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General discussion2039specifically attended to flanged males. Their attentional bias might be driven by positive-valenced interest, but could also be the result of vigilance. This is also reflected in the fact that the participating male also had a bias towards flanged male stimuli. Therefore, future studies should further investigate the motivational drives behind this attentional bias.While Chapters 6 and 7 focused on visual processing, Chapter 8 reports the link between auditory signals and sociosexual behaviour of Bornean orangutans. Because orang-utans are characterised by a dispersed social system, vocalisations might be crucial for social organisation and reproductive strategies (Delgado, 2006). I report anecdotal evidence of two zoo-housed Bornean orangutan females which supports this idea. First, I described the behavioural response of Temmy to male long-call playbacks while she was ovulating. She showed a strong tendency to approach the origin of the sound, whereas she did not do so when I played control sounds. Furthermore, she responded by producing vocalisations. Second, I described the case of Eloise, who consistently showed an increase in proceptive behaviour during ovulation for at least over a decade. Similar to Temmy, she produced vocalisations during her ovulation. However, in contrast to Temmy, she proactively produced them. Both the vocalisations produced by Temmy and Eloise seem distinct from previously described orangutan vocalisations. Thus, these observations suggest that the female vocal repertoire may be more extensive than previously thought. While conclusions about the function of these calls remain tentative, they may facilitate coordination between ovulating females and flanged males.Overall, this thesis has shown that (1) sexually selective cognition has rarely been studied in non-human primates, even though suitable test paradigms are available, (2) human initial mate choice is strongly determined by visual attractiveness, and not auditory or olfactory attractiveness, (3) human attentional processes are attuned to visual attractiveness, both when using general ratings and idiosyncratic ratings of attractiveness, (4) Bornean orang-utans have an attentional bias in immediate and voluntary attention towards flanged males in an eye-tracking task, but no attentional biases or choice biases towards male flanges and/or facial symmetry in touchscreen tasks, and (5) Bornean orangutan females can show both a behavioural and vocal response to visual or vocal indicators of flanged male presence during ovulation. Although these results have relevant theoretical and practical implications, methodological considerations and important knowledge gaps remain. This is discussed in the following section.Tom Roth.indd 203 08-01-2024 10:42