S6 Marine mammals

Topic

This marine mammal possesses a sensory system highly specialized for the aquatic environment. A dolphin produces high-intensity, broadband acoustic clicks that allow it to echolocate objects with very fine spatial resolution—similar to the echolocation system of a bat, which is described in another model within this set. The structure of the returning echo also makes it possible to distinguish the properties of different objects, including differences in their material composition.

The maximum range of this echolocation is determined by the balance between the signal and background noise—a concept established in another model in this set regarding signal detection theory: an object ceases to be detectable when the returning echo falls below the detection threshold. Acoustic propagation in water allows for long ranges at low frequencies, but absorption increases rapidly as frequency rises. While the dolphin's high-frequency click yields more detailed information, it also increases attenuation, thereby reducing the effective echolocation range.

Whales resolve this trade-off between frequency and range differently. Their low-frequency sounds can travel vast distances and, under specific oceanic conditions, take advantage of regions where the sound-speed structure favors confinement through refraction. In the deep sound channel—where the speed of sound reaches a minimum—waves can remain confined around a specific depth and propagate over enormous distances with relatively little loss. The lower frequency of these signals results in reduced absorption and increased potential range, facilitating long-distance communication.

In contrast, the dolphin's eye features adaptations that allow it to function both underwater and in the air. Underwater, where the difference in refractive index between the water and the cornea is minimal, the cornea contributes little optical power. The lens—which is nearly spherical and highly powerful—handles most of the necessary focusing. Additionally, the pupil and other optical characteristics help maintain functional vision across widely varying lighting and refractive conditions. These adaptations allow the dolphin to maintain visual acuity both while submerged and when observing its surroundings above water.

Finally, cetaceans can combine acoustic signals with other sources of information during their extensive migrations. It has been proposed that some possess magnetoreception mechanisms capable of providing information related to the geomagnetic field, possibly through structures sensitive to magnetic materials. This mechanism would be physically distinct from the cryptochrome-associated magnetoreception already established in another model regarding migratory birds. Gradients in the Earth's field intensity and inclination could provide complementary spatial information during oceanic navigation. When a whale is far from coastal or visual landmarks, globally available signals—such as acoustic and geomagnetic cues—can become relatively more important for maintaining orientation during long-distance migration.

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