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Chapter 9208pads on the side of the face. Therefore, I investigated whether Bornean orangutan cognition would be biased towards stimuli depicting males bearing this trait. However, the results are mixed, with no evidence of cognitive attunement towards flanged males in Chapter 6, but evidence of immediate and voluntary attentional bias towards flanged males in Chapter 7. The eye-tracking results from Chapter 7 do indeed suggest that orang-utan cognition is attuned towards flanged males. This corroborates previous work in rhesus macaques, where systematic studies have established links between visual attention and visually conspicuous traits, such as facial masculinity (Rosenfield et al., 2019), facial coloration (Dubuc et al., 2016; Waitt et al., 2003), and facial symmetry (Waitt & Little, 2006). Thus, attentional biases towards conspicuous facial traits that are thought to play a role in mate choice seem not to be restricted to rhesus macaques, but might be more widespread across primate species, including great apes. It is important to note, though, that the results of Chapter 6 do not provide evidence for the notion that orang-utan cognition is biased towards flanged males. However, I will further elaborate on this in the section “Methodological considerations”.Furthermore, Chapter 8 suggests that the first filter of orang-utans might also be sensitive to auditory information. From a functional cognition perspective, this would make sense, as orang-utans are considered semi-solitary (Roth et al., 2020; van Schaik, 1999). Because of their dispersed social system, auditory communication has a pivotal role in inter-individual coordination (Delgado, 2006). Consequently, orang-utan females will often have access to auditory information from males, but not visual information. Therefore, auditory information may have an important role in female orang-utan mate choice. While some evidence for this has been found in Sumatran orang-utans (Mitra Setia & van Schaik, 2007), results for Bornean orang-utans do not support this notion (Mitani, 1985; Spillmann et al., 2010). Nevertheless, this point underlines the need for a biocentric approach to animal cognition (Bräuer et al., 2020). While presenting identical tasks to different species may seem an objective approach to comparative cognition, this approach ignores the fact that each species has its uniquely adapted sensory systems. Therefore, it is crucial to consider the specific sensory modalities and environmental contexts of each species when studying animal cognition.Caveats in interpreting the results of cognitive bias tasksThus, it is possible that orang-utans indeed possess an attraction-filter, and that sexually selective cognition is one of the mechanisms through which this filter Tom Roth.indd 208 08-01-2024 10:42