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In-Depth Information
By contrast, the horizontal forces needed for propulsion through air are
far lower than those needed for horizontal movement through water. Air
is less dense than water, so there is far less friction in moving through the
air than there is in moving through water and therefore the energy costs
are considerably lower. The denser the medium, the higher are the energy
costs to an animal moving through that medium. The more streamlined the
profile an animal presents, the less drag its movement will incur and the
less effort it will need to propel it forward. Fish such as sailfish and tuna are
very streamlined, thereby making them capable of very high speeds. When
carrying a fish in its talons, an osprey will maneuver the fish to face forward,
creating a more streamlined profile, reducing drag, and therefore reducing
the effort needed to fly while carrying its prey.
Some animals other than fish manage to get around in and on water. Ducks
and other water birds manage to swim on water's surface due to buoyancy,
but some animals use the natural surface tension of water to support them.
Liquids do allow animals to travel along their surface in this manner, but due
to the limitations of the forces that the surface tension exerts on objects, this
practice is limited to very small animals. Once again we find that size really
does matter. A large fisher spider (
Dolomedes
) weighing in at around 1g can
easily get around on the surface of water by distributing its weight over a
wide circumference on legs that are around 50mm in length. Each leg makes
contact with the water around 15mm. To put this in some kind of perspective,
if a spider weighing in at around 1kg chose to walk on water in the same way,
its legs would have to form a perimeter of 140m.
Swimming on the surface of water also requires more energy than submerged
swimming due to wave drag effects. Measurements of energy expenditure
in sea otters showed that 70% more energy was required to swim on the
surface than swimming at the same speed when completely submerged.
Boats traveling at very high speeds along the water's surface experience
hydroplaning
. This results in hydrodynamic lift, which greatly reduces wave
drag effects and makes energy consumption far more efficient.
Swimming with Hydrofoils
Many animals, including fish, have adapted to use their limbs, paired fins, and
tails as hydrofoils to propel themselves through the water. Sea lions, dolphins,
turtles, and penguins as well as fish all use various parts of their anatomy
to propel themselves in this way. The difference between a hydrofoil action
and a rowing action is that during a rowing action, the oar moves more or
less at a right angle to the body, whereas in a hydrofoil action the limb or
fin moves at a much shallower angle to the body. Lift may be generated on
either the up- or downstroke and is determined by the angle of attack. A
steep angle provides the thrust; a shallower angle prepares for the next power
stroke. Penguins and marine turtles use their wings and their fore flippers,
respectively, as hydrofoils and gain lift on both the up- and the downstroke.
To achieve this effect, the angle of attack for both up- and downstrokes are
