Rats spontaneously show categorical responses toward familiar or unfamiliar conspecifics in a habituation-dishabituation task using multiple habituation stimuli

Discriminating between familiar and unfamiliar conspecifics is a fundamental aspect of social recognition. Previous studies have shown that familiarity strongly influences rat social behavior, and that rats can be trained to discriminate between olfactory cues from cage-mates and non-cage-mates in a digging task. However, such discrimination did not occur in a lever-pressing task using live conspecifics as stimuli. In the present study, we investigated whether rats could discriminate social familiarity in a simple, less-demanding and naturalistic context using whole-animal stimuli. We employed a habituation-dishabituation paradigm in which multiple cage-mate or non-cage-mate individuals were presented during the habituation phase. We examined whether rats would habituate to the familiarity category among these multiple stimuli and subsequently show dishabituation when the stimulus class switched (cage-mate to non-cage-mate or vice versa). During the habituation phase, exploratory responses declined over successive sessions, suggesting habituation to the common element among multiple cage-mates or non-cage-mates, i.e., familiarity/unfamiliarity. When all subjects were considered together, clear dishabituation was not observed, indicating individual variation in habituation. However, a post-hoc analysis restricted to subjects showing sufficient habituation revealed significant dishabituation to the stimulus class switch. These findings suggest that rats can spontaneously classify multiple conspecifics according to their familiarity without training. Rats may flexibly employ different forms of social recognition and information such as individual identity or familiarity-based heuristics to guide adaptive social behavior in varying contexts.