Play vocalizations induce affective bias in reward learning and memory in orangutans.

Affective biases, or shifts in learning and decision-making driven by affective states, are central to human cognition. Studies in rodents have shown that pharmacological and stress manipulations alter reward valuation, but how positive social signals shape these processes remains poorly understood. Here, we adapted the affective bias test, a translational rodent assay of affective biases in depression and antidepressant therapy, to ask whether positive social signals influence reward valuation in orangutans (Pongo spp.). We first validated the task by manipulating reward magnitude during learning and found that participants showed a significant preference for substrates previously associated with higher reward value, confirming sensitivity to reward valuation. We then tested whether play vocalizations influence reward valuation by presenting either conspecific play vocalizations or control sounds before learning reward-substrate associations, with identical reward values. In subsequent preference tests, orangutans showed a significant choice bias for substrates previously associated with play vocalization recordings. These findings demonstrate that orangutans experience positive affective states when hearing conspecific play vocalizations, as indicated by affective biases in reward valuation and memory.