1.    Multiple mating by females is common and often driven by social constraints on female mate choice. However, females mate with multiple males even in systems without these social constraints and rates of multiple mating tend to be highly variable within and between populations. In lek mating systems, females are able to assess multiple males and their choice is unrestricted by pair bonds or the need for biparental care, yet some females mate with multiple males. 
2.    To better understand the factors affecting variation in multiple mating, we investigated the occurrence of multiple paternity within clutches in a highly polygynous lek mating system. 
3.    Using long-term data on genetic paternity, survival, social status, and individual age from a population of lance-tailed manakins (Chiroxiphia lanceolata), a species where males lek in cooperative alpha-beta pairs, we tested five non-exclusive hypotheses about causes of variation in multiple mating and its benefits to females. 
4.    We found that inexperienced males, including new alphas and rare beta sires, were disproportionately likely to share paternity when they sired any chicks. In contrast, female age (experience) was unrelated to multiple paternity. Multiple mating did not result in higher reproductive success or reduced variance in success for females, and there were neither consistently promiscuous females nor males that consistently shared paternity.
5.    The occurrence of multiple paternity in this lek mating system was best explained by female choice related to male characteristics that change with male experience. Our results support the hypothesis that there is a developmental component to the occurrence of multiple mating, and suggest females choose to mate multiply when their choices are not optimal.