Understanding testosterone variation in a tropical lek-breeding bird.

Male reproductive coalitions, in which males cooperate to attract females, are a rare strategy among vertebrates. While some studies have investigated ultimate aspects of these relationships, little is known about the mechanistic role that hormones play in modulating cooperative behaviours. Here, we examined male testosterone variation in a tropical lekking bird, the wire-tailed manakin (Pipra filicauda), which exhibits cooperative male-male display coalitions. We found that testosterone levels in territorial males were comparable to those of temperate breeding birds, a surprising result given their environmental, social and reproductive dynamics. In addition, social status rather than plumage was a strong predictor of testosterone variation. Territorial males had significantly higher testosterone levels than did two other plumage classes of floater males, who do not hold territories. We hypothesize that testosterone variation plays an important role in the establishment of male dominance hierarchies (competition), while concurrently facilitating stable display partnerships (cooperation).