The U-shaped rostrum is the main characteristic feature in the skull of the blue whale, Balaenoptera musculus. In his preceding study on balaenopterid mandibles, one of the authors, Oishi, indicated that the morphology of the mandible has a close relevancy to that of the rostrum, as follows. The mandible of the blue whale has unique characteristics such as the condylar region rotating inward and the coronoid process extending upward when the mandible is laid on the horizontal floor. In contrast, the Antarctic minke whale, Balaenoptera bonaerensis, has a mandible showing a non-rotating condylar region and the coronoid process extending upward and outward. The difference between the mandibles of the blue whale and the Antarctic minke whale corresponds with rostral morphology. The U-shaped rostrum of the blue whale requires no inward rotation of the mandible at the long axis, however the mandible of the Antarctic minke whale rotates inward when its mouth is closed so that the hiatus between the mandible and the V-shaped rostrum is minimized. In this situation, the condylar region of the mandible of the Antarctic minke whale rotates inward. In other words, the state of articulation in the condylar region is equivalent in both the blue and Antarctic minke whales when the mouths are closed. In this study, the authors validated the evidence noted above by using 3DCG software. The results of our 3D simulations show the validity of the preceding study by one of the authors. Furthermore, we newly recognized the weak inward rotation of the mandible when the mouth of the blue whale is closed. 3D computer models enable us to conduct virtual tests on enormous materials such as whale skulls.
Following a previous paper (Honma and Kogawara 2007), sighting records of whales by Sado Line (Sado Kisen) jetfoils and car ferry boats operating on the Niigata－ Ryotsu route, Sado Straight, Sea of Japan, were compiled from February to December, 2006. Thirty-six instances, comprising 25 jetfoil and 11 car ferry observations, were enumerated. These numbers were larger than those of the previous year, 2005. Two or three sighting records of different individual in a day were secured on six occasions throughout the investigation period. The greatest number of sightings was in March, 1 or 2 months earlier than would be expected in a normal year, with an exceptional sighting in December. The time zone of higher frequencies was concentrated in 10 a.m. and in 2 p.m. The directions in which the whales were swimming (= migrating route) were noted in 16 instances, consisting of 8 north, 4 south, and 4 others. There was no occurrence of collision with cetaceans in 2006.
In spring, Pacific white-sided dolphin (PWS) come to the Tsugaru Strait, Sea of Japan, and is observed mostly in April and May. In this study, the data of sighting surveys conducted from ferry vessels between Hakodate and Aomori from May 2003 to August 2007 was analyzed. Differences in the appearance period of PWS in various years in the strait were discussed. The number of PWS and other cetaceans observed, both groups and individuals, were counted, and the relationship was examined between the appearance of PWS and water temperature and between the appearance of PWS and of its typical prey, anchovy. The first appearance of PWS has become earlier from 2003 to 2007. The relationships of appearance time to water temperature and the high season of anchovy catch were suggested, and it supported past studies.