The changing shape of a common dolphin’s skull throughout its growth process

Yamamoto Satoshi T., Yatabe Akiko

doi:10.5181/cetology.0.23_7

<p> “Telescoping” is a unique morphological feature of cetacean skulls which previous studies have suggested is the result of aquatic adaptation of abilities such as breathing, sound production, and feeding. We think these abilities are necessary for both newborns and adults, but telescoping is not yet complete at birth. Hence we examined changes to shape during the growth process, and why those changes occurred, through observation of external features and actual measurements of the skulls of 93 common dolphins (Delphinus delphis) in different stages of growth, from newborns to adults. According to observation of external features on the dorsal aspect of the skull, in young common dolphins the braincase was relatively large compared to the rostrum, and large areas of the interparietal, parietal and frontal bones were visible. These results are the same as for previous studies. According to the measurements, condylobasal length (CBL) and length of rostrum (LR), and CBL and distance from tip of rostrum to external nares (DRE) had an almost linear positive correlation. However, the ratios of LR and DRE to CBL did not increase constantly, having a tendency to increase in three phases. This was especially the case for the ratio of DRE to CBL. In the first phase, the braincase extended little due to the interparietal bone being overlapped by the frontal bone. Extension of the rostrum was faster than the braincase after the first phase, and in the last phase, rostrum and braincase extended with proportions maintained. These results suggest that the interparietal bone has an effect on the braincase appearing relatively large and on changing proportions throughout growth. One merit of incomplete telescoping of the skull, given that frontal bone and interparietal bone are components of the braincase, would be to ensure brain volume. On the other hand, a possible demerit is insufficient ability to perform echolocation given that the concave part of the skull, which functions as an acoustic mirror for echolocation sounds, is incomplete. However, this demerit may have little effect on newborns, which are nursed by their parents. We considered the possibility that the incomplete telescoping of a newborn’s skull is due to ensuring brain volume having priority over feeding-related organs and all abilities.</p>

The changing shape of a common dolphin’s skull throughout its growth process

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