Functional Relationships Between Wrist Joint Morphology and Ulnar Deviation in Strepsirrhine Primates

Ulnar deviation is a fundamental hand movement reflecting different positional behaviors that characterize primates and other arboreal mammals. Few experimental data exist on the relationship between wrist joint morphology and ulnar deviation of the hand in living primates. This study tests functional relationships between carpal joint anatomy and the degree of ulnar deviation for eight strepsirrhine species representing major locomotor groups.

Passive ranges of ulnar deviation were measured from radiographs of the hand of 25 anesthetized subjects using a motion analysis software. Position of the carpal elements was also compared in two different anatomical positions (i.e., neutral and maximal ulnar deviation).

On average, the hand of pronograde quadrupeds (Cheirogaleus, Eulemur, Hapalemur, Lemur, and Varecia) showed ulnar deviation angles ranging between 31° and 50°. Compared to pronograde quadrupeds, ulnar deviation of the hand was greater (but not statistically significant) in the vertical clinging Propithecus (49° to 59°) and greatest (and statistically significant at p < 0.01) in slow climbing Loris and Nycticebus (64° to 95°).

These differences in ulnar deviation of the hand closely match reported differences in proximal carpal and midcarpal joint shape in strepsirrhine primates. In pronograde quadrupedal and vertical clinging lemurs, ulnar deviation takes place mainly at the midcarpal joint, with some movement of the scaphoid and lunate at the proximal carpal joint of Propithecus. In slow climbing lorises, ulnar deviation of the hand is accompanied by equally notable movements of the carpals at both proximal carpal and midcarpal joints.