It makes it easier for them to travel through the water.
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So it can keep it's direction
One example is that the dolphin has a stream-lined body.
A fairing makes the bike more stream lined, reduces wind resistance.
boats and aeroplanes are given special shapes inorder to reduce the loss of energy in overcoming friction
Boats that float on the surface of water have shapes similar to the bodies of waterfowl. Birds like ducks, swans and geese. Animals who are adapted to floating on water try to displace as much water as they can. So, just like boats or ships that float on water, they have very wide bodies. Boats that go underwater, called submarines, are very similar in shape to dolphins or whales. These animals have very stream lined, long and tapering shapes because they do not need to displace water to float as they are moving about underwater. So instead, they try to reduce how much resistance they offer to the water that is passing all around them by being thin. The "Portuguese Man o' War" is an animal who lives at sea and is named after an old Portuguese warship because part of the animal's body sticks above the water and is shaped like a triangle, looking just like the sail of the old Portuguese sailing ship.
viking long boats were shapes like a half oval
The most streamline shape is the teardrop-shape or the better known as the aerofoil.That is the reason why the airplane body and wings is shaped like a teardrop.This shape reduces air pressure on the object thereby increasing air flow in return increasing the speed of the body.
Birds have streamlined bodies and wings that enable them to move efficiently through the air, while fishes have hydrodynamic shapes and fins that reduce water resistance, allowing them to move easily through water. These adaptations help both birds and fishes to navigate their respective environments with speed and agility.
The great white shark needs its stream lined body so that it can swim easier and not be pushed away by strong currents. I hope that answered your question!!
Streamlined bodies reduce drag as they move through a fluid, allowing for faster movement with less resistance. This is particularly advantageous in swimming, cycling, and other sports where speed is a key factor. Streamlined bodies also help to improve efficiency and reduce energy expenditure.