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Chapter 9202both male and female participants (N = 35 participants). Our results complement previous studies that have found an association between general attractiveness ratings on attention (Ma, Zhao, et al., 2015; Roth et al., 2022) by showing that the same applies for idiosyncratic attractiveness ratings. Furthermore, our findings suggest that biases in voluntary attention concord with mate choice.In Chapter 6, I moved away from humans and focused on Bornean orang-utans (Pongo pygmaeus). Across three touchscreen experiments, I investigated whether Bornean orang-utans showed an implicit attentional bias towards males with larger flanges or more symmetrical faces using the dot-probe paradigm. Furthermore, I presented orang-utans with a choice task that allowed them to view a picture of either a flanged or unflanged male. Previous studies have suggested that orangutan females prefer to mate with flanged males (Knott et al., 2009). Furthermore, facial symmetry has been linked to health in multiple primate species (Little et al., 2012; Sefcek & King, 2007). Therefore, I expected individuals to immediately attend to males with larger flanges and symmetrical faces and to choose to look at flanged males. However, none of our expectations were confirmed: individuals had no clear attentional biases in the dot-probe paradigm and chose to look at flanged and unflanged males equally often in the choice task. Furthermore, the pattern in which they selected flanged and unflanged stimuli seemed random, as I did not find evidence for temporal clustering. However, I did find evidence for energy efficiency trade-offs and a colour bias in the preference task. The findings of this chapter cast doubt on the suitability of extrinsically rewarded touchscreen experiments to test partner preferences of zoo-housed orang-utans.In Chapter 7, I employed non-invasive eye-tracking to investigate immediate and voluntary attention in Bornean orang-utans across two experiments. To this effect, I presented individuals (N = 4) with two stimuli that were presented simultaneously for four seconds. In Experiment 1, these presentations always consisted of one flanged and one unflanged male. In Experiment 2, I also presented pairs consisting of two flanged or two unflanged stimuli. Across the two experiments, I found that orang-utans (1) were more likely to fixate first on stimuli of flanged males, and (2) spent a larger proportion of time looking at flanged males. Furthermore, I found some indications of individual preferences in Experiment 2. Taken together, our results suggest that orang-utan attention is attuned toward flanged males, which suggests that sexual selection shaped attentional processes in orang-utans. However, our results should mainly be considered at the proximate level, as I cannot explain why orang-utans Tom Roth.indd 202 08-01-2024 10:42