Long-term-stability of relationship structure in family groups of common marmosets, and its link to proactive prosociality.

Cooperatively breeding, group-living common marmosets show differentiated relationships, where more strongly bonded dyads within a group engage more in affiliative interactions than less strongly bonded ones. Intriguingly, recent results suggest that strong bonds do not only occur between breeding partners but between individuals from any sex or status, and that strong-bond partners exhibit correlated oxytocin fluctuations (dyadic oxytocin synchrony, OTS) over a period of six weeks. To date, it is unclear whether such relationships are stable over time and whether they are also reflected in higher partner-specific proactive prosociality. To assess the long-term stability of the relationship structure of common marmoset family groups, we investigated whether hormonal and behavioral markers of group structure (dyadic OTS, dyadic affiliation, and individual group integration) in common marmoset families remained stable over a period of six months. We collected baseline urinary OT and social behavior of 36 dyads from three family groups in a non-reproductive period (period A), and again six months later, around the birth of new infants (period B). Patterns of dyadic OTS, dyadic affiliation, and individual group integration were consistent between the two study periods. Oxytocin data from a fourth group (10 dyads), collected in two non-reproductive periods separated by a period of more than five years, could replicate this finding. Furthermore, OTS was also correlated with proactive prosociality that was assessed experimentally for 38 dyads during an earlier study. These results suggest that differentiated relationships are stable over time, even between group members other than the breeding pair, and that more strongly bonded partners also show higher levels of proactive prosociality. Future studies are necessary to identify whether these relationships have an adaptive function, perhaps with regard to positive consequences on cooperativeness.