Oxytocin-mediated empathy internally facilitates cooperative behaviors in rats.

Reciprocity is considered one of the vital mechanisms that sustain the evolution of cooperative behavior. However, free-riding, where assistance is received but not reciprocated, poses a serious threat to reciprocity behavior, which relies on future payback. Previous theories proposed that third-party punishment plays a vital role in preventing free-riding behavior. However, this external mechanism has inherent limitations, particularly in situations where third parties are absent. Empathy, the ability to perceive and share the emotional states of others, has long been considered a driving force behind prosocial behavior, yet its role in cooperative behavior remains underexplored. In this study, we have designed a new reciprocity paradigm, and demonstrate that rats' reciprocity behavior can stably establish even in the absence of the external mechanisms. Additionally, reciprocity experiences can enhance the empathy of wild type rats, but not oxytocin-deficient rats, towards their partners. Furthermore, oxytocin-deficient rats exhibit more free-riding behaviors. Through fiber photometry recording of oxytocin probe, we found that oxytocin is remarkably released in the orbitofrontal cortex during the reciprocity task, significantly exceeding levels observed in both mutualism and individual tasks. Based on our results, we suggest that oxytocin-mediated empathy enhancement reduces rats' free-riding behavior towards their partners, thereby making reciprocity behavior more stable. This empathy-mediated internal driving force complements the previously proposed external mechanisms, providing new theories and perspectives for understanding the evolution of cooperative behavior.