Carrying and hand-use patterns in Panamanian white-faced capuchins (Cebus capucinus)

Hand preference in wild crab-eating capuchin monkeys (Sapajus libidinosus) in the coastal area of Northest Brazil

Handedness is a fundamental human trait, although recent research, especially on nonhuman primates, has shown that it is displayed by other animals as well (e.g., chimpanzees, gorillas). In this study, we explore hand preference in wild crab-eating tufted capuchin monkeys (Sapajus libidinosus) inhabiting a mangrove forest located on the coastal area of Northeast Brazil (Maranhão State). Tufted capuchin monkeys at our site use facultatively wooden tools to crack open crabs. We observed hand preference in 12 subjects who spontaneously participated in experiment sessions, in which we provided crabs and tools on wooden platforms. We recorded (using events and bouts) two unimanual tasks, (tool or crab) grabbing and (tool or crab) pounding, and one bimanual task, crab pulling, where one hand kept the crab in place while the other pulled off parts of the crab. Hand preference increased with greater strength needed to perform the task and its complexity. While only 17%-25% of capuchins showed hand preference during grabbing, 44%-64% showed hand preference during pounding, and most subjects 64%-80% displayed a right-hand preference when performing the bimanual task, for which all lateralized individuals were right-handed. Hand preference did not vary between adults and juvenile individuals and was not consistent across tasks. Group-level hand preference was found only for the bimanual task, for which all lateralized individuals were right-handed. Our findings are in concordance with those of other primate studies showing the emergence of hemispheric specialization for bimanual actions, highlight the importance of conducting such studies on diverse type of tasks, and show the feasibility to conduct experimental manipulation under natural conditions.

Observations of hand preference in wild groups of white-faced sakis (Pithecia pithecia) in Suriname

Hand preference is well observed in humans and some primates. Unlike many other primates, however, humans show a consistent hand preference across a variety of tasks, and a distinct right-handed skew at the population level. Although there are a moderate number of published studies, primate hand preference literature is unbalanced by the large number of studies on only a few species. No previous studies have addressed hand preference in white-faced sakis (WFS; Pithecia pithecia). We followed three habituated groups of wild WFS in Suriname and recorded individual hand preference for six different manual behaviors. There was no consistent hand preference across a range of uni-manual behaviors for any individual. Likewise, there were significantly more ambidextrous individuals in the population than expected (χ(2) (df = 2) = 11.2, P = 0.004) and thus, no population level hand preference. Our findings contribute baseline data to the debate of primate hand lateralization, and support the notion that lateralization of hand function does not characterize all species.

Experimental field study of hand preference in wild black-horned (Cebus nigritus) and white-faced (Cebus capucinus) capuchins: evidence for individual and species differences

In this experimental field investigation, we compare the degree to which wild capuchins in Brazil (Cebus nigritus) and Costa Rica (Cebus capucinus) exhibit individual- and population-level handedness during three visually-guided tasks. These tasks required reaching to remove a large leaf covering a hidden food reward, seizing the food reward, and manipulating a tool (pulling a wooden dowel) in order to obtain access to an embedded food reward. Studies in some populations of captive capuchins indicate evidence for both individual hand preferences and population-level handedness. In this study, six of eight wild C. capucinus and six of seven wild C. nigritus exhibited a significant hand preference during individual tasks, but no individual exhibited a consistent preference across all three tasks. Task-specialization, or the tendency for most individuals in the same group or population to use the same hand to accomplish a particular task, also was evaluated. Cebus nigritus showed a significant bias toward the use of the right hand in removing the leaf. Although the number of individual capuchins in both species that manipulated the dowels was limited (N = 7), each individual that manipulated the dowels in eight or more instances had a positive handedness index, suggesting a greater use of the right hand to accomplish this task. Overall, our results provide preliminary support for individual- and population-level handedness in wild capuchin monkeys.

A Meta-Analysis of Primate Hand Preferences, Particularly for Reaching

P. F. MacNeilage, M. G. Studdert-Kennedy, and B. Lindblom (1987) proposed a progression for handedness in primates that was supposed to account for the evolution of a right bias in human handedness. To test this proposal, the authors performed meta-analyses on 62 studies that provided individual data (representing 31 species: 9 prosimians, 6 New World monkeys, 10 Old World monkeys, 2 lesser apes, and 4 greater apes), of the 118 studies of primate handedness published since 1987. Although evidence of a population-level left-handed bias for prosimians and Old World monkeys supports P. F. MacNeilage et al., the data from apes, New World monkeys, and individual species of prosimians and New World monkeys do not. Something other than primate handedness may have been the evolutionary precursor of the right bias in hand-use distribution among hominids.