Answer:
a. Because tooth shape varies with fracture properties of the foods that primates eat, relationships between dental form and function can teach us about the diets of our distant ancestors.
b. Most plant parts and animals have not evolved to be eaten (with a few exceptions, such as primate-dispersed angiosperm fruits). They are typically protected from being broken by either stress-limited or displacement-limited defenses. Stress-limited foods include hard-brittle items, such as some nuts and palm fronds, which require substantial stress (force per unit area) to initiate a crack, but once a crack starts, it is easily propagated. Displacement-limited foods, on the other hand, are typically tough items that may require little work to start a crack, but substantial energy to spread the crack through. Examples include raw meat and many mature leaves. This dichotomy is clearly an over-simplification, but it serves us well for understanding the challenges that teeth face during chewing.
Explanation:
DENTAL FUNCTIONAL MORPHOLOGY
An understanding of relationships between tooth form and function requires a way of comparing dental morphology between species with different diets. Researchers have developed several approaches to quantifying functional aspects of occlusal shape in primates.
The traditional approach has been the study of shearing quotients (Kay, 1984). This involves calculation of the summed lengths of crests running up and over individual cusps relative to the mesiodistal length of the tooth as a whole (Figure 3). The longer the crests (i.e., taller or sharper the cusps), the higher the shearing quotient (SQ). Among closely related primates, folivores have higher SQs than frugivores, and among fruit-eaters those that consume hard objects have the lowest SQ values. This fits well with our predictions for occlusal morphology given the fracture properties of leaves, fruit flesh, and hard objects. We should therefore be able to infer diets of fossil species based on where they fall on a plot of molar-crest length against tooth length for living primates with known food preferences (Figure 3).
