Challenges

Because water is so dense (up to 800 times denser than air), it can easily support an animal's body, eliminating the need for weight-bearing skeletons like terrestrial animals. Water is also more viscous than air, and this coupled with the high density has resulted in aquatic animals adapting a very streamlined shape, particularly the carnivores. This makes them very fast and powerful swimmers, enabling them to catch their prey.

Many of the adaptations of aquatic organisms have to do with maintaining suitable conditions inside their bodies. The living "machinery" inside most organisms is rather sensitive and can only operate within a narrow range of conditions. Therefore, aquatic organisms have devised ways to keep their internal environments within this range no matter what external conditions are like.

Thermoregulation

Most aquatic animals are ectotherms, or poikilotherms, or what is often referred to as "cold-blooded." As the temperature of the surrounding water rises and falls, so does their body temperature and, consequently, their metabolic rate. Many become quite sluggish in unusually cold water. This "slowing down" caused by cold water is a disadvantage for active swimmers. Some large fish, such as certain tunas and sharks, can maintain body temperatures that are considerably

The fluke of the humpback whale (Megaptera novaeangliae) has evolved to be wide with scalloped edges, which enables the mammal to reach great heights when breaching. (Photo by John K. B. Ford/Ursus. Reproduced by permission.)

warmer than the surrounding water. They do this by retaining the heat produced in their large and active muscles. This allows them to remain active even in cold water.

Aquatic mammals are able to keep their body temperatures more or less constant regardless of water temperature. Marine mammals deposit most of their body fat into a thick layer of blubber that lies just underneath the skin. This blubber layer not only insulates them but also streamlines the body and functions as an energy reserve. The fusiform body shape and reduced limb size of many marine mammals and organisms decreases the amount of surface area exposed to the external environment. This helps conserve body heat. An interesting example of this body form adaptation can be seen in dolphins: those adapted to cooler, deeper water generally have larger bodies and smaller flippers than coastal dolphins, further reducing the surface area of their skin.

Arteries in the flippers, flukes, and dorsal fins of marine mammals are surrounded by veins. Thus, some heat from the blood traveling through the arteries is transferred to the venous blood rather than the outside environment. This coun-tercurrent heat exchange also helps to conserve body heat.

The Antarctic fur seal (Arctocephalus gazella) and her pup have a thick layer of blubber to keep them warm. (Photo by Animals Animals ©Johnny Johnson. Reproduced by permission.)

than two seconds, half the time humans take, even though the whale breathes in 3,000 times more air. Exhaling and inhaling takes about 0.3 seconds in bottlenosed dolphins (Tursiops truncatus). When swimming quickly, many pinnipeds and dolphins jump clear out of the water to take a breath. Cetaceans have the advantage of having a blowhole on top of the head. This allows them to breathe even though most of the body is underwater. It also means that cetaceans can eat and swallow without drowning.

The long, deep dives of aquatic mammals require several crucial adaptations. For one thing, they must be able to go a long time without breathing. This involves more than just holding their breath, for they must keep their vital organs supplied with oxygen. To get as much oxygen as possible before dives, pinnipeds and cetaceans hold their breath for 15 to 30 seconds, then rapidly exhale and take a new breath. As much as 90% of the oxygen contained in the lungs is exchanged during each breath, in contrast to 20% in humans. Not only do diving mammals breathe more air faster than other mammals, they are also better at absorbing and storing the oxygen in the air. They have relatively more blood than nondiving mammals. Their blood also contains a higher concentration of red blood cells, and these cells carry more hemoglobin. Furthermore, their muscles are extra rich in myoglobin, which means the muscles themselves can store a lot of oxygen. To aid in diving, marine mammals also increase buoyancy through bone reduction and the presence of a layer of lipids (fats or oils).

Aquatic mammals have adaptations that reduce oxygen consumption in addition to increasing supply. When they dive, their heart rate slows dramatically. In the northern elephant seal, for example, the heart rate decreases from about 85 beats per minute to about 12. A bottlenose dolphin's average respiratory rate is about two to three breaths per minute.

The bottlenosed dolphin (Tursiops truncatus), with its streamlined shape and powerful tail, can swim at speeds of up to 33.5 mph (54 kph). (Photo by François Gohier/Photo Researchers, Inc. Reproduced by permission.)

The air-breathers

Some water birds, such as cormorants and pelicans, simply hold their breath until completely out of the water. However, it is not appropriate for all air breathers to leave the water to breathe, especially if only a small portion of them can do it. This also has two evolutionary advantages: it reduces the amount of time at the surface of the water so they can spend more time feeding, and it reduces the amount of wave drag they encounter. The external nares of aquatic mammals, such as beavers, hippopotamuses, and dolphins, are always dorsal in position, and the owner seems always to know when they are barely out of water. A ridge deflects water from the blowhole of many whales. When underwater, the nares are automatically tightly closed. Sphincter muscles usually accomplish this, but baleen whales use a large valve-like plug, and toothed whales add an intricate system of pneumatic sacs so that great pressure can be resisted in each direction.

A manatee's (Trichechus manatus) heartbeat slows while diving, en-

To avoid inhaling water, aquatic mammals take very quick abling it to stay underwater for a longer period of time. (Photo by Phillip breaths. Fin whales can empty and refill their lungs in less Colla/Bruce Coleman, Inc. Reproduced by permission.)

The bottlenosed dolphin (Tursiops truncatus), with its streamlined shape and powerful tail, can swim at speeds of up to 33.5 mph (54 kph). (Photo by François Gohier/Photo Researchers, Inc. Reproduced by permission.)

Blood flow to nonessential parts of the body, like the extremities and the gut, is reduced, but it is maintained to vital organs like the brain and heart. In other words, oxygen is made available where it is needed most.

Another potential problem faced by divers results from the presence of large amounts of nitrogen in the air. Nitrogen dissolves much better at high pressures, such as those experienced at great depths. When nitrogen bubbles form in the blood after diving, they can lodge in the joints or block the flow of blood to the brain and other organs. Aquatic mammals have adaptations that prevent nitrogen from dissolving in the blood, whereas human lungs basically work the same underwater as on land. When aquatic mammals dive, their lungs actually collapse. They have a flexible rib cage that is pushed in by the pressure of the water. This squeezes the air in the lungs out of the places where it can dissolve into the blood. Air is moved instead into central places, where little nitrogen is absorbed. Some pinnipeds actually exhale before they dive, further reducing the amount of air, and therefore nitrogen, in the lungs.

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