Taste

The vast array of chemical and mechanical defenses in plants shows evidence of economy when viewed at a fine enough scale (Choong, 1996), meaning tissues with mechanical defenses (e.g., thick cell walls) tend not to have chemical defenses and vice versa. As described in an earlier section, "hard" stress-limited defenses are designed to stop cracks initiating. If cracks do not start, then cells are not opened. Accordingly, plants or plant parts designed this way would be predicted to have few chemical defenses. Many monocotyledonous plants are an example of this. Even though silica and cell wall materials are relatively cheap components for a plant, "hard" defenses have a major drawback: they are (generally) dead. On the other hand, "tough" displacement-limited defenses rely not on resisting crack initiation, but on opposing crack propagation. So cells will inevitably be opened, suggesting immediately that chemical defenses have a role as herbivore deterrents. The major problem with displacement-limited defense is that they are slow to develop, leaving young tissue vulnerable. The greatest concentration of chemicals is found in young tissues, an example being tannins and phenolic compounds in young leaves (Coley, 1983).

The amount of cell wall in a food is called by nutritionists its fiber content. Fiber is a key element of food quality for a primate because cell walls either have to be fractured (by the teeth or by gut peristalsis) to expose cell contents or else microorganisms need to be housed in the gut so as to achieve this enzymatically. The efficiency of digestion depends on fiber. Unfortunately, cell walls are largely colorless, odorless, and tasteless, so the major means of sensing them must be via their texture. As shown by Lucas et al. (2000), the volume fraction of a tissue that is occupied by cell wall is not proportional to its toughness and this must displace foraging away from the optimum.

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