Texture is another physical characteristic that primates can use to identify food items for ingestion. Texture is a term that refers to the physical hand or mouth "feel" of objects and provides important information about the potential edibility of plant parts. Most of the food textures that primates encounter are actually mechanical defenses erected by plants to deter herbivores. These defenses are of two types (Lucas, 2004: Lucas et al., 2000), aiming either to prevent cracks in structures from starting (crack initiation) or to prevent small cracks from growing (crack propagation). A primate attempting to detach a plant part could face one of two possible limits preventing its success: either it could not generate sufficient stress—a limit that relates to problems with crack initiation or, it could run out of displacement—a criterion relevant to crack propagation. Defenses that rely on these limits to protect the plant are called stress-limited and displacement-limited defenses, respectively. These limits are valuable concepts for mechanical analyses in any context, devised by Ashby (1999) to help engineers select materials for specific applications. In a biological context, we can recognize them as forming the basis of plant defenses against herbivore damage.

Lucas et al. (2000) gave an example of these two defenses that is appropriate in the context of this chapter and so we repeat it here. Consider attempting to fracture a horizontal branch on a small tree. Grasping it with your arm, you may bend it downwards with the intention of fracturing and detaching it purely with this downward motion. However, there are two possible limits that could prevent you from succeeding. Firstly, you could bend it through 90°, right against the trunk without success. This is displacement limitation: there was sufficient stress but you ran out of displacement. Alternatively, the branch may resist with little deflection, but not crack because you cannot generate sufficient stress. This is stress limitation: the displacement was sufficient displacement, but you lacked the force to generate the stress.

Three major material properties of plants largely determine the effectiveness of these defenses. Young's modulus is the ratio of stress to strain in which stress is the concentration of a force (obtained by dividing a force by the surface area over which it acts) and strain is the intensity of the force's effect on dimensional change (measured as the ratio of linear displacement in the direction of the force to the size of the structure). Young's modulus is an elastic measure; it holds only for small elastic strains and when there is a proportionate change in strain with stress. Toughness is the ability to resist crack growth and defined as the energy consumed in growing a crack of given area. The boundary between distortion or deformation and fracture (i.e., crack initiation) is not marked by any particular property and depends on the size of object that is breaking and the manner by which it is loaded. The yield stress is the boundary between elastic (recoverable) and plastic (permanent) strain in any material that exhibits it.

As shown elsewhere (Ashby, 1999, Lucas et al., 2000), stress-limited defenses are typified either by a high-yield stress or by a high value of KIC, which is roughly equal to the square root of the product of toughness to Young's modulus. These "hard" defenses contrast with "tough" displacement-limited defenses, which are controlled by the square root of the ratio of toughness to Young's modulus.

The structure of plants designed in these two ways look very different. Stress-limited defenses, which try to prevent cracks from initiating, tend to involve hard outer surfaces. Dense outer layers raise Young's modulus, particularly in bending, which is the most likely way in which high stresses are realized during ingestion. Hard, sharp features also act to deter herbivores due to their high-yield strengths. Familiar contemporary examples are spines, thorns, and stiff hairs—the latter being common in plant parts that primates feed on. These structures often have amorphous silica incorporated in them because this is harder than plant cell wall material. Some hard tissues are, however, defended by their very thick cell walls—many seed shells are like this. In contrast to this, displacement-limited defenses, that try to prevent cracks growing, are typified by a woody structure. Elongated woody cells have a cellular arrangement whereby the cell wall buckles plastically into the cellular lumen absorbing large amounts of energy, thus impeding crack growth. This greatly increases the toughness of tissue containing such cells while lowering the Young's modulus slightly. Rather than be disposed at the plant surface, wood is disposed throughout the plant interior wherever possible.

These two defenses are actually incompatible in plants, the simplest example being the mechanism of toughening (Lucas et al., 2000). The toughness of woody cells depends on the arrangement of the cell wall, but also on the presence of a cell lumen into which the wall must buckle. In contrast, the exact arrangement of the cell wall is immaterial to its hardness, which is maximized by infilling virtually the whole cell with cell wall, as in seed shells. Very dense seed shells can have cell walls organized exactly as in wood, but their toughness is very much lower.

Stress-limited defenses are actually antiingestion defenses that will evolve whenever major herbivores are large enough to threaten the survival of a plant by a single bite. Just one fracture is then life threatening for the plant. Savannah grasses are a familiar contemporary example. These small plants have long thin parallel-veined leaves reinforced by thick-walled sclerenchyma barely hidden under superficial ridges. Resistance to the tensile element of the bite of large ungulates is optimized by the sclerenchyma being aligned parallel to the direction of the bite force, maximizing the stresses at which the leaves start to fracture. This stress-limited design appears to explain the parallel venation of the leaves of many plants, including most monocotyledonous angiosperms. They often also contain large amounts of superficially located silica, consistent with the same defensive strategy. Such grasses must have been subject to predation by relatively large herbivores for much of their evolutionary history. Recent extinctions can, however, confuse the contemporary picture because plants evolve slower than animals. The survival of many stress-limited defenses in the neotropics, such as spines, thorns, and hard seeds, has been associated with megafauna, like the gomphotheres of the neotropics, which died out about 10,000 years ago (Janzen and Martin, 1982).

The spacing of thorns and spines seen on many stress-limited plants, such as the acacias of the African savanna, is probably linked to herbivore size—the larger the spacing, the larger the predator. The small siliceous hairs found on some angiosperm leaves can be interpreted as miniature spines that probably represent a stress-limited defense against some very small herbivores, probably cell suckers, which are often significant causes of leaf damage (Leigh, 1999). In contrast, displacement-limited defenses are antifragmentation (antimastication) defenses that typify plants attacked by very small "chewing" herbivores. It is extremely difficult to stop all damage at small scale. In particular, leaves are very difficult to protect because photosynthesis is compromised by any structures positioned in the light path leading to chloroplasts. The major predators of most angiosperms are "chewing" invertebrates (Leigh, 1999). In consequence, displacement-limited defenses predominate in most angiosperms.

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