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Chapter 7170flanged males, which are morphologically distinct from adult females and unflanged males, and are preferred partners of females (Knott et al., 2009; Utami et al., 2002). Here, we presented four Bornean orang-utans with flanged and unflanged stimuli (Experiment 1) or combinations of different morphs, that is, flanged-flanged, unflanged-unflanged, or flanged-unflanged (Experiment 2), while we non-invasively tracked their gaze. Across two experiments, we found that the orang-utans were more likely to first fixate on stimuli depicting flanged males, and spent longer fixating on flanged male stimuli. Furthermore, we found that orang-utans had an immediate attentional bias toward the left side of the screen. Below, we discuss our findings in the context of human and primate literature on attention and provide suggestions for future research.In two separate experiments, we discovered that orang-utans exhibited a higher likelihood of immediately fixating on stimuli depicting a male with flanges. This attentional bias towards flanges is in line with the general phenomenon of immediate attention towards evolutionarily relevant stimuli. By immediately detecting and processing biologically salient stimuli, an individual can effectively cope with situations that are relevant to their biological fitness (Carretié, 2014; New et al., 2007). Accordingly, humans have been found to immediately attend to the emotional expressions of conspecifics (Carretié, 2014; Kret & van Berlo, 2021; van Berlo et al., 2023), potential threats (Öhman et al., 2001), and attractive conspecifics (Lindell & Lindell, 2014; Roth et al., 2022, 2023). Similarly, recent evidence suggests that primates also immediately attend to biologically salient stimuli such as emotional scenes (King et al., 2012; Kret et al., 2016; van Berlo et al., 2023; but see Laméris et al., 2022). Our results suggest that immediate attentional bias may also be present for stimuli that are relevant to mate choice in primates. Immediate attention is mostly driven by bottom-up processes and low-level stimulus properties (Theeuwes, 2010). We attempted to control for luminance and contrast at the stimulus level by comparing the luminance and contrast of flanged and unflanged stimuli in Experiment 1 and standardizing the stimuli in Experiment 2. However, local differences in contrast and luminance within stimuli may have affected immediate attention and influenced our results. Nonetheless, it is important to emphasise that differences in low-level properties, such as contrast, may provide a perceptual mechanism that makes specific traits stand out (Rosenfield et al., 2019). Therefore, rather than an alternative explanation, low-level properties may be the mechanism by which attention is attracted to biologically meaningful traits. In general, sexual selection favours traits that Tom Roth.indd 170 08-01-2024 10:42