What skeletons do

Skeletons are support systems; they keep animals from collapsing, and they provide levers on which muscles can act. Skeletons are hard and mechanically rigid, and they can be either internal structures (endoskeletons, as in vertebrates, from fish to mammals) or external (exoskeletons, as in arthropods, from crabs to spiders and insects).

Skeletons are mechanical structures that must withstand the forces that impinge on them, or they fail. The skeleton must support the weight of the animal, or it will fail in compression by crushing. It must withstand the forces due to locomotion, which produce bending and torsion that may cause failure in buckling. Also, a skeleton must withstand the forces of impact, and this may be the most critical requirement.

A heavier skeleton will be stronger, and for animals that do not move about (corals, for example), most of the organism can consist of skeletal material. For animals that move about, a heavy skeleton increases the cost of moving about and also reduces agility and the chances of escape from predators. The size and structure of the skeleton will therefore involve a compromise between the various demands: What should be optimized, strength or lightness?

Scaling mammalian skeletons

Consider a group of vertebrates, mammals, for example, that are similarly organized, although they are not alike. It is familiar to all of us that the bones of an elephant are proportionately heavier and stockier than those of a mouse.