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Reproductive effort should negatively correlate with reproductive value, yielding a pattern of increased effort with age. According to the terminal investment hypothesis, females near the end of their reproductive life span should devote more resources to reproduction than those near the start of their reproductive careers. We tested predictions of the terminal investment hypothesis by evaluating 38 years of reproductive life-history data collected from Nile lechwe (Kobus megaceros), an ungulate species living at San Diego Zoo's Wild Animal Park. The maximum reproductive success of Nile lechwe matched predictions of models of lifetime reproductive effort, with the relative mass of newborn calves providing an accurate indicator of the costs of reproduction. Newborn mass was significantly correlated with maternal age, and neonatal males tended to be heavier than neonatal females. Older dams were more likely to produce sons than daughters, dams that produced sons were more likely to die than were dams that produced daughters, and male calves were less likely to survive than were female calves. We conclude that young females endure a fertility cost while breeding, whereas older females encounter a survivorship cost, associated with progeny production. Our findings support the terminal investment hypothesis, and we suggest that secondary sex ratio bias among older female Nile lechwe reflects the evolution of a flexible life-history strategy promoting production of costly male calves when reproductive value is declining. [ABSTRACT FROM AUTHOR]