The Distinctive Forehead Cleft of the Risso’s Dolphin (Grampus griseus) Hardly Affects Biosonar Beam Formation

SIMPLE SUMMARY: Risso’s dolphins have a sophisticated biosonar system. However, unlike other dolphins that have a round and smooth forehead, Risso’s dolphins have a distinctive vertical crease (or cleft) along the anterior surface of the forehead. Researchers have speculated how the cleft may affect biosonar beam formation given its location on the biosonar sound propagation pathway. It is almost impossible to test this experimentally. To fill this gap, this study built 2D numerical sound propagation models based on CT scans of a Risso’s dolphin. We digitally filled the cleft with neighboring soft tissues, creating a hypothetical “cleftless” head, representing a Risso’s dolphin with a round and smooth forehead as other dolphins. After comparing the sound propagation process through the original head and cleftless head, we found that the cleft played an insignificant role in forehead sound propagation and far-field beam formation. Moreover, the cleft was not responsible for the bimodal click spectrum that has previously been reported from this species. Our study presents a promising approach to advance our understanding of the function of the internal biological structures in biosonar beam formation, specifically in the absence of experimental methods to measure tissue functions directly in situ. ABSTRACT: The Risso’s dolphin (Grampus griseus) has a distinctive vertical crease (or cleft) along the anterior surface of the forehead. Previous studies have speculated that the cleft may contribute to biosonar beam formation. To explore this, we constructed 2D finite element models based on computer tomography data of the head of a naturally deceased Risso’s dolphin. The simulated acoustic near-field signals, far-field signals, and transmission beam patterns were compared to corresponding measurements from a live, echolocating Risso’s dolphin. To investigate the effect of the cleft, we filled the cleft with neighboring soft tissues in our model, creating a hypothetical “cleftless” forehead, as found in other odontocetes. We compared the acoustic pressure field and the beam pattern between the clefted and cleftless cases. Our results suggest that the cleft plays an insignificant role in forehead biosonar sound propagation and far-field beam formation. Furthermore, the cleft was not responsible for the bimodal click spectrum recorded and reported from this species.