A few Considerations
Size
For small living things, surface area considerations are important,
relatively speaking. For instance, keeping the heat in (insulation) and
nourishment (generating internal heat)
tend to be proportionally more important for smaller animals. Smaller animals
tend to experience more air resistance; larger animals tend to fall faster.
Volume-related issues tend to predouminate for larger living things:
- gravity (weight). Larger animals tend to require larger and stiffer
bones to support them.
- inertia - it takes more work to get started or to stop moving
Also, solutions for oxygen exchange that work for
small living things may exploit the fact that there is more surface area per unit
of interior volume and, so, may not scale well to larger organisms.
Some "economies of scale" exist, also, where some solutions can be
harder to make happen in a smaller form, such as the digestive systems
of grazing animals.
Composition
Unlike man-made structures, nature prefers composite materials. This allows the
same raw materials to serve multiple purposes within a living thing. Living
things also have to be able to maintain and repair themselves.
Reproductive Strategy
Some organisms produce many offspring in the hope that a few will survive. This
works best in harsh or unstable environments or situations where availability of
nourishment is variable. Others produce fewer long-lived descendants, which works
best when mortality is low and conditions are relatively stable.
Some plants and animals reproduce infrequently in large numbers. This probably
suceeds because predators are not able to increase their numbers quickly enough
to take advantage of this overabundance and so some offspring will survive.
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