How the structures of the Risso’s Dolphin (Grampus griseus) affect sound propagation and reception

Abstract

Like other odontocetes, Risso’s dolphins actively emit clicks and passively listen to the echoes during echolocation. However, Risso’s dolphin head anatomy differs from other odontocetes by a unique vertical cleft along the anterior surface of the forehead and a differently shaped lower jaw. In this study, 3D finite-element sound propagation and reception models were constructed based on CT data of a deceased Risso’s dolphin. Model results of sound propagation and reception were verified by finding good agreement with practical recording data from previous biosonar beam measurements and hearing sensitivity measurements. The cleft was filled with neighboring soft tissues, creating a hypothetical “cleftness” head. Comparison between sound travelling through a “cleftness” head versus an original head indicate that the distinctive cleft plays a limited role in biosonar sound propagation. Additionally, we digitally simulated the acoustic pathway for sounds to travel from the water into dolphin’s tympanoperiotic complexes (TPCs). The gular reception mechanism, previously discovered in Delphinus delphis and Ziphius cavirostris was also found in this species. Sound pressure levels and displacement (motion) of the TPCs were compared between the cases with/without the mandibular fats or mandible. The results demonstrate a wave-guiding role of the mandibular fats and a bone conductor role of the mandible to transfer acoustic energy to the TPCs.