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, Leontopithecus 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 within the Order Primates. Small-bodied platyrrhines which frequently forage among small-diameter substrates are expected to possess 1) relatively expanded apical pads, 2) well-developed epidermal ridges, 3) distally broad terminal phalanges, and 4) reduced flexor and extensor tubercles compared to those species which use large-diameter arboreal supports more frequently for their locomotor and postural behaviors. Results show that as the frequency of small-branch foraging increases among taxa within this sample, relative distal phalanx breadth also increases 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 breadth and epidermal ridge complexity are both positively correlated with apical 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 the pad to resist disruptive forces from different directions by friction and interlocking (Hildebrand, 1995). The observed morphoclines demonstrate that a gradient in form from claw- to nail-like tegulae exists among these taxa. Thus, the distinction between claw- and nail-bearing platyrrhines is essentially arbitrary. These observations corroborate Cartmill's (1972) functional and adaptive model for the loss of claws in primates: namely, expanded apical pads are required for habitual locomotor 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 the keeled tegulae of callitrichines represent a derived postural adaptation rather than a primitive retention from an ancestral eutherian condition.