Cetaceans and evolution
Cetaceans and evolutionCetaceans
There are many forms of cetacean life living in the worlds waters. How an why they came into existence is uncertain, but there have been many theories. Some say that dolphins are aquatic forms of goats, and others believe they evolved just like everything else, from a single cell, and
not from an actual multi-cellular being. Because most cetaceans have some vestigial structures such as under-developed os coxae, the former of these two theories remains prevalent in most
researchers minds.
The theory that cetaceans evolved from a goat-like being is not at all farcical. The similarities between the skeletal systems are more than coincidental. The only difference is that the cetaceans adapted to a marine lifestyle. The front limbs became modified as paddle-shaped flippers, the bones of which are still reminiscent of jointed limbs and digits, but the hind limbs were lost. The broad horizontal tail flukes that provide the main propulsive thrust bear no anatomical connection to the lost hind limbs, but are a seperate and distint development. They contain no bone, and owe their firm and yet flexible shape to underlying fibrous elastic tissue. The body is enveloped in a thick layer of blubber that aids in bouyancy, helps to preserve body heat, and is a source of stored energy. A cetacean�s skin is free of sweat glands, oil glands, or hair, and feels much like smooth, wet rubber to the touch.
Cetaceans, like other mammals, have lungs. They breathe air through a single nostril, or pair of nostrils, located on the top of the head; but contrary to a popular image, they do not �spout� water when they exhale. The visible spout, the size and shape of which is unique to many species, is simply water vapor in the lungs and a small amount of water present in the depression
around the blowhole, which is blown into the air as th cetacean exhales.
A number of physiological adaptations enable whales to perform deep dives. First, they have a larger blood volume than land mammals of comparable size and weight, and they also have a greatly increased capacity to store oxygen in their blood and muscle tissue. Second, each breath provides an 80 to 90 percent renewal of air in the whale�s lung, compare with only 10 to 20
percent in most land mammals. Third, cetaceans have a resistance to the metabolic by-product...
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