Reciprocal social ties drive stable cooperation within a social network

Social reciprocity is thought to be the most important driver of cooperation among non-kin, but its effects within complex social networks where individuals choose their partners have not been studied. Here, we test whether reciprocation can explain social dynamics in a bird species where males form coalitions to court females, the wire-tailed manakin (Pipra filicauda). In our study population, approximately a third of male-male social interactions involve one male leaving his territory to interact and display with the owner of another territory; this directional behaviour provides benefits to the recipient at a cost to the visiting male. We characterize social ties among pairs of territory holders according to the directionality of their interactions, and show here that territory holding males engaged in far more reciprocated (bidirectional) partnerships with other territory holders than expected by chance. Reciprocated partnerships were also stronger (i.e., the partners interacted more frequently) than non-reciprocated partnerships, controlling for spatial proximity of the territories. An individual-level analysis revealed that a males social contribution to a given partner was predictive of the number of social interactions he received from that same partner. Finally, we show that reciprocation predicted the long-term stability of social partnerships one year later. Together, these results demonstrate that reciprocity is associated with stability in complex social networks where partner choice is flexible. LAY SUMMARYO_ST_ABSWhen animals are free to choose their partners, how does reciprocity influence social dynamics?C_ST_ABSIn the wire-tailed manakin, males form coalitions with other males to help attract mates. We tracked male social networks and found that some males form reciprocal partnerships where each male leaves its territory to display at the others territory. These bidirectional partnerships are particularly strong and stable, indicating that reciprocity is associated with stability in complex social networks.