Free The Dolphin in the Mirror by Diana Reiss

a certain ineffable presence about them, which is partly what draws people to aquariums and drives them to seek swim-with-dolphins experiences. Despite this visceral connection, people recognize how different humans are from dolphins in physical form and, of course, in aquatic abilities. But most people are unaware of how very different dolphins are from humans in mundane realms, such as breathing and sleeping and the perception of their environment.
    I'll start with the differences in how dolphins and humans breathe and sleep, which, odd as it might seem, are actually closely linked. A dolphin breathes through a blowhole that is positioned on the top of the head, an important aspect of dolphins' evolution from land animal to marine mammal. The nostrils of dolphins' ancestors slowly "migrated" from the front to the top of the head. Nostrils atop one's head makes breathing easier during swimming or resting at the surface. Having your nostrils on top of your head might seem bizarre, but that is not the most important difference between us and them. The most important difference is the
manner
in which dolphins breathe. For humans, and for most terrestrial mammals, breathing is an involuntary process. The rate and timing of human breathing is governed by receptors in the brain that monitor the level of carbon dioxide in the blood and respond appropriately. Humans don't have to think about it. But for an air-breathing mammal who spends most of its time underwater, exactly
when
it breathes can be a matter of life or death. When receptors in a dolphin's brain indicate it needs to breathe, the dolphin must first swim to the surface. In other words, for dolphins, breathing is a voluntary process, not an involuntary one.
    A human wouldn't make a very good dolphin because he or she would have to surface half a dozen times each minute to breathe, which would be very inconvenient for a creature that forages and conducts its social life underwater. Dolphins, however, are superbly adapted to life under the waves. Under normal circumstances, they breathe two to four times a minute, although they can hold their breath as long as fifteen minutes; they exhale (just as they reach the surface) and inhale in a fraction of a second; they exchange up to 90 percent of the air in their lungs, compared with our much more limited 10 percent; and they extract much more oxygen from each breath than we do. The anatomy and physiology of their breathing is much more efficient than ours.
    So when we ask what it's like to be a dolphin, part of the answer is that they have to think about breathing. Also, they sleep very differently than we do. All mammals sleep, and dolphins are no exception. When humans go to sleep, we become unconscious, and our physiological systems run on autopilot. Dolphins have no such autopilot; at least, not in the realm of sleeping. To keep the dolphin from drowning, there has to be some part of the dolphin brain that remains awake. And yet, they also need to sleep. How do they do it? Dolphin brains, like ours, are divided into two hemispheres. When we humans sleep, both hemispheres go into unconscious mode. Through evolution, dolphins have arrived at another strategy: one hemisphere sleeps or rests while the other maintains breathing duties, motor behaviors, and possibly also some vigilance against predators. When dolphins are in sleep mode they often lie motionlessly at the water surface or at the bottom of a pool, or they might swim slowly, sometimes with one eye closed, surfacing from time to time to breathe. Dolphins appear to sleep or rest many times throughout the day and night. We don't know if they dream.
    Another big difference is in how they perceive the world. Dolphins, like us, face complex environmental and social challenges, and they use their senses and perceptions to survive and thrive in their dynamic environment. Dolphins use a rich constellation of multisensory signals to communicate with one another, including