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General discussion2099functions. However, I want to mention three caveats. First, the eye-tracking results cannot be considered evidence for an attentional bias that is driven by positive evaluation. It could also be the result of vigilance towards flanged males, because they can be a threat to both males, with whom they may compete, and females and their offspring. Therefore, future studies should further investigate this, for example by using physiological measures in combination with experiments (see “Future directions”). Second, it is possible that orang-utan females will especially employ an attraction-filter during ovulation. In Chapter 8, I have described two cases that suggest increased sensitivity for flanged male signals, visual or auditory, during ovulation. This idea is also in line with behavioural evidence from wild Bornean orang-utan females, who selectively mate with flanged males only during their ovulation (Knott et al., 2009). Third, the idea of an attraction-filter does not take into account potential socio-ecological constraints, even though such constraints are essential in shaping female sexual behaviour. In the case of Bornean orang-utans, populations consist of a relatively large proportion of flanged males, many of which are semi-nomadic (Kunz et al., 2022). Furthermore, the dominance hierarchy between flanged males is often rather unstable (Spillmann et al., 2017). This results in a situation where females can be confronted with a large number of different males, none of which achieves absolute local dominance, and some of which may be infanticidal. Consequently, it may not be adaptive for female Bornean orang-utans to rely purely on an attraction-filter mechanism for mate selection. Instead, females need to strategically spread the probability of paternity across multiple flanged males, to decrease risk of infanticide (Kunz et al., 2022; Palombit, 2015). Still, employing such a sexual counterstrategy against infanticide does not fully rule out an attractionfilter. Because orang-utan females are characterised by concealed ovulation, they might still exert female preference by mating with their most preferred flanged male around ovulation. This would imply that their attraction-filter is activated specifically during their peri-ovulatory period, while outside of this period, females do not discriminate based on attractive traits as much. Importantly, this would allow females to ensure a mate with good genetic quality during their most fertile period, while at the same time reducing infanticide risk by spreading the probability of paternity.The above-mentioned points are summarised in Figure 2. First, the attraction-filter of Bornean orang-utans encompasses both visual and auditory attractiveness. Second, the sensitivity of this filter is strongly modulated by the Tom Roth.indd 209 08-01-2024 10:42