Dolphins have a number of extraordinary features so that they can thrive in their watery world.
The most obvious thing that dolphins need is air. They glide through the water so effortlessly, surfacing every few minutes to take a breath. Dolphins can dive to 200 m (600 ft).
Marine mammals take more air with each breath than other mammals, and they exchange more of the air in their lungs with each breath. Their red blood cells can hold more oxygen and they have a much higher tolerance for carbon dioxide than we do. (This means that they have to handle wider blood pH ranges than we do.) During each breath they exchange 80% of the air in their lungs, while humans only exchange 17%.
Even so, given the size of their lungs, they should run out of oxygen and drown before they can get that deep! How do they do it? When diving, they cut off blood circulation to their skin digestive system and extremities, leaving only the heart, brain and tail muscles working. However, even these measures give insufficient time to plummet to those depths. Using a video camera strapped to a dolphin's back, researchers have seen that when the dolphin reaches the 70 m mark, it just stops moving and sinks like a rock. It turns out that at this depth the dolphin's lungs collapse and it becomes denser than water. Since it doesn't need to move any muscles, oxygen consumption is greatly reduced. At the bottom of the dive the dolphins heart races for a few seconds and it swims back up to 70 m. At this point, it becomes less dense than water and just again can just glide to the surface.
When the dolphin's lungs collapse, its rib cage does also. This abruptly squeezes its heart - which would cause instant death in humans as the extra blood rushing through our cartoid artery would burst all the blood vessels in our brain. In dolphins, blood flows from the aorta in to a sponge-like mesh of capillaries that slows the blood and reduces the pressure before it reaches the brain (just put a sponge over your garden hose to see how it works).
Dolphins and other marine mammals don't get the bends (nitrogen narcosis) when they plummet to the depths of the ocean. In human lungs, air remains all throughout the lungs and gas exchange continues in the alvoli, allowing nitrogen to be forced into the blood. The alvoli of doplhins collapse at 3 atm of pressure, forcing the air back into the bronchioles where gas exchange does not take place.
How do dolphins (and whales) sleep without drowning?
Marine mammals have two basic methods of sleeping: they either rest quietly in the water, or sleep while swimming slowly next to another animal. Dolphins also enter a deeper form of sleep at night where they become like a log floating on the water. When a baby dolphin is born it does not have enough body fat to float easily. The baby stays afloat by being towed in its mother's slipstream or wake even when it is sleeping. This means that the mother cannot stop swimming for the first several weeks of her baby's life!
To avoid drowning, it is crucial that cetaceans retain control of their blowhole and recognize when it is at the surface. When sleeping, dolphins shut down half of their brain and one eye. The other half stays awake at a lower level of alertness. The semi-conscious side watches for predators, obstacles, and signals when to rise to the surface for a breath of air. After 2 hours, things are reversed, the active side goes to sleep and the rested side looks after vital functions. Amazing!
Dolphins are the most efficient swimming mammals at using energy for transport. They are famous for surfing on the bow waves of ships. This is not just for fun, it is like hitchhiking, allowing the dolphin to use about half the normal amount of energy that they use when swimming. Normally they spend as little time as possible on the surface where they waste energy battling surface waves and can't swim as efficiently because their dorsal fin is sticking up into the air.
The Navy has had a longstanding interest in dolphins - they achieve a speed and maneuverability that physics says should be impossible. (Of course, for a long time, physics couldn't explain how a bumble bee was able to fly). Navies would like to design submarines that could slip through the water with the ease and silence of Flipper. As far as we know, all of their research has been in vain. There are also various military ideas of using dolphins to look for mines or patrol underwater installations. Yuck!
Unlike fish, dolphins and whales (cetaceans) are warm blooded. Warm blooded creatures are typically more active with a higher metabolism than cold-blooded ones. Unfortunately, this causes another problem: temperature regulation.
Dolphins are warm blooded and their body temperature is normally 36 degrees C degrees, while the water in which they swim is typically 10 to 25 degrees. Like whales, they have a layer of blubber insulation under their skin. In some cases, this can provide too much insulation and they can overheat. To control their temperature, dolphins have blood vessels that run through their dorsal fin and their tail flukes. All that they need to do to cool down is to redirect blood to these areas - just like we do when we are hot (when blood goes to the surface of our hands and feet). This presents a contradiction: we saw that to conserve oxygen, blood flow to all extremities is cut off when under water. Research has shown that as soon as a dolphin surfaces to breathe, large amounts of heat are released in their tail and fins. This brings its body temperature back down for the next dive.
This is great, except that, as in all male mammals, the sperm needs to be kept cool in order for it to be viable (stay alive). For hydrodynamic reasons, dolphin's testes cannot be outside the body as they are in land mammals. Instead they are between massive muscles that work continuously as they swim. Logically, all male dolphins should be sterile. Female dolphins have an even bigger problem when they are pregnant. The baby dolphin acts like an extra heater inside it. If this heat is not removed, the fetus can be damaged or killed. What researchers have discovered is that the first place that cooled blood from the tail and dorsal fin goes is directly to the womb or to the testes. This amazing design keeps the temperature low enough to ensure the survival of both the fetus and sperm. We still don't know exactly how a dolphin can refrigerate its uterus and yet release its body heat only when it surfaces to breathe.
Marine biologists have found that as dolphins swim harder they use up more oxygen. However, after a certain point, they can increase the force of their strokes - even doubling the power - without using any more oxygen. Now, in order to swim so effectively, dolphins have a specialized muscle system. Cables of muscle run from the neck to the base of the tail. On a blue whale, this muscle (the longissimus) is 80 feet long, the longest muscle on Earth! However, something more than this is needed in order to explain the amazing effectiveness of dolphins at moving through water. The first clue came from looking at kangaroos! They use the spring-like tendons in their legs to recover 93% of the energy back out of each jump. So, is there an equivalent spring in a dolphin and if so where?
As mentioned earlier, under the skin of a dolphin is a layer of blubber. This blubber is attached to a sheath of connective tissue which encases the dolphin from head to tail. This sheath is made up of crisscrossing fibres in a spiral pattern. Tendons attach muscles to this sheath just as they do to bone. When dolphins swim, one set of muscles tightens this sheath until it is stiff. Then another set of muscles uses it as another spine to transmit their forces down the length of the body.
Mathematical models of cylinders made by spiraling fibres show that the angle of the fibres is important. At an angle of 55 to the long axis they can enclose the most volume. At an angle of 60 they are the springy and bounce back when bent. Guess what? The angle of the fibres around the dolphin's torso is 55, while near the tail it is 60! At the right swimming frequency, the upstroke stores energy in the fibre sheath, which then springs back helping push the tail through its downstroke.
Furthermore, it turns out that blubber is not just plain fat used for insulation. It also has a tapestry of fibres woven into it which function as springs just like of the main connective tissue sheath underneath. Ordinary fat will return 35% of the energy that you put into it. Blubber is much more springy and will return 87% of the initial energy. It is hard to imagine a more perfectly designed aquatic mammal than the dolphin.
Our bodies are equally or even more amazing than those of dolphins - it is just that we take things for granted so much. Our eyes are a marvel of engineering and optics. Our hands cannot be duplicated by any robots. Our balance system is exceptional in that we can walk on two legs, and ice skate and do ballet gracefully. Our minds are also unparalleled in its complexity and ability: we have abstract reasoning, we are creative, we can invent and communicate with languages (written, spoken, non-verbal, secret codes, Braille). Our mind-body combination is so flexible that we can train ourselves to do almost anything - play the piano, swim, work in weightlessness, climb vertical cliffs, devise solutions to problems that have never been encountered before. Why do our souls appreciate the beauty of Beethoven's Pastoral symphony, Niagara Falls, or a breath-taking view? How do we feel each other's joy and sorrow so deeply? We are truly fearfully and wonderfully made!