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Chapter 6148dependent measure in cognitive tasks. Instead, more fine-scaled methods such as non-invasive eye-tracking could be considered to study attentional preferences in primates. These methods are relatively easy to implement in primates (Hopper et al., 2021), and provide a more direct measure of attention (Armstrong & Olatunji, 2012). Correspondingly, we did find an immediate attention bias towards flanged males in an eye-tracking task (see Chapter 7). This suggest that eye-tracking allows us to probe cognitive biases that are potentially too subtle to identify using reaction time tasks, at least in orang-utans.In the preference task, we used a previously developed paradigm (Watson et al., 2012) to test whether Bornean orang-utans would choose to be presented with flanged or unflanged stimuli. However, all individuals selected flanged and unflanged stimuli equally often. Our results are in contrast with the results that Watson et al. (2012) found in rhesus macaques, who specifically selected stimuli depicting faces of high-ranking individuals or stimuli showing coloured perinea. While we made some minor adaptations to the original paradigm (longer stimulus presentation, no fixed dot locations to avoid anticipatory responses, no indirect comparison of stimulus categories), we do not consider it likely that these changes explain the null results. One potential explanation relies on the fact that both of the choices were rewarded equally, meaning that there was no incentive to choose one category over the other in principle. Because Bornean orangutans are often confronted with long periods of fruit scarcity (Vogel et al., 2017), they might be especially sensitive to food reward. Potentially, the anticipation of reward during the trial was so salient for them that the means to get to the reward became relatively unimportant. This raises the question whether extrinsically rewarded touchscreen experiments like the one we used here are suitable to study Bornean orang-utan cognition. We also found that individuals had a higher tendency to choose the flanged male stimulus when it was associated with a red-coloured dot instead of the green-coloured dot, despite the fact that the dots were similar in saturation. This preference for red may indicate a general sensory bias towards the colour red, which could be attributed to the evolutionary pressure on primates to select ripe fruits or young leaves (Fernandez & Morris, 2007). This bias for red objects might extend beyond fruits, possibly explaining why the individuals in the study were more likely to select the red dot. However, previous reports present conflicting evidence regarding the colour bias in food preferences among orangutans. While one report suggested a preference for red food in a juvenile orang-utan (Barbiers, Tom Roth.indd 148 08-01-2024 10:41