Exceptional bone density DXA values of the rostrum of a deep-diving marine mammal: a new technical insight in the adaptation of bone to aquatic life

Abstract

The use of dual energy X-ray absorptiometry (DXA) to measure bone mineral density (BMD) has been applied to several animal species to examine bone and body composition in longitudinal studies. Although the rat is the most frequently used mammal in DXA-based investigations, the BMD of several species of larger-sized nonhuman mammals has already been reported [1]. Studies on a single specific bone or total body BMD have been performed on nonhuman primates [2, 3], sheep [4], bovine calves [5, 6], horses [7], pigs [8], dogs [9, 10], cats [11], and striped [12] and bottlenose dolphins [13]. All BMD values (expressed as g/cm) obtained from the species listed above ranged between 0.463 (canine III metacarpal bone) and 2.12 (pig skull). Other values to consider for reference are 1.1 (dolphin arm) and 1.85 (equine III metacarpal bone). In terrestrial mammals, the most important intraspecific factors affecting BMD are body weight and age, whereas in aquatic mammals, the arm and forearm BMD correlated to body dimensions and age of the animal [10–13]. For the first time, we radiographed the whole skull and scanned by means of a DXA device (Hologic QDR1000, Hologic Inc., Waltham, MA, USA) the rostrum of a male Blainville’s beaked whale Mesoplodon densirostris (Blainville, 1817). This specimen was found stranded by Lamberto Loria in Papua New Guinea (ex British New Guinea) at the end of nineteenth century and is now preserved at the “G. Doria” Civic Museum of Natural History in Genova (Italy) with the number MSNG 16. The rostrum of M. densirostris is made up by maxillary and premaxillary bones, pterygoids, palatines, and vomer and in mature animals is supported axially by ossification of the mesorostral canal. In adult males, the mesorostral ossification is due to the gradual replacement of mesethoid cartilage with secondary bone [14]. The rostrum consists of dense Haversian tissue, the secondary osteons of which are oriented longitudinally, themselves constituted by hypermineralized parallel-fibered osseous tissue [15]. The ultrastructure of the walls of the secondary osteons is characterized by a strong volumetric reduction of the collagenous network, composed by very thin fibrils oriented longitudinally. The biomechanical behavior of the rostrum is that of an exceptionally rigid, hard but also apparently fragile material [15]. The BMD values of this bone (2.612 to 2.686 g/cm) obtained by simple measurements based on Archimedes’s principle have been reported as the highest values hitherto observed for mammalian bone and the rostral mineralization has been quantified up to 22% above that of “normal” adult mammalian bone [15]. We divided the scan of our specimen into three regions of interest (RoI) because the length of the bone exceeded the DEXA-arm motion track (approximately 70 cm). The proximal (pRoI), medium (mRoI), and distal RoI (dRoI) were then scanned in a dorsoventral projection starting from infraorbital foramina toward the foremost extremity of Skeletal Radiol (2009) 38:1123–1125 DOI 10.1007/s00256-009-0647-4