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mammals do not undertake such epic journeys, but small birds do. Swallows,
swifts, and martins regularly travel from their summer homes in northern
Europe to their winter homes in South Africa. Albatross cover even greater
distances. Flying is a far more economic mode of travel than travel by land,
which is why even small birds such as the reed warbler, weighing in at
around 13 grams, are able to migrate from northern Europe to North Africa
each year.
The use of energy and the demands for energy usage can differ by species
within the same animal family group. A racehorse is clearly built to
travel at high speeds; a shire horse is built for strength in pulling power
and endurance. I'm not a gambling man, but if both of these horses
were entered in the Grand National, I think my money would be on the
racehorse.
For a few animals, a combination of endurance and speed is the norm. Swifts
spend the vast majority of their lives flying and only return to land again to
nest; they even sleep on the wing. For them a sustained period of movement,
much of it at speed, is the normal condition.
The nature of movement has an impact on the energy intake of the animal,
the frequency with which it feeds, and its diet. The very nature of flight
in hummingbirds entails a rapid action that burns energy quickly, in turn
demanding regular high-energy intake. We will look at this phenomenon a
little more closely later in the chapter.
Stability and Gait
Stability in an object is achieved when it is either at rest or moving at a
constant speed when the forces acting on it are balanced. An object is most
stable when it is balanced across a broad base with the center of gravity
supported across that base. Stability is less easy to achieve when the center
of gravity is over a single point located directly below the center of gravity.
A pyramid is most stable if supported by its base; invert it and the support on
the point becomes very much less stable.
FIG 4.5 The pyramid on the left
is a stable structure due to its wide
“footprint” and distribution of weight.
The inverted pyramid on the right
represents a far more unstable structure
with a much smaller “footprint.”
Center of balance is linked to stability
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