How capuchin monkeys (Cebus apella) behaviorally cope with increasing delay in a self-control task

Displacement activities are behavior patterns apparently irrelevant to the situation in which they are performed that facilitate transitions between behavioral states. Scratching is one of the most commonly described displacement activities in primates and is related to frustration and anxiety. In chimpanzees scratching during cognitive tasks increases with task difficulty. We analyzed behavioral data obtained from video clips on nine capuchin monkeys tested in a delay choice task, a potentially stressful self-control task where subjects choose between a small immediate and a large delayed option. Scratching, response latency, and motor impulsivity scored during the delay decreased across sessions, as capuchins became indifferent between options, whereas the bias for choosing the option placed on one side of the apparatus increased. Capuchins might have found the delay choice task less stressful over time because they shifted from slower discriminatory responses in the earlier sessions to more 'automatic', faster responses in the later sessions. However, it cannot be excluded that they habituated over time to the contingencies of the task. In contrast with previous data on chimpanzees, handedness in scratching behavior or preferential scratching on one side of the body did not emerge, but further studies are needed to corroborate this finding.