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Marine mammals (cetaceans, pinnipeds) have developed extraordinarily sophisticated sensory systems for the aquatic environment. Dolphins produce very high-level broadband clicks (30120 kHz) (SL 220230 dB re 1Pa@1m, the most intense biological sound known) that allow echolocation with distance resolutions of ~1 cm and discrimination of objects of different material.

The active sonar equation determines echolocation range: maximum range occurs when SNR_out = DT (detection threshold). The path losses TL = 20·log(r) + ·r in water are lower than in air for low frequencies (_water << _air), but the absorption grows with f²: high frequencies are absorbed in meters, limiting echolocation to short distances.

Blue and humpback whales use the SOFAR (Sound Fixing And Ranging) channel to communicate over distances of thousands of kilometers. In the SOFAR channel (depth of minimum sound speed, ~1000 m), sound is trapped by refraction and the losses are only cylindrical (TL = 10 log(r), instead of 20 log(r) for spherical propagation), which extends the range by 100× compared to free propagation.

The dolphin's eye is adapted to function in air and water: the cornea is almost flat (does not contribute to refraction underwater), and the lens is almost spherical (very powerful) to compensate. Vision is bifocal: the U-shaped pupil allows you to focus simultaneously in two different directions. Underwater, vision is moderate (acuity ~30 arcmin); in air, surprisingly better.

Cetaceans have deposits of magnetite (FeO) in cranial soft tissue, which suggests magnetoreception capacity for ocean navigation. Whales follow migratory routes of thousands of kilometers with precision of ~10 km, possibly using the geomagnetic field as a map (intensity) and compass (inclination) for two-dimensional navigation in the ocean without visual references.

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