Principles and levels of laterality in unimanual and bimanual stone handling patterns by Japanese macaques

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.

Hand preferences in coordinated bimanual tasks in non-human primates: A systematic review and meta-analysis

The evolutionary significance of hand preferences among non-human primates and humans has been studied for decades with the aim of determining the origins of the population-level tendency. In this study, a meta-analysis was conducted to statistically integrate data on hand preferences in non-human primates performing the tube task and other bimanual tasks to determine the presence and direction of manual laterality. Significant individual-level lateralization was obtained for these bimanual tasks. In nonhuman primates, 82% of the animals analysed showed right or left-hand preference performing the tube task, this figure being 90% for other bimanual tasks. In contrast with humans, no asymmetry was found at the population level. Additionally, population-level preferences were not found in either of the tasks, although a strong manual preference was found when performing the tube task and other bimanual tasks. Species was studied as a variable moderator throughout the meta-analysis. These results highlight the importance of standardized testing methodologies across species and institutions to obtain comparable data and fill the gaps in the taxonomy.

Handedness in nature: first evidence on manual laterality on bimanual coordinated tube task in wild primates

Handedness is a defining feature of human manual skill and understanding the origin of manual specialization remains a central topic of inquiry in anthropology and other sciences. In this study, we examined hand preference in a sample of wild primates on a task that requires bimanual coordinated actions (tube task) that has been widely used in captive primates. The Sichuan snub-nosed monkey (Rhinopithecus roxellana) is an arboreal Old World monkey species that is endemic to China, and 24 adult individuals from the Qinling Mountains of China were included for the analysis of hand preference in the tube task. All subjects showed strong individual hand preferences and significant group-level left-handedness was found. There were no significant differences between males and females for either direction or strength of hand preference. Strength of hand preferences of adults was significantly greater than juveniles. Use of the index finger to extract the food was the dominant extractive-act. Our findings represent the first evidence of population-level left-handedness in wild Old World monkeys and broaden our knowledge on evaluating primate hand preference via experimental manipulation in natural conditions.