Bone strain gradients and optimization in vertebrate skulls.

It is often stated that the skull is optimally designed for resisting feeding forces, where optimality is defined as maximum strength with minimum material. Running counter to this hypothesis are bone strain gradients--variation in bone strain magnitudes across the skull--which in the primate skull have been hypothesized to suggest that different parts of the skull are optimized for different functions. In this paper strain gradients in the skulls of four genera of primates, Sus, and Alligator were documented and compared. Strain gradients were pervasive in all taxa sampled. Patterns of strain gradients showed inter-taxon differences, but strains in the mandible and zygomatic arch were always higher than those in the circumorbital and neurocranial regions. Strain magnitudes in Alligator were twice as high as those in mammals. Strain gradients were also positively allometric; i. e., larger primates show steeper gradients (larger differences) between the mandible and circumorbital region than smaller primates. Different strain magnitudes in different areas of the same animal are hypothesized to reflect optimization to different criteria. It is therefore hardly surprising that the skull, in which numerous functional systems are found, exhibits very steep gradients. Inter-specific differences in strain magnitudes at similar sites also suggest inter-specific differences in optimality criteria. The higher strain magnitudes in the Alligator skull suggest that the Alligator skull may be designed to experience extremely high strains less frequently whereas the primate skull may be designed to resist lower strains more frequently.