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can be used as wings. Their fins, once held out at right angles to the body and
flayed out to the full, provide a large surface area that acts much like the wing
of a bird and provides enough lift to create their distinctive flying action. As
a result the usual short periods above water that leaping provides have now
become extended periods of flying over considerable distances to escape
predators.
Porpoising
A number of animals engage in a “porpoising” action that forms only a part of
the locomotion that makes up swimming. During this action the animal leaps
periodically out of the water while swimming at speed. Both dolphins and
penguins do this on a fairly regular basis, simply as part of their swimming
action, though some fish do this as a strategy to avoid predators.
The benefit of porpoising is difficult to assess in terms of efficiency of
locomotion and economy of energy consumption. Some experts think
that it does indeed show an efficiency benefit for animals that travel at
high speeds. For penguins one of the possible benefits of porpoising is the
frequency with which it allows them to breathe. The speed at which they are
able to travel underwater is much greater than they can travel on the water's
surface. Underwater they are able to propel themselves using their wings as
hydrofoils; on top of the water they are restricted to using their feet, a far less
powerful tool. As with other forms of locomotion, the option of porpoising
is available only to animals under a certain size. The energy required for such
a gait is within reach of dolphins, but it would seem to be beyond larger
animals such as whales.
A number of animals propel themselves through the water by jet propulsion.
Water is drawn in at the front of the animal and driven out under pressure
from the rear end. Animals that use this mode of locomotion experience
fluctuating velocity; they accelerate quickly as the water is expelled under
pressure and decelerate as water is taken in. In some animals the frequency at
which the water is taken in and then expelled is so high that any fluctuations
in speed are negligible.
Shoaling
Rather like birds, fish gather together in large numbers for a variety of reasons,
the main one being for safety against predation, with individual fish in a large
group being far less likely to fall prey to a predator. Some predators have
become capable of exploiting the defensive shoaling trait of prey fish when
they form large bait balls. Sailfish, tuna, seals, dolphins, and many others
round up small prey fish such as sardines for ease of feeding.
Other fish shoal together in large numbers during spawning. Shoaling also
forms part of a migratory pattern in some fish. Salmon return to the rivers
of their birth in large numbers to spawn; elvers (small eels) swim in huge
numbers from the Sargasso Sea to rivers in Europe. Unlike their journey as
