales. Four hypotheses have been proposed in the last decades to explain the evolution of this peculiar structure: 1) for favoring deep diving (the ballast hypothesis); 2) for intraspecific fights between adult males; 3) for facilitating sound transmission; 4) for intraspecific sexual display. According to the ballast hypothesis a heavy pachyosteosclerotic rostrum, combined with a light postcranial skeleton, moves the center of mass toward the head of the whale helping to rotate body for vertical diving. To better test this hypothesis, we computed the center of mass of the skeleton of three beaked whales (Hyperoodon ampullatus, Mesoplodon bowdoini, and Ziphius cavirostris) and one dolphin (Delphinus delphis), all kept at the Museo di Storia Naturale, Università di Pisa. Moreover, analyses about the density of the vertebral column and radiodensity of the first thoracic vertebra, first lumbar vertebra, first caudal vertebra and phalanxes were performed. Radiodensity was estimated by means of Computed Tomography (CT) and Micro-CT. Our analyses confirm the significant displacement of the center of mass to the anterior part of the skeleton in beaked whales, with the most notable shift occurring in M. bowdoini, exhibiting the center of mass in correspondencee of the cervical vertebrae. The skeleton of M. bowdoini also exhibits the least dense trabecular bone tissue, both in the vertebral bodies and the phalanx. The common dolphin, on the contrary, shows the posteriormost position of the center of mass and the densest trabecular bone tissue of vertebral bodies. Although this preliminary study seemingly supports the ballast hypothesis, these results should be further strengthened by increasing the number of investigated species and also by analyzing the whole bodies in order to estimate the center of mass of the entire living whales.