,1 Reprinted from THE JOURNAL OF EXPERrMENTAL ZOOLOGY Vol. 188, No .3, June 1974 " The Wistar In s titute Press 1974 Surgical Approach to the Dolphin's Ear SAM H. RIDGWAY , JAMES G. McCORMICK 1 AND ERNEST GLEN WEVER Naval Undersea Cent e,/" San Diego, CalifoTnia 92132 , and AuditOl-Y Resea1'Ch Laboratories, Princeton UniveTSity, Pl'ince ton, New Jersey 08540 ABSTRACT A surgical approach to the cetacean (TuTSiops truncatus and Lagenorhynchus obliqui.dens) ear represented a unique problem. The first obstacle was the development of a safe and humane anesthesia procedure and techniques of long-term physiological maintenance of the animal during surgery . The surgical anatomy was a challenge especially because of the extensive venous plexuses that invest the entire ear and adnexa. The corpus cavernosum carotidis and other small arterial networks course through this venous mass. Sodium pentothal-halothane anesthesia and careful physiological monitoring were determined to be adequate for long-term maintenance during surgery and cochlear recording experiments. A specially designed surgical tank held the animal during the procedure. Immersion in water relieved the lungs of external pressure that ordinarily occurs , when the animal is out of the water, due to the flexible thorax. Thermoregulation was provided for by regulating the water temperature in response to the dolphin 's core temperature. This arrangement also provided for underwater auditory stimulation and easy manipulation of the animal since dolphins with their lungs inflated are close to neutral buoyancy. It was necessary to ligate the external and internal carotid arteries, neither of which supplies blood to the ear or brain . Hemostasis in the massive venous plexus could be achieved only by partial removal and subsequen t application of oxycel and surgical cement to effectively dam off the area of the round win- dow and other ear structures for electrical potential recording and experi- mental manipulations. With this technique the round window of the cochlea was exposed. Fully anesthetized anim a ls were maintained for periods up to 24 hours while electro- physiological measurements were being made. Some of the auditory measure- ments required further surgical manipulation of the auditory meatus, bulla , ossicular chain and other middl e- ear structures. Cetaceans lack an external ear or pinna , but it has been known for a long time that they can hear. Pindar, who lived from about 522 to 422 BC, claimed that dolphins could be attracted by a flute or lyre, and Aristotle was surprised that they fled from all kinds of noise despite their apparent lack of an auditory passage (Slijper, ' 58) . Several theories have been put forward to explain how cetaceans hear. These vary Widely, from the early suggestion of Cam- per (1787) that these animals hear only sounds produced in air and then only when the head is out of the water, to the more recent hypotheses of Norris ('68). Most of the theories of cetacean hearing have been treated in the additional works of Yamada (,53), Reysenbach de Haan (,58), Fraser and Purves ('6 0) and Dudok van Heel ('62). Schevill and Lawrence (' 53) repor ted that bottlenosed dolphins responded to fre- quencies above 100 kHz . Later Johnson ('66) produced the first detailed audio- gram of a cetacean and showed that TUT- siops truncatus had broad-band hearing extending to 150 kHz. Through these and numerous other investigations , it has been established that cetaceans have a keen sense of hearing. It was clear, however, that the theories of how the cetacean au- ditory mechanism worked had multiplied in the absence of detailed phYSiological and anatomical observations in the acoustic ap- paratus. Therefore, we set out to study auditory function in the living animal. The cochlear-potential method was chosen as J. Exp. ZOOL.. J 88 .. 265-276. I Present address: S ec ti on of Otolaryngology, Bow- m an Gray Sch oo l of Medicine, Wake Forest University, W LrlSto n-Salem, North Carolina 27103. 265