Commerson's dolphins have a well-developed, acute sense of hearing.
Mechanics of sound reception.
- A toothed whale's small external ear openings don't seem to be important in conducting sound. They lead to reduced ear canals that are not connected to the middle ears.
- Soft tissue and bone conduct sound to a toothed whale's middle and inner ears. In particular, fat lobes in a toothed whale's lower jaw appear to be an adaptation for conveying sound to the ears.
- In toothed whales, ears aren't attached to the skull. Ligaments hold each ear in a foam-filled cavity outside the skull. This separation of the ears allows a killer whale to localize sound, which is important for echolocation.
Commerson's dolphins have acute vision both in and out of the water.
The lens of a marine mammal's eye is stronger than that of a land mammal.
- In the eye of a land mammal, the cornea focuses light rays toward the lens, which further focuses the light rays onto the retina. Underwater, the cornea isn't able to adequately focus waves into the lens because the refractive index of water is similar to that of the interior of the eye.
- The eye of a marine mammal compensates for this lack of refraction at the cornea interface by having a much stronger, spherical lens. It is more similar to the lens of a fish's eye than the lens of a land mammal's eye.
- In air, a marine mammal's eye compensates for the added refraction at the air-cornea interface. At least in bright light, constricting the pupil helps, but it doesn't fully explain how a whale achieves visual acuity in air. Research is ongoing.
DNA from several other species of toothed whales indicated that the eyes of these whales do not develop pigment cells called "S-cones," which are sensitive to blue light. Researchers theorize that all modern cetaceans lack these visual pigments and therefore aren't able to discriminate color in the blue wavelengths.
Olfactory lobes of the brain and olfactory nerves are absent in all toothed whales, indicating that they have no sense of smell.