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

Abstract

Objectives: 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.

Methods: 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).

Results: 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°).

Conclusions: 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.

Keywords: carpals; hand; lemurs; locomotion; lorises.

FIGURE 1

Hand position during positional behavior in three strepsirrhine primates. In pronograde quadrupeds like Lemur catta (A), the hand is held in a more neutral position with the substrate usually gripped between the second and third digits or along the second digit of the hand. The hand of vertical clingers like Propithecus coquereli (B) and slow climbers like Nycticebus coucang (C) shows more ulnar deviation, and the substrate is usually gripped between the first and second digits or along the second digit.

FIGURE 2

Transverse view of the distal radius (R) and dorsal view of the capitate (Ca) and hamate (Ha) of a pronograde quadruped ( Eulemur fulvus ) (A), a vertical clinger ( Indri indri on top and Avahi laniger on bottom) (B), and a slow climber ( Nycticebus coucang ) (C). On top, note the greater articular curvature of the distal radius of the slow climber compared to other taxa (arrows). At the bottom, note the progressively steeper sloping of the articular surface of the proximal hamate from the pronograde quadruped, vertical clinger to slow climber (arrows). Redrawn from Hamrick (1996a).

FIGURE 3

Radiograph of the hand in maximum ulnar deviation for Lemur catta (Scale bar = 1 cm). Two lines superimposed onto the image were drawn to measure the maximum ulnar deviation angle (α) as indicated.

FIGURE 4

Angle values (°) for maximum ulnar deviation of the hand in eight strepsirrhine species. Note the steady increase in angle values from pronograde quadrupeds (blue triangles), vertical clinger (yellow squares), to slow climbers (red circles).

FIGURE 5

Radiographs of the hand in neutral position (left) and maximum ulnar deviation position (right) in a pronograde quadruped ( Varecia variegata ) (A, B), vertical clinger ( Propithecus coquereli ) (C, D), and slow climber ( Nycticebus coucang ) (E, F) (Scale bars = 1 cm).

FIGURE 6

Inverted radiographs of the hand in maximum ulnar deviation position in Varecia variegata (A), Propithecus coquereli (B), and Nycticebus coucang (C) (Scale bars = 1 cm). Ca: capitate, Ce: os centrale, Ha: hamate, Lu: lunate, Pi: pisiform, Sc: scaphoid, Tr: triquetrum. Note the relative position between taxa of the carpals of the proximal row (in green) and distal row (red) involved in the proximal carpal and midcarpal joints. Extreme ulnar deviation of the hand observed in Nycticebus is accompanied by ulnar rotation and translation of the hamate onto the triquetrum, radial translation of the scaphoid and lunate onto the distal articular surface of the radius, and lack of an apparent ulnotriquetral contact.

FIGURE 7

Sequential images of a Nycticebus coucang climbing up an oblique branch. The arrows indicate the extreme degree of ulnar deviation of the hand at touchdown (A) and the wide range of radioulnar deviation, flexion‐extension, and likely rotation of the hand at the wrist joints throughout the support phase (A–D). Note as well the position of the forearm parallel with the substrate at touchdown. Each image represents 1/30 s.