Structure of male cooperation networks at long-tailed manakin leks

Social networks arise from complex interactions among multiple individuals and affect the emergent properties of groups (e.g. cooperation, disease spread, information transfer, etc.). Cooperation among nonkin is generally predicted to be favoured in structured social networks where individuals primarily interact only with certain individuals. Long-tailed manakins, Chiroxiphia linearis, form lek groups of as many as 15 unrelated males, whose members can attend multiple leks. At each lek, several top-ranked males perform the majority of obligate cooperative courtship displays. We used exponential random graph (ERG) modelling to analyse manakin cooperation networks constructed from 2-year time intervals over a 14-year study period. ERG modelling evaluates how local processes contribute to formation of global social network structure. We found that four local processes of link formation largely explained the overall structure of male manakin cooperation networks: (1) the spatial proximity of birds: males were more likely to cooperate if they primarily displayed at the same or neighbouring leks; (2) social status of birds: males were more likely to cooperate as they moved up the social queue at leks; (3) triad closure: males were more likely to cooperate with a 'friend of a friend' than with males with which they did not share a mutual partner; and (4) link persistence: males were more likely to cooperate with males whom they had cooperated with in the past. Other plausible mechanisms, such as selective mixing (the tendency to interact with individuals of similar or dissimilar social status) and preferential attachment by degree (whereby individuals with many social links gain additional links) did not consistently explain the structure of male cooperation networks at leks. These local processes may facilitate cooperation among long-tailed manakins by creating structured social networks in which males interact with only a subset of the population.