Sex and Age Differences in Decision-Making Under Risk by Wild Balinese Long-Tailed Macaques (Macaca fascicularis fascicularis): A Field Experimental Study

This study examines risky decision-making in a free-ranging population of Balinese long-tailed macaques (Macaca fascicularis fascicularis), addressing gaps in research that have largely focused on captive primates and have rarely considered individual differences by age and sex. Thirty-three macaques of different age–sex classes were tested using a choice task contrasting a guaranteed small reward with a probabilistic larger reward. At the group level, macaques showed no preference for safe or risky options. However, substantial individual variation emerged: some individuals were risk-prone, others risk-averse, and many indifferent. Notably, age and sex interacted in shaping risk preferences. Among males, adults and juveniles were more risk-prone than younger adults, whereas among females, adults were more risk-prone than juveniles. Juveniles also displayed outcome-dependent flexibility, choosing the risky option more often after a previous successful risky choice, consistent with a win–stay strategy. Like in rodents, this pattern may reflect adaptive learning during developmental transitions. Importantly, the observed behavioral differences were not due to misunderstanding of the task, as macaques reliably chose the larger option when outcomes were visible. This pronounced individual variability in primate risk preferences underscore the importance of considering demographic factors when characterizing species-typical risk preferences.