Abstract

Dissections of preserved specimens revealed air cavities around the ear. Air cavities are especially well-developed medial and posterior to the ear bones. Air is supplied to these spaces by Eustachian tubes from each nasal cavity. Air cavities were confirmed in live dolphins by computed tomography (CT) and magnetic resonance imaging (MRI) scans. We took tissue samples from preserved specimens for histology. Histological evaluation of tissue around the ear revealed epithelial lined air sinuses and expansible tissue. This tissue contains nerves, ganglia, smooth muscle bands, vascular spaces, and periarterial venous networks.

We studied the metabolic activity of the ear, cranial tissue and brain with positron emission tomography (PET) scans. A total of five scans were done with two trained dolphins. In three experiments, 20 target tones were presented through a jaw phone after injection of 18 Fluro-deoxy-d-glucose (FDG). In two experiments, pulses similar to echolocation pulses were presented through jaw phones. PET scans revealed high levels of metabolism in auditory areas of the brain and in the periotic tissue medial and posterior to one or both ears. Dolphin periotic structures are rich in nerves, vascular spaces, arteries, smooth muscle and erectile tissue that may actively shape sinus air cavities. The acoustic reflectance of air cavities together with active vascular and muscular shaping may allow periotic cavities to form an active internal pinna and possibly an "acoustic tapetum" for dolphins.

Acknowledgements

We thank Donald A. Carder who assisted in the dolphin and equipment management for the PET experiments. Experiments were approved by the Institutional Animal Care and Use Committee of the U. S. Navy Marine Mammal Program.

* Presenting author
+ Student presenter

Bibliography

1.  Houser DS, Finneran J, Carder D, Van Bonn W, Smith C, Hoh C, Mattrey R and Ridgway SH. 2004. Structural and Functional Imaging of Bottlenose Dolphin (Tursiops truncatus) Cranial Anatomy. J Exp Biol 207:3657–3665.

2.  Houser DS, Moore PW, Johnson S, Lutmerding B, Branstetter BK, Ridgway SH, Trickey J, Finneran JJ, Jensen E and Hoh C. 2010. Relationship of blood flow and metabolism to acoustic processing centers of the dolphin brain. J Acoust Soc Am 128:1460–1466.

3.  McCormick JG, Wever EG, Palin J, and Ridgway SH. 1970. Sound conduction in the dolphin ear. J Acoust Soc Am 48:1418–1428.

4.  Møhl B, Au WL, Pawloski J and Nachtigall PE. 1999. Dolphin hearing: Relative sensitivity as a function of point of application of a contact sound source in the jaw and head region. J Acoust Soc Am 105:3421–3424.

5.  Moore PW, Pawloski DA and Dankiewicz L. 1995. Interaural time and intensity difference thresholds in the bottlenose dolphin (Tursiops truncatus). In: Kastelein RA, Thomas JA, and Nachtigall PE editors. Sensory Systems of Aquatic Mammals. Woerden, The Netherlands: De Spil Publishers. p 11–24.

6.  Ridgway SH. An illustration of Norris' acoustic window. 1999. Marine Mammal Science 15:926–930.

7.  Ridgway SH, Blankenship B, Houser D, Carder D and Hoh C. 2006. A dolphin acoustic tapetum. J Acoust Soc Am 119: 3276.

8.  Ridgway SH, Houser D, Finneran JJ, Carder DA, Keogh M, Van Bonn W, Smith CR, Scadeng M, Mattrey R and Hoh C. 2006. Functional Imaging of Dolphin Brain Metabolism and Blood Flow. J Exp Biol 209: 2902–2910.

9.  Sassu R and Cozzi B. 2007. The External and Middle Ear of the Striped Dolphin Stenella coeruleoalba (Meyen1833). Anat Histol Embryol 36:197–201.