Prevent Fracture/Rupture
High force impact or stress can cause materials that comprise living systems to separate into two or more pieces (called fracturing) or to break or burst suddenly (called rupturing). For example, a scallop prevents structural failure from fracture because its shell is comprised of two materials of varying stiffness. When a crack moves from the scallop’s stiff material to the less stiff one, the latter reduces the force at the tip of the crack, thereby stopping it from spreading farther.
Modify Material Characteristics
The materials found in living systems are variable, yet often made from the same basic building blocks. For example, all insect exoskeletons consist of a material called chitin. Because material resources are limited, each material within or used by a given living system must frequently serve multiple purposes. Therefore, living systems have strategies to modify materials’ softness, flexibility, and other characteristics. To ensure survival, the benefits of these modifications must outweigh the living system’s energy and material expenditure to generate them. For example, spiders store the liquid components of spider silk in a gland, converting them into silk thread when needed. Some threads have different characteristics, such as elasticity and UV reflectance, than others.
Modify Size/Shape/Mass/Volume
Many living systems alter their physical properties, such as size, shape, mass, or volume. These modifications occur in response to the living system’s needs and/or changing environmental conditions. For example, they may do this to move more efficiently, escape predators, recover from damage, or for many other reasons. These modifications require appropriate response rates and levels. Modifying any of these properties requires materials to enable such changes, cues to make the changes, and mechanisms to control them. An example is the porcupine fish, which protects itself from predators by taking sips of water or air to inflate its body and to erect spines embedded in its skin.
Prevent Deformation
When a living system undergoes compression, tension, shear, bending, or twisting, its internal inter-molecular forces can often resist these forces and even change shape temporarily, returning to the original shape when the forces stop. However, if the force is too strong or lasts too long, permanent deformation or structural failure can occur, resulting in death. Therefore, living systems have strategies to resist deformation or help ensure limited deformation. For example, bones have thin crystals and proteinaceous fibers that provide strength and flexibility, protecting them from forces that would otherwise cause deformation on a daily basis.
Protect From Temperature
Many living systems function best within specific temperature ranges. Temperatures higher or lower than that range can negatively impact a living system’s physiological or chemical processes, and damage its exterior or interior. Living systems must manage high or low temperatures using minimal energy, which often requires controlling responses along incremental temperature changes. To do so, living systems use a variety of strategies, such as avoiding high or low temperatures, removing excess heat, and holding heat in. Insulation is a well-known example of managing low temperatures by retaining heat using thick layers of hair, fur, or feathers to hold warm air next to the skin.
