Abstract Variation in adult sex ratios (ASR) offers important insights into evolutionary and ecological dynamics, as it reflects demographic processes and predicts sex-specific reproductive strategies. In most mammals, ASRs tend to be female-biased at older ages. Yet, Madagascar’s group-living primates and some other mammals consistently show male-biased ASRs, but the underlying mechanisms remain unclear. Wild red-fronted lemurs (Eulemur rufifrons) exhibit even birth sex ratios, no sex-specific early life mortality, higher female disappearance rates within the first 8 years of life and male-biased ASRs. Since female reproductive activity peaks between the ages of 3 and 8 years, we hypothesized that reproductive costs might be associated with higher female disappearance rates, which are indicative of likely mortality, thereby contributing to the male-biased ASRs. Event-history analyses of 27 years of demographic data revealed that reproducing younger females and older mothers who successfully raised their offspring to independence were both less likely to disappear, suggesting that short-term reproductive success is closely linked to individual condition. By contrast, females with higher cumulative lifetime reproductive effort faced an increased risk of disappearing, highlighting the long-term costs of reproduction. By linking key components of reproductive investment to sex-specific mortality, this study contributes novel insights into the demographic processes underlying ASR variation among mammals.
Pethig et al. (2026) studied this question.