AbstractDegus (Octodon degus) are a highly gregarious species of caviomorph rodent native to South America. Kinship does not appear to play a role in degu social structure, and alloparenting is often observed between unrelated females. We hypothesize that female degus readily establish new, cooperative peer relationships. Here, we examined changes in dyadic behavior as individuals became more familiar, testing the prediction that interactions between female strangers would quickly resemble those of cagemates. Adult degus underwent a several week series of 20 min “reunion” social exposures, interleaving reunions with initial strangers and, as a control, familiar cagemates. Males showed initially higher levels of interaction with strangers that converged with cagemate levels over experience. Females could be split into two groups: those that consistently interacted more with strangers (SC‐HIGH) and those that did not (SC‐LOW); however, unlike males, the higher interaction levels observed between strangers did not change with familiarity. Following 10 reunion sessions female strangers were housed together to create “new cagemates.” Even after co‐housing, SC‐HIGH (but not SC‐LOW) females continued to interact more with the relatively unfamiliar peer than their prior cagemate, particularly in face‐to‐face and rear‐sniffing interactions. A final set of reunions with new strangers found that individual differences in female responses to social novelty were preserved. These results reveal sex differences in the rules relating familiarization to social relationships in degus, and that female predispositions toward cooperation may be due to inherent responses to new individuals more than to how they negotiate relationships over time.