Several studies have investigated response latency in different tasks, showing that more complex choices usually require more time than less complex ones. In both humans and non-human primates self-directed behaviours, such as scratching, are related to frustration and anxiety. A few studies demonstrated that in great apes the occurrence of self-directed behaviours during cognitive tasks increase with task difficulty. Here, we investigated how response latency, scratching and other potentially stress-related behaviours vary in nine capuchins faced with an intertemporal choice task, an inhibition task where subjects face choices between a small immediate option and a large delayed option. We scored: (i) response latency, (ii) scratching, alarm calls, and pointing at chosen/not chosen option (a behaviour indicating motor impulsivity) during the delay associated to the large option, and (iii) scratching and alarm calls during the intertrial interval. Overall, there was a significant decrease of response latency across sessions. Moreover, in the course of the study, capuchins adjusted their behaviour to the hard task requirement (waiting for a desired reward), becoming less stressed during the delay. In fact, (i) scratching decreased both within each session and across sessions, and (ii) pointing at chosen/not chosen option decreased across sessions. In contrast, during the intertrial interval alarm calls increased across sessions. Thus, experience helps capuchins to cope with productive delays (i.e. instrumental to obtaining a reward), but has a negative impact on their tolerance for meaningless delays (i.e. non related with the specific task, as in the intertrial interval).
