Bone Density Distribution Patterns in the Rostrum of Delphinids and Beaked Whales: Evidence of Family‐Specific Evolutive Traits

Abstract

Toothed whales have undergone a profound telescopic rearrangement of the skull, with elongation of facial bones and formation of a hollow rostrum, filled in vivo by the mesorostral cartilage. In most species of the family Ziphiidae, this latter cartilage becomes secondarily ossified, producing in some cases the densest bone existing in nature. Starting from this observation, we wanted to investigate the patterns of distribution of bone mineral density (BMD) in the rostrum of two families of toothed whales with different ecological and behavioral traits: Delphinidae and Ziphiidae. We analyzed BMD non invasively by means of the dual energy X-ray absorptiometry technology, and found two different density distribution patterns that distinctly set the two families apart. Namely, BMD values decrease from the proximal to the distal region of the rostrum in delphinids, whereas the beaked whales show a BMD peak in the central region. Possible functions such as ballast or protection against clashes might be likely, although more data about the species of both families is needed to give better evidence. Anat Rec, 293:235-242, 2010. V V C 2009 Wiley-Liss, Inc.

Records of beaked whales (family Ziphiidae) in Western Australian waters

“…Studies of both strandings and at-sea sightings of Cuvier's beaked whale in the Mediterranean have shown certain areas are more favourable (Podesta et al, 2006;Gannier, 2011) with strandings of Cuvier's beaked whale being associated with steep bathymetry (Podesta et al, 2006;Cozzi et al, 2010). Undersea canyons are known to be present off the south coast and Perth regions of WA (Harris et al, 2003) which correlates well with the location of Cuvier's beaked whale strandings.…”

“…The dense bone structure of the rostrum and head area are diagnostic features of beaked whales but this only develops as the animal ages (Heyning, 1984;MacLeod, 2002). The head density is thought to assist their deep diving habits to protect them during male to male aggression (MacLeod, 2002;Cozzi et al, 2010). The beaked whales with rostrum injuries may have been too young for their bones to have been reinforced and therefore were more susceptible to injury.…”

Hypermineralized whale rostrum as the exemplar for bone mineral

“…The ziphiid rostrum is often modified compared to that of other toothed whales, i.e., the thickened vomer usually fills the mesorostral groove [21,[23][24][25]. Although many aspects of the rostrum of this whale are shared by most of the members of the beaked whale group, in M. densirostris, the vomer, together with the maxilla and premaxilla, are all hypermineralized [21,26].…”

Fetal and Early Post-Natal Mineralization of the Tympanic Bulla in Fin Whales May Reveal a Hitherto Undiscovered Evolutionary Trait

“…Elongation of the facial skull, coalescence of the cervical spine, and absence of pelvic limbs, are well known key features of the whale and dolphin structure. Imaging evidenced peculiar features of cetacean bones, including absence of a medullary cavity in the humerus, radius and ulna substituted by an hour-glass shaped trabecular architecture [8], [9]; modification of the mandible to increase sound-receiving properties [10]; presence of atypical gradients of density of the rostrum in deep-divers [11], [12]. These morphological features are considerably different from the equivalent of terrestrial mammals, and constitute a peculiar acquisition of the order.…”