FUNCTIONAL AND ADAPTIVE SIGNIFICANCE OF PRIMATE PADS AND CLAWS - EVIDENCE FROM NEW-WORLD ANTHROPOIDS

This study tests predicted morphoclines in fingertip morphology among four small-bodied (<1 kg) New World monkeys (Saimiri sciureus, Leontop ithecus rosalia, Callithrix jacchus, and Saguinus oedipus) in order to test previous functional and adaptive explanations for the evolution of flattened nails, expanded apical pads, and grasping extremities wit hin the Order Primates. Small-bodied platyrrhines which frequently for age among small-diameter substrates are expected to possess 1) relativ ely expanded apical pads, 2) well-developed epidermal ridges, 3) dista lly broad terminal phalanges, and 4) reduced flexor and extensor tuber cles compared to those species which use large-diameter arboreal suppo rts more frequently for their locomotor and postural behaviors. Result s show that as the frequency of small-branch foraging increases among taxa within this sample, relative distal phalanx breadth also increase s but distal phalanx length, height, and flexor tubercle size decrease . Moreover, epidermal ridge development becomes more pronounced as the frequency of small-branch foraging increases. Terminal phalanx breadt h and epidermal ridge complexity are both positively correlated with a pical pad size. The large, flexible apical pad increases stability of the hand and foot on small-diameter arboreal supports because the pad can contact the substrate in several planes which, in turn, enables th e pad to resist disruptive forces from different directions by frictio n and interlocking (Hildebrand, 1995). The observed morphoclines demon strate that a gradient in form from claw-to nail-like tegulae exists a mong these taxa. Thus, the distinction between claw-and nail-bearing p latyrrhines is essentially arbitrary. These observations corroborate C artmill's (1972) functional and adaptive model for the loss of claws i n primates: namely, expanded apical pads are required for habitual loc omotor and postural behaviors on small-diameter supports whereas claws are more useful for positional behaviors on large-diameter substrates . Finally, results from this study support previous suggestions that t he keeled tegulae of callitrichines represent a derived postural adapt ation rather than a primitive retention from an ancestral eutherian co ndition. (C) 1998 Wiley-Liss, Inc.