BIOMECHANICAL ADAPTATION OF ULNAR CROSS-SECTIONAL MORPHOLOGY IN BRACHIATING PRIMATES

The forelimbs of hylobatids (gibbons and siamang) are distinctive amon g tetrapods in that they are loaded in overall tension during normal l ocomotion. While hylobatid ulnae must also encounter bending stresses in the course of their full range of locomotor behavior, their loading regime differs from that of quadrupedal anthropoids in that these ben ding stresses are distributed evenly along the bone, are not exerted i n a preferred plane, and are probably of generally lower magnitude. Th is study examines the degree to which hylobatid ulnae are adapted to t his suspensory loading regime. We obtained cross-sections of ulnae at various increments along the length of the bone using CAT scans. The s ample comprises 476 cross-sections representing the ulnae of 25 indivi duals from five species of comparable body size. We show that in gibbo ns and siamang, the patterning of ulnar cross-sectional area and resis tance to bending in the dorsoventral plane along the ulnar diaphysis d iffer from that of similarly sized quadrupedal anthropoids in the mann er predicted by a suspensory loading regime. We also find the same pat tern for the ulnae of Ateles, whose loading regime may be fairly simil ar to that of hylobatids. However, we find that the cross-sectional sh ape of the ulnar diaphysis in hylobatids and Ateles does not differ fr om that of quadrupedal monkeys in the manner predicted by a suspensory loading regime. (C) 1995 Wiley-Liss, Inc.