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, 2010. © 2009 Wiley-Liss, Inc.