GROWTH-RELATED CHANGES IN CRANIUM OF KILLER WHALES IN THE WESTERN NORTH PACIFIC

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Abstract

<p>One of the crucial questions for using the skull morphology for classification purposes in cetaceans is whether the skull features have growth-dependent differences. This question was addressed by examining cranial specimens collected from killer whales in different localities of the western North Pacific. The present study is the first to investigate developmental change in the skull morphology of western North Pacific killer whales. A total of 24 cranial measurement characters were examined from 22 animals collected between 1937 and 2011. Two kinds of analyses were conducted 1) the relationships between body length (BL) and cranium length (condylobasal length CBL), and 2) between CBL and each measurement character. For these analyses, an allometric equation was used. The relationship between BL and CBL showed a negative growth pattern, consistent with previous studies in delphinid species. The length of the lacrimal bone and the width of the internal nasal cavity showed isometric change. Therefore, these characters can be used as criteria for sex and/or species/subspecies classification as their proportions are free from relative change with respect to skull growth in CBL. Regarding ontogenic growth, this study showed that the anteroposterior length of the temporal fossa, the zygomatic process and the space where the temporal muscle passes became proportionally larger as their skull grows. The development and activity of temporal muscles would produce these changes. On the morphometric features involved in the generation and modification of acoustic signals, this study found the following changes: the width of the posterior regions of the rostrum and preorbital bone became wider; the ventral surface of the preorbital process became thick and sturdy, and the maxillary crest became more apparent with the skull growth. Therefore, these morphometric features might be used to characterize species/subspecies of the killer whales, which have highly divergent foraging behavior and vocalization.</p>

Journal

  • Cetacean Population Studies

    Cetacean Population Studies 3 (0), 175-188, 2021-12-28

    Publication Committee for Cetacean Population Studies

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