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Eye-tracking reveals bias to flanges in orang-utans1717exploit the sensory system of receivers because a salient trait should stand out against environmental noise (Ryan & Cummings, 2013). Orang-utan flanges may be an example of a trait that exploits the sensory system of receivers for sexual selection purposes. Their unique flanged features may stand out, making them more salient to their potential mates. This may explain why orang-utans fixated first on stimuli depicting a male with flanges in our experiments, as these stimuli stood out against background noise due to their distinct low-level properties. We also found that the orang-utans exhibited a first fixation bias towards the left stimulus in both experiments irrespective of the type of stimulus that was shown on the left. These results are consistent with those of previous human experiments. For instance, a study on attractiveness bias in humans found that 82% of the first fixations were directed towards the left face when two faces were paired (Leder et al., 2016). Importantly, this bias does not appear to be restricted to humans. Guo et al. (2009) presented facial stimuli to humans, dogs (Canis lupus familiaris), and rhesus macaques, and found that these species have a strong left bias in their first fixation when presented with faces. However, this bias may extend beyond the faces. In humans, it seems that an initial attentional bias towards the left visual field can be found across contexts, which suggests a more general perceptual bias driven by asymmetries in the attentional system between the two brain hemispheres (Ossandón et al., 2014). Overall, our findings regarding the first fixation of orang-utans suggest that they also have a perceptual bias towards the left during their first fixation. It is essential to consider this when designing studies to explore immediate attention in primates.With regard to voluntary attention, we found that orang-utans fixated on flanged male faces for a larger proportion of each trial than on unflanged male faces across the two experiments. While immediate attention is mainly driven by automatic bottom-up attentional processes, the results for the total fixation duration were probably more reflective of top-down attention because the orangutans could actively and voluntarily divide their attention during the four seconds that each trial lasted (Theeuwes, 2010). Therefore, it seems that the orang-utans volitionally fixated more on the flanged male faces than on the unflanged males. These results complement those of a previous study where it was shown that when scanning faces, orang-utans pay attention to the flanges (Kano et al., 2012). Furthermore, these results are in line with those of previous studies in humans and macaques. Previous research has shown an attentional bias for masculinity in male faces in humans (Garza & Byrd-Craven, 2023; Yang et al., 2015). Additionally, Tom Roth.indd 171 08-01-2024 10:42