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Chapter 7164formulas for phi, coi, and zoi. For all variance parameters, we kept the default Student’s t priors with 3 degrees of freedom. Furthermore, we kept the default logistic priors for the Intercepts of zoi and coi, and default Student’s t prior with 3 degrees of freedom for the Intercept of phi.After running the models, we used the emmeans-package (Lenth, 2023) to integrate the different model components and to provide estimates based on the posterior predictive distribution. Using these values, we calculated multiple quantitative measures to describe the effects (see “First fixation”).Ethics statementThis study employed only non-invasive methods, and animals were not harmed or punished in any way during the study. Participation was voluntary, animals were tested in a social setting, and animals were never deprived of food or water. The care and housing of the orang-utans adhered to the guidelines of the EAZA Exsitu Program (EEP). Furthermore, our research complied with the ASAB guidelines (ASAB Ethical Committee/ABS Animal Care Committee, 2023), was carried out in accordance with the national regulations, and was approved by the zoological management of Apenheul Primate Park (Apeldoorn, The Netherlands).ResultsExperiment 1First fixationThe outcomes of the Bayesian binary logistic regression (Supplementary Table 1; see Appendix G for model stability checks) suggested that the orang-utans had a first fixation bias towards flanged male stimuli (bIntercept = 0.599 [0.033], 89% CrI [0.546; 0.651], pd = 0.998). However, this was not the case for all individuals (Figure 1A): Amos (bIntercept = 0.690 [0.066], 89% CrI [0.586; 0.787], pd = 0.999) and Sandy (bIntercept = 0.641 [0.059], 89% CrI [0.548; 0.733], pd = 0.994) showed a robust bias towards flanged male stimuli, while this was not the case for Samboja (bIntercept= 0.512 [0.073], 89% CrI [0.403; 0.627], pd = 0.565) and Wattana (bIntercept = 0.557 [0.064], 89% CrI [0.450; 0.650], pd = 0.800). Furthermore, we found that the orangutans had a leftward bias: they were more likely to first fixate on the flanged male stimulus if it was on the left side of the screen (bleft-right = 0.332 [0.064], 89% CrI [0.226; 0.433], pd = 1.00).Tom Roth.indd 164 08-01-2024 10:41