Decrease in tooth count in melon‐headed whales

A total of 62 skulls from two herds of melon-headed whales stranded on Japanese coasts were examined. Although the species originally had around 22 teeth in each tooth row, older specimens (30–35 years old) possessed only an average of 9.2 teeth on each side of the upper jaw. Significant shortening of the root caused the tooth loss observed in this species. In addition, we observed the following trends with respect to the pattern of tooth loss: (1) teeth are lost from the posterior to the anterior part of the tooth row, (2) the loss of teeth in the upper jaw precedes that in the lower jaw, and (3) tooth count decreases with age. The above-mentioned tendencies were found in both herds. Bony tissues at the alveoli seem to push out the teeth: the bottom of the tooth socket rises both during the root-shortening process and after the tooth falls out. There were no signs of accidental tooth loss such as resorption of alveolar bones due to gingival disease or injuries on alveolar bones by physical impacts. Therefore, the postnatal tooth count decrease is considered an ontogenetic event of this species. The ontogenetic decrease in tooth count coincides with the tendency of evolutionary tooth number reduction in odontocetes at two particular points, the secondary reduction from polyodontia and the more progressed decrease in teeth in the upper jaw than in the lower jaw. Therefore, the evolutionary tooth count reduction seems to be repeated in the lifetime of the melon-headed whale.