Quantifying 'surprise' in capuchin monkeys (Sapajus apella)

Violation-of-expectation (VoE) events offer a fascinating lens into the intersection of cognition and emotion. Research suggests that, like humans, many animals are susceptible to VoE events in many physical domains, susceptibility typically assessed through behaviours such as visual and physical exploration of a stimulus. In humans, such susceptibility has been further documented through emotional surprise, such as delight following positive surprise and frustration following negative surprise. However, evidence in animals remains relatively limited. To address this gap, this study examined behavioural, cognitive and emotional responses of capuchin monkeys (Sapajus apella) to bait-and-switch VoE events of food items. Capuchins were chosen for this study not only for their exploratory and information-seeking behaviours, but also for known emotional responses to VoE events (e.g. frustration following inequity aversion). Contrary to prior research, this study found no evidence for violation susceptibility via behavioural, cognitive or emotional responses. Overall, such findings contribute valuable information to the VoE literature in primates regarding methodology and information salience, as well as underscore the current challenges of investigating emotional surprise in animals. Future research may benefit from establishing stronger expectations (e.g. more salient stimuli), to better investigate and quantify surprise to VoE events in animal species such as capuchin monkeys.