The Anatomy, Bioacoustics, and Neural Physiology of Dolphin Biosonar

Light is rapidly attenuated in sea water, and the habitats of dolphins and other toothed whales are characterized by reduced visual stimuli relative to terrestrial habitats. Sound is efficiently transmitted under water; however, and these species have undergone significant modifications to head structures that facilitate both passive and active (echolocation) acoustic sensing of marine environments. The nasal anatomy involved in the creation of dolphin biosonar signals is intricate. Structures immediately beneath the blowhole – termed the phonic lips – create short‐duration, directional pulses that travel through a fat‐filled melon (forehead) and into the water. The exact mechanism by which these “clicks” are generated in the phonic lips is not well understood. Recent electrophysiological measurements suggest that neural potentials may precede the production of every click. Thus, dolphins may actively mediate click generation on a click‐by‐click basis. It is not known if this is possible at the highest rates observed in dolphin echolocation (hundreds of clicks per second), as this would require very rapid motor activity. Dolphins also appear to control the direction of the outgoing biosonar beam independent of large head movements. This may be facilitated by deformations of the melon or manipulation of a complicated system of air sacs surrounding the click generation structures. One of the most obvious anatomical modifications to sound reception structures is the complete absence of external pinnae, and only a small remnant of the ear canal is apparent on the surface of the skin. Although lower frequencies (below 20 kHz in the case of bottlenose dolphins) may primarily be received in this region, higher‐frequency sound (up to approximately 140 kHz) is thought to primarily travel through the mandible. The mandible is partially hollow and filled with specialized “acoustic fats” that connect to the auditory bullae. The cochlea is similar in construction to those of terrestrial mammals; however, there are bony laminae present that result in increased basilar membrane stiffness that facilitates very high‐frequency hearing. Recent data suggest that these laminae may drastically increase the traveling wave speed along the high‐frequency basal end of the basilar membrane, and result in near simultaneous stimulation of the 40–140 kHz region that is critical to echolocation. Refinement for echolocation is further reflected in the neural anatomy of dolphins; e.g., the auditory nerve and brainstem are hypertrophied for rapid transmission of sonar signals. Ongoing research is aimed at determining how the auditory regions of the cortex process acoustic stimuli, but little is currently known about these phenomena. Support or Funding Information Funded by the Office of Naval Research