Marine mammals also need a streamlined head for fast and easy movement through water that external ears would impede. Instead, they have ear holes located right behind the eye. In bottlenosed dolphins, the ear hole is only 2-3 millimeters in diameter.
Some scientists believe that the toothed whales have ear channels that are open and filled with sea water. Others think that the ear channel is closed off, and both the channel and ear hole have become a vestigal organ--useless since they evolved (much like our appendix). Hearing in toothed whales, they suggest, occurs either by the bones of the skull transmitting sound to the inner ear--"bone conduction," or through "tissue conduction," where sound is conducted to the inner ear by deposits of fat that run up from the lower jaw.
Go to marine mammal acoustic article.