Age-dependent drivers of sex-specific disappearance in wild redfronted lemurs: the role of behavior

Abstract In humans, frailty, risk-taking behaviors, and unhealthy lifestyles have been linked to shifts in adult sex ratios (ASR), which are heavily female-biased at older ages. Conversely, Madagascar’s group-living primates frequently exhibit male-biased ASRs, defying patterns commonly observed among polygynous mammals. To explore these unusual sex ratio dynamics in wild redfronted lemurs ( Eulemur rufifrons ), we examined three behavioral proxies of mortality risk. We conducted 150 flight initiation distance (FID) experiments to assess sex differences in risk-taking and collected locomotion and proximity data to gauge variation in frailty and social integration. To achieve the observed male-biased ASR, we expected females to be less physically active than males, to have shorter FIDs, and to maintain less proximity to group members, thereby increasing their risk of predation. Using observational data from six years, we found that males spent less time being physically active than females. Locomotor quality followed an inverted U-shaped pattern, with very young individuals and those over eight years jumping less far than young adults. Older lemurs also had fewer nearby individuals. Individual FIDs decreased with height above the ground. Moreover, females with longer FIDs had a slightly lower risk of disappearing sooner, indicating a relevant behavioral adaptation that may enhance survival. However, weak evidence for sex differences in these proxies suggests that other intrinsic and extrinsic factors more strongly shape mortality and ASR in this species. The predicted effects of risk-taking, agility and social integration on survival therefore merit further study in additional taxa and using alternative proxies.