Page 134 - Demo

Chapter 6132Furthermore, for the preference task, we retrospectively decided to explore (i) whether individuals had a colour bias, (ii) whether individuals made choices that might reflect conservation of energy, and (iii) whether individuals showed temporal clustering in their choices. We investigated colour bias because evolutionary theories of colour vision have suggested that the ability to see red co-evolved with frugivory (Bowmaker et al., 1991). With regard to energy conservation, Bornean orang-utans are characterised by extremely low rates of energy use (Pontzer et al., 2010), potentially an adaptation to habitats with long periods of fruit scarcity resulting in negative energy balance (O’Connell et al., 2021; Vogel et al., 2017). Potentially, such energy conservation mechanisms could also influence their responses during the task. Lastly, we also investigated temporal clustering, because flanged males are not only preferred mating partners (Knott et al., 2009), but might also pose a threat (e.g., infanticide: Kunz et al., 2022) or are perceived as threatening (Beaudrot et al., 2009). Consequently, individuals may show temporal clustering in their choices during our task, by either opting for a less arousing picture of an unflanged male after seeing a flanged male stimulus (i.e., more switching, temporal dispersion) or by mostly sampling flanged male stimuli, until arousal reaches a certain threshold and individuals switch to unflanged stimuli instead (i.e., fewer switches, temporal clustering). Thus, because of socio-ecological characteristics, we decided to explore these three topics in addition to our main questions.MethodsSubjects and housingThe animals that participated in this study were part of a population of 9 Bornean orang-utans (Pongo pygmaeus) at Apenheul Primate Park, The Netherlands (Table 1). They were kept in a fission-fusion housing system consisting of 4 enclosures, meaning that they were in small subgroups with changing composition over time, in order to mimic the natural social system of the species. Some individuals never shared enclosures to avoid conflict (e.g., the two adult males). Each enclosure consisted of an inside part and an outside part. The orang-utans were fed multiple times a day, and had ad libitum access to water. Most of the orang-utans had previously been exposed to touchscreens for a previous dot-probe study (Laméris Tom Roth.indd 132 08-01-2024 10:41