Does distinctiveness enhance memory for cooperative events in Norway rats?

Reciprocity – defined as selectively helping those who were previously cooperative – is ubiquitous in humans, but its prevalence in other species is debated due to its apparently high cognitive demands. To effectively direct help to others via direct and indirect (but not generalised) reciprocity, individuals must memorise and correctly recall details of potentially many interactions with different partners, which can be challenging. However, the distinctiveness effect, whereby memory is enhanced for stimuli or events that are rare, unique, or surprising, could lower these demands. Using a reciprocal allogrooming paradigm, we tested whether distinctiveness could increase memory of experienced help in Norway rats. Focal rats experienced more cooperative and less cooperative allogrooming partners in either a distinct or a routine environment. After a time delay with intervening social interactions to increase memory demands, focal rats were given the opportunity to return allogrooming to their previous partners. In the routine environment, focal rats groomed more cooperative partners longer than less cooperative partners. In the distinct environment, focal rats started grooming more cooperative partners earlier than less cooperative partners, whilst the opposite was true in the routine scenario. These results demonstrate that rats allogroom partners in accordance with reciprocity. Whilst our results suggest a potential distinctiveness effect in the focal rats’ latency to reciprocate help, further research is needed to disentangle distinctiveness from confounds such as neophobia.