Effects of size on the skull shape of the bottlenose dolphin (Tursiops truncatus)

We measured 145 bottlenose dolphin skulls to examine the effects of size on skull shape. We found that the rostral length and the width of the temporal fossa showed highly positive allometry. These differences, as a result of size, reflect the growth of the feeding apparatus. The positive allometry of the rostral length implies an extension of the range for food catching and an increase in mouth-closing speed. The positive allometry of the temporal fossa width reflects the development of the temporal muscle. On the other hand, the positive allometry of both the rostral length and the temporal Fossa width seem to indicate a contradictory effect on occlusal force. However, the occlusal force estimated by bony characters increased by the condylobasal length. These trends were observed not only in immature specimens but also in mature specimens of different sizes. This fact means that the change of the skull shape in immature specimens is caused by the development of the feeding apparatus, while that the variation in mature specimens of different sizes is due to a result of ontogenetic changes. Therefore, it is suggested that mature specimens also have morphological variation depending on size, and this fact sometimes confuses the taxonomic studies based on the morphological data.