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The hand of two ‐toed sloths ( Choloepus ): Its anatomy and potential uses relative to size of support
Author(s) -
Mendel Frank C.
Publication year - 1981
Publication title -
journal of morphology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.652
H-Index - 74
eISSN - 1097-4687
pISSN - 0362-2525
DOI - 10.1002/jmor.1051690102
Subject(s) - anatomy , claw , wrist , hook , interphalangeal joint , biology , ulna , phalanx , joint (building) , medicine , architectural engineering , ecology , dentistry , engineering
Abstract Hands of two‐toed sloths ( Choloepus ) are long, narrow, hook‐like apparatuses with only two functional digits (II and III); rays I and IV are represented only by metacarpals. The proximal phalanges of digits II and III are shortened to essentially proximal and distal articulating surfaces, and all but distal interphalangeal joints of these digits are restricted by interlocking surfaces to minimal ranges of flexion and extension. Several intercarpal joints and the wrist joint, however, allow wide ranges of movement in several axes. Wide excursion at the wrist is permitted by an extremely lax joint capsule, the manner of insertion of several prime movers of the carpus, and the reduced participation of the ulna in the wrist joint. Several extrinsic digital muscles, particularly extensors, are absent and others have unusual actions. Intrinsic musculature consists primarily of mm. interossei and m. extensor digitorum brevis, although other, inconstant muscles do occur. Hands of Choloepus are used as flexible hooks on supports less than 52 mm in diameter and as fixed grapnels on larger supports. In both cases, distal phalanges (and covering claws) form the “hook” element. Whereas bare volar pads seem to be adjunctive on supports smaller than 52 mm in diameter, they are essential on those larger than 65 mm. Two‐toed sloths may prefer supports 50 mm in diameter or smaller. The potential importance of vines as supports is discussed.