The present study describes the flexor and extensor muscles in Cebus libidinosus' forearm and compares them with those from humans, chimpanzees and baboons. The data is presented in quantitative anatomical indices for similarity. The capuchin forearm muscles showed important similarities with chimpanzees and humans, particularly those that act on thumb motion and allow certain degree of independence from other hand structures, even though their configuration does not enable a true opposable thumb. The characteristics of Cebus' forearm muscles corroborate the evolutionary convergence towards an adaptive behavior (tool use) between Cebus genus and apes.

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.

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Among primates, only chimpanzees and orang-utans are credited with customary tool use in nature. Among monkeys, capuchins stand out with respect to the number of accounts of tool use. However, the majority of capuchin tool use observations reported in nature is anecdotal or idiosyncratic. In this report, we documented the stone pounding of dry fruits (Hymenea courbaril and Acrocomia aculeata) in two wild free-ranging groups of Cebus libidinosus in the Brasilia National Park, a preserved area representative of the Cerradobiome of Central Brazil. In 2004, we noted 2 episodes at which 4 monkeys used stones to crack open nuts. In 2005, we recorded 5 pounding episodes involving 2 different monkeys. Observations of tool use over the course of 2 consecutive years by some individuals, as well as other indirect evidence, indicate that this behaviour could be habitual in the studied groups. We propose that the probability of the emergence of the use of pounding stones as tools may be dependent on the ecological variables that influence the degree of terrestriality and extractive foraging and the complex interaction of these factors.

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