Age-related dominance helps reduce male aggressiveness in great bustard leks

Stable dominance hierarchies have evolved in many socially living animals to reduce the negative consequences of agonistic interactions. To evaluate whether established dominance relationships reduce aggressive encounters among males during the mating season, we studied aggressive behaviour in lekking male great bustards, Otis tarda. This sexually dimorphic bird species is potentially subject to strong sexual selection operating through intramale competition. The strongly skewed mating success of males is mainly determined by their age and weight, which are reliably signalled through plumage traits. We observed that adult males lived in stable groups at traditional leks to which they remained faithful throughout their lives, a prerequisite for the establishment of stable dominance hierarchies. Males substantially reduced aggressive interactions during the mating period. Males involved in more aggressive interactions during the premating period had weakly expressed sexual traits and low courtship success, which suggests the existence of an age- and possibly also weight-determined lek hierarchy. This is supported by the increase with age in a male’s dominance index, and the absence of aggressive interactions at advanced ages. During the mating period, older males and those with higher courtship success were involved in fewer aggressive encounters, suggesting that other males respected their status. Subdominant males disrupted other males’ copulation attempts, perhaps to reduce their success or to gain copulations themselves. Our study supports the hypotheses that social dominance is largely age determined in the great bustard and that established dominance relationships help reduce intramale aggression during the mating period.