A Young Manakin Knows His Place: Evidence for an Age-Graded Dominance Hierarchy Among Long-Tailed Manakins

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In lek-breeding systems where many males gather at display sites, males benefit from the establishment of dominance hierarchies to reduce intrasexual aggression and the associated risk of injuries. Long-tailed manakins (Chiroxiphia linearis) exhibit an exploded lek-breeding system wherein the two top-ranking males at each display site team up to perform elaborate coordinated courtship displays for females. Young males undergo delayed plumage maturation whereby they acquire distinct pre-definitive plumage patterns each year until they attain definitive plumage in their fifth year. This unique characteristic is thought to have evolved as a status-signalling mechanism to aid in the establishment of an age-graded dominance hierarchy in which older males are dominant to younger males. Previous research has shown evidence for such a dominance hierarchy among alpha and beta males; however, the presence of this hierarchy among males of other age classes has never been quantified. In this study, we investigated the presence of an age-graded dominance hierarchy by determining whether older males direct more aggressive behaviours towards younger males. We also investigated whether status signalling is less clear within age classes than between age classes, by determining whether males within the same age class exhibit more aggression towards each other. We found that older males performed aggressive behaviours towards younger males much more frequently than younger males performed aggressive behaviours towards older males. We also found that some aggressive interactions occurred between males within the same age class more frequently than between males from different age classes. Our study provides some evidence for an age-graded dominance hierarchy among male long-tailed manakins of all age classes and also provides some support for the status-signalling hypothesis. However, further research is needed to conclusively establish the presence of a linear dominance hierarchy among younger male manakins. This research may help us better understand the evolution of complex hierarchical systems in animals.