Abstract
The northern elephant seal (Mirounga angustirostris) is found in great numbers off of the coast of southern California. For 8–10 months out of the year, they are out at sea and only return to land to molt and reproduce.1 Northern elephant seals (NES) forage for food in the open ocean and 80–90% of their time is spent submerged in very deep waters, making them highly pelagic animals. In fact, they are one of the deepest diving marine mammals, reaching depths of 1500 meters for time spans of 20–30 minutes to find food.1 Such deep depths puts them in the aphotic zone of the ocean, implying that hearing is an important sense for these animals.1 It stands to reason that in order to maintain this deep diving lifestyle, NES must have structural adaptations in their special senses, some of which have been previously described. For example, high densities of photoreceptors and a well-developed tapetum in pinnipeds serve as visual adaptations.2 It is also noted that the vibrissae aid in environmental recognition e.g., hydrodynamic trail following from escaping fish which enables them to forage in deep dark seas.2 Further assessing the ear of NES provides a more complete picture of their special sense organs.
The purpose of this research was to investigate the NES ear to better understand adaptations they possess to maintain their lifestyle. We established that NES have a narrow, collapsed external acoustic meatus. Pseudostratified ciliated epithelium lines the mucosa in the middle ear which is supported by thick vascular cavernous sinuses. Exceptionally large middle ear ossicles are partially located in these vascular cavernous sinuses. We determined that the cochlear nerve was comprised of approximately 58,861 axons with an average axonal diameter of 4.57 µm which enables a high conduction velocity of 30 m/s. Above listed adaptations support a deep diving lifestyle. The axonal number and diameter were significant since their numbers lied between obligate marine mammals and obligate terrestrial mammals, indicating a complex auditory system (due to axonal number) and fast reaction time (due to large axonal diameter). All in all, NES adaptations help them to survive by giving them means to hunt in deep waters and fast reaction times in life and death situations.
Acknowledgements
The authors wish to thank the office of Dr. Henriksen for funding this unique project as well as Western University College of Veterinary Medicine.
* Presenting author
+ Student presenter
Literature Cited
1. Reeves RR, Stewart BS, et al. 2002. Northern elephant seal. In: National Audubon Society Guide to Marine Mammals of the World. New York, NY: Alfred A. Knopf; 162–165.
2. Wartzok D, Ketten DR. 1999. Marine mammal sensory systems. In: Reynolds J, Rommel S, eds. Biology of Marine Mammals. Washington, DC: Smithsonian Institution Press; 117–175.