Components of a hypothetical odontocete biosonar signal generator

Abstract

The site and physiologic mechanism(s) responsible for the generation of odontocete sonar signals have eluded investigators for decades. Examination of postmortem odontocete heads with medical imaging devices (x‐ray CT and MRI) across diverse taxa has identified a complex of structures that may function as the essential components of a biosonar signal generator. Each monkey lips/dorsal bursae complex (MLDB) is associated with one (right and left) air passage. Each complex is composed of at least two fatty bursae that are embedded in the monkey lips along the airway, a cartilagenous “stiffening” rod, and a stout ligament. The size of the fatty bursae varies with the species but tends to suggest a relationship with wavelengths for peak frequencies of published signals. In addition, the location of these bursae makes them excellent candidates for sonar signal transducers. The details of this complex and its morphologic surroundings will be shown in examples from six extant odontocete families. Preliminary results from an endoscopic investigation of an echolocating dolphin support the hypothesis that this anatomic complex is the site of sonar signal generation.