The Anatomy of Acoustic Structures in the Spinner Dolphin Forehead as Shown by X-Ray Computed Tomography and Computer Graphics

Abstract

The dolphin’s unusual skull is characterized by profound displacement and modification of the bones composing the snout and pares, resulting from an evolutionary process referred to as “telescoping” (Miller, 1923). Deep within the forehead and acting as valves in the superior bony nares are two muscled nasal plugs. These both have connective tissue flaps (lips) on their lateral margins that may be involved in sound generation (Evans and Prescott, 1962). Above the nasal plugs the nasal passages empty into a single airway (spiracular cavity) which exits at the blowhole on top of the head. Along this airway are at least six blind-ended air sacs which are presumed to act as nearly perfect reflectors of sound propagated in the tissues of the forehead (Norris, 1964). The fatty melon is an ellipsoid structure anterior to the nasal passages that grades from a tough exterior shell, rich in connective tissue, to a pellucid fatty core.