Testosterone drives status-specific patterns of cooperation and transmission of behavior in a social network

Stable social partnershipsrequire physiological mechanisms that maintain a balance between competition and cooperation. Although the endocrine systemis known to facilitate competitive behavior, we know very little about how steroid hormones affect cooperation and the stability of partnerships. Here, we examine how testosterone (T) modulates social behavior in wire-tailed manakins( Pipra filicauda), a species in which territorial and non-territorial males form cooperative coalitions within a larger social network. Our approach links repeated hormone sampling and a hormone-manipulation experiment with an automated telemetry system that captured >30,000 male-male social interactions. Using 528 repeated T samples from 209 males, we find that circulating T is repeatable with individual differences accounting for 19% of the total variation, and that among-individual differences in baseline T can explain 17% of the variation in cooperative behavior. Patterns of hormone-mediated behavior were status-specific whereby T significantly increased cooperative behaviors in floaters whereas it tended to decrease the same behaviors in territorial males. Experimental elevation of T caused significant decreases in both the number of partners and frequency of cooperative interactions received by territory-holders, with the strongest effect observed in lower-T individuals. This status-specificity suggests that hormone-mediated behavior is under stabilizing selectionand that ontogenetic plasticity in hormone-regulatory networks facilitated the evolution of a stable cooperative society. Our analyses also reveal that one male-s T can indirectly affect the behavior of other individuals with whom he interacts (explaining 1-3% of variation), indicating that hormones are a mechanism underlying the transmission of behavior in social networks.