Free The Dolphin in the Mirror by Diana Reiss Page A

acoustic, visual, tactile, and possibly chemical (taste) signals. In the course of the dolphins' adaptation to a totally marine existence, they have evolved specialized sensory systems that enable them to perceive their world even when vision is limited and vast distances separate them. Dolphins have the basic senses of hearing, vision, touch, and taste, but they have lost their olfactory receptors and the olfactory areas in the brain—thus they are unable to smell. Terrestrial mammals use scent trails and territorial scent marks, but these do not exist in the aquatic world, and the unneeded sense was eventually lost.
    The visual systems of animals are adapted to each species' environmental requirements. As primates, humans have excellent vision (not as good as some birds', of course), and it is our primary sensory guide in the world. Humans, like most other primates, have trichromatic vision; this means the eye has three types of color cones, enabling an individual to perceive a myriad of colors. The dolphins' sensory world is highly acoustic, so many people believed that they had inferior visual abilities. But in fact, dolphins have excellent vision. Although they are monochromats—they have only one type of color cone, and thus are colorblind—they are very sensitive to light in the blue region, which is the color of their aquatic world. The dolphin eye has many more rod cells than the human eye, making it very sensitive to low-light conditions, an important adaptation for life in the sea. Underwater, a human can't see very well without a facemask, but the lens of the dolphin eye is highly specialized and affords the dolphin vision that's just as good in the air as it is in the water.
    Humans have at best a mediocre sense of hearing when compared with that of many other animals, but our hearing really pales in comparison to the acoustic processing abilities of dolphins. Their marine world is filled with sound; sounds travel farther and about 4.3 times faster in water than in air. Our world is made up of visual images; the dolphins' world is one of acoustic images, as well as images.
    For creatures whose perceptual world is so dominated by sound, the absence of any obvious external ears might seem odd. The ears have been lost over evolutionary time, sacrificed in the radial streamlining of their bodies in their adaptation to a superbly efficient hydrodynamic physical form. (Dolphins are one of the swiftest creatures in the oceans.) The external ears have vanished, but the inner ears are present and again exquisitely adapted to the species' needs. For instance, dolphins can locate the source of a sound underwater, which humans find almost impossible, and their range of hearing is much greater than ours—it stretches from 150 to 150,000 Hz or higher, compared with humans' 20 Hz (a little lower than the first key on a piano) to 20,000 Hz (about two octaves higher than the last key on a piano). It is their high-frequency range that also sets dolphins apart from humans.
    In the course of evolving into fully aquatic mammals, dolphins have developed an exquisite and highly sophisticated biosonar system known as echolocation that allows them to navigate in a world without particularly good vision. It's a biological version of man-made radar and sonar, but much more powerful. (Dolphin echolocation is the envy of the military.) Dolphins have air sacs beneath their blowholes, and a dolphin echolocates by moving air between these air sacs at an extremely fast rate; this produces rapid sequences of high-frequency clicks, each of which lasts less than a thousandth of a second, that are directed forward in a narrow beam (the beam is shaped partly by the contours of the dolphin's bony skull and partly by a fatty "melon" that acts as an acoustic lens to focus sound). When the sound waves hit a solid object, echoes bounce off the object, travel back to the dolphin, are collected through the bones of the lower jaw (and a few other areas of