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Eye-tracking reveals bias to flanges in orang-utans1697CrI [0.015; 0.265], pd = 0.964). While other contrasts revealed no robust differences, Sandy tended to prefer Bako to Jingga (Supplementary Table 7). Lastly, Wattana had no clear preference: she spent most time fixating on Bako, closely followed by Jingga and Sibu. Interestingly, she spent the least amount of time fixating on Wousan compared with the other three males (Figure 4). However, pairwise contrasts revealed a clear difference only between Bako and Wousan (bBako-Wousan = 0.156 [0.090], 89% CrI [0.011; 0.294], pd = 0.960). Furthermore, Wattana tended to prefer both Sibu and Jingga over Wousan (Supplementary Table 7).Figure 4. Results of the individual preference-models based on Experiment 2. The figures depict which male received most attention for each female participant separately. Grey areas represent the posterior predictions for each iteration of the model, black dots indicate the median posterior prediction, and black lines indicate the 95% credible interval.DiscussionThe effect of facial characteristics on attentional processes in a mate choice context has been extensively studied in humans (Leder et al., 2016; Roth et al., 2022, 2023; Yang et al., 2015). However, relatively few studies have explored this topic in primates. In this study, we investigated how fully developed secondary sexual characteristics affect attentional processes in Bornean orang-utans. This arboreal great ape species is an ideal model for studying the effect of secondary sexual characteristics on attention processes because they are characterised by male bimaturism, meaning that adult males exist in two distinct morphs: unflanged males, who are reproductively active but not fully developed, and Tom Roth.indd 169 08-01-2024 10:42