Seals, sea lions and walruses (pinnipeds), sea otters, and polar bears spend time on land as well as in the water. Consequently, their ears have not evolved great differences from terrestrial mammal ears.

True seals such as the harbor seal (on left) have no external ears. Eared seals such as the California sea lion (on right) have external ear flaps. Photos ©Tom Kieckhefer

In pinnipeds the pinnae (external ear flaps) are reduced or absent. Otariids (eared seals) have small ear flaps. Phocids (true seals) have no pinnea. Muscles and a cartilage valve along the external ear canal function to close the ear canal to water. In general, the middle and inner ears of pinnipeds, polar bears, and otters are similar to those of humans and other terrestrial mammals. The mechanisms for hearing are essentially the same (See Hearing in Land Mammals). These animals that spend time both on land and in the water have a mix of adaptations. Depending on their life styles, some species hear best in air, whereas others hear better underwater.

References
• Wartzok, D. and Ketten, D.R. 1999. Marine Mammal Sensory Systems. Pages 117-175 in Reynolds, J.E. III and Rommel, S.A. (eds.). Biology of Marine Mammals. Smithsonian Institution Press, Washington D.C.

Additional Resources
• 2 dimensional and 3 dimensional CT scan images of cetaceans and other animals: Computerized Scanning and Imaging Facility at Woods Hole Oceanographic Institution