Abstract
Capuchin monkeys present a wide variety of manipulatory skills and make routine use of tools both in captivity and in the wild. Efficient handling of objects in this genus has led several investigators to assume near-human thumb movements despite the lack of anatomical studies. Here we perform an anatomical analysis of muscles and bones in the capuchin hand. Trapezo-metacarpal joint surfaces observed in capuchins indicate that medial rotation of metacarpal I is either absent or very limited. Overall, bone structural arrangement and thumb position relative to the other digits and the hand’s palm suggest that capuchins are unable to perform any kind of thumb opponency, but rather a ‘lateral pinch’ movement. Although the capuchin hand apparatus bears other features necessary for complex tool use, the lack thumb opposition movements suggests that a developed cognitive and motor nervous system may be even more important for high manipulatory skills than traditionally held.
Highlights
It was traditionally held that no New World primate genera are able to perform precision grips [1,2]
The Trapezium-metacarpal Joint With regards to its articular surface, we found that the capuchin trapezium-metacarpal joint is concave at the base of metacarpal I and convex at the articular face of the trapezium, yielding a 90u angle between them
A few primates, for instance Saimiri, Tupaia and Tarsius (a ’prosimian’, or non-anthropoid primate), present a true saddle joint for the thumb whereas marmosets and other prosimians present a shallower joint. This joint is less shallow in New World primates when compared with the Old World primates, including the great apes and modern humans [16]
Summary
It was traditionally held that no New World primate genera are able to perform precision grips [1,2]. Functional grips to grab objects using lateral (thenar) sides of the digits have been attributed to a relative capacity to move fingers independently [6]. Spinozzi et al [9] reported that the capuchins grasp food objects in the palm by closing all digits simultaneously, with the thumb flexed parallel with the fingers. They reported that the capuchins, less frequently, grab peanuts between a flexed thumb and the palm, or in their thenar and hypothenar eminences. Christel and Fragaszy [4] indicated that the capuchin monkeys use all digits together for flexion and extension, they were able to perform some form of fine grips
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