2.1 The skeleton includes all the structures that function to absorb energy without breaking or permanently deforming, to transfer energy from one part to another, or to store-and-release energy

Most people (even many biologists!) think only of the bones when they think of the skeleton but skeletal structures include any structure that functions to support and physically protect the body and to transfer, store, or absorb mechanical energy. For animals generally, and vertebrates specifically, the skeleton includes the bones and other mineralized tissues, cartilages, tendons, ligaments, the dermis of the skin, and the connective tissues of the walls of the heart, the blood vessels, the respiratory tubes, and other fluid filled tubes in the body. Even fluids can sometimes act as skeletal elements (a hydrostatic, or hydrostatic skeleton). And, while the bones of vertebrates are part of the skeleton, as organs, the bones have other, non-skeletal, functions, for example Ca\(^{2+}\) and PO\(_4^{3-}\) homeostasis, the storage of fatty acids, and the site of the development of blood cells. In plants, all plant cell walls act as skeletons at the cell level but certain cells organize into distinct skeletal tissues, including collenchyma cells in growing parts of the plant, sclerenchyma cells called fibers, and xylem cells (including the wood in woody plants).

Figure 2.1: Inverted ankle joint

A vertebrate skeleton has to do lots of things. Hyena’s crush the bones of scavenged prey to get to the fatty marrow inside. When crushing a bone, the hyena’s teeth, mandible (the bone of the lower jaw), and maxilla (a bone of the upper jaw) need to transfer the force of muscle contraction to crack the prey bone. Imagine if hyena teeth were made of rubber, like a solid rubber ball. When biting bone, their teeth would simply be squished (or compressed). Instead the bones of the jaw and the teeth need to resist deformation, or shape change, in order to transmit the force.

Figure 2.2: Some tendons or ligaments act like springs

Unlike humans, most vertebrates run on uneven surfaces filled with rocks, roots, and small depressions and mounds. Landing on an uneven surface will frequently put the ankle joint into bending, which applies a force that stretches ligaments on the convex side of the bent ankle (figure~). A tight (difficult to stretch) ligament inhibits excessive bending and thus maintains joint stability. But in addition to be unstretchy, the ligament needs to be tough, to keep from tearing.

An under-appreciated function of the vertebrate skeleton, especially in introductory textbooks, is the ability of some skeletal structure to act like a spring. Squash or stretch a spring and it springs back to its starting length and this spring can be used to do work on something, such as launching a man, or a kangaroo, against gravity. In the vertebrate body, the skeleton of many structures act like springs and are used to reduce the energy cost of activity, such as hopping in kangaroos. Unlike skeletal structures used for transmitting forces or resisting deformations, structures that act like springs should be easily stretch or squashed.

The ability of a structure to resist stretching, or bending, or breaking is a function of both the geometry of the structure (its size and shape) and of the material in the structure. In this chapter, we will focus on the latter – the material properties.