A measure of asymmetry of the skulls of odontocete whales

Linear measurements were made of the asymmetry of the skull which is characteristic of odontocete whales. Of the living genera (except Physeter, Tasmacetus, Stenella, Feresa and Lissodelphis) at least one specimen was measured; 317 skulls in all. One species, Stenodelphis blainvillei , proved symmetrical. All other species had leftward deviation of the nasal prominence. No specimen showed inverse asymmetry. When, for species or families, regressions were formed of the asymmetry measure on skull length the slopes formed a sequence such that Kogia >monodontids>delphinoids (except Orcinus)>ziphiids> Orcinus. In some species of delphinoid, in the delphinoids as a group, and more strongly in Monodon , the proportion of asymmetry to skull length increased with increasing size. In the ziphiids, as a family, asymmetry increased roughly linearly with bodily size. Orcinus orca proved an anomalous delphinoid in having little asymmetry for its size. The asymmetry of the skull of Monodon monoceros is independent of the possession of a tusk. It is suggested that the leftward deviation of the dorsal aspect of the skull acts to conserve the symmetry of the body surface by countering an opposite asymmetry of the upper breathing passages, an asymmetry which is primary but whose function is unknown.