Page 159 - Demo

Eye-tracking reveals bias to flanges in orang-utans1577(Petersen & Higham, 2020). Some primate species, such as orang-utans (Pongospp.), exhibit extreme forms of sexual dimorphism that are thought to be the result of both intense male-male competition and female choice (Utami Atmoko et al., 2008). In addition, these arboreal great apes are characterised by male bimaturism, meaning that there are two distinct types of male morphs: unflanged males and flanged males (Utami et al., 2002). Unflanged males experience arrested development. They are sexually mature and produce offspring but are not preferred mates for adult females (Knott et al., 2009; Kunz et al., 2022). However, flanged males have fully developed secondary sexual characteristics such as an extremely large body size, long hair, a large throat sac, and conspicuous flanges on the sides of their faces (Kunz et al., 2022). Furthermore, ovulating females prefer mating with flanged males (Knott et al., 2009). Female choice for flanged males potentially reflects selection for good genes because the transition from unflanged to flanged males is energetically costly (Knott, 2009). Moreover, flanged males exhibit mutual aggression, and fights between flanged males can lead to serious bodily harm. Thus, by mating with flanged males, orang-utan females may select for males of good genetic quality.In this study, we investigated whether Bornean orang-utans have an immediate and sustained attentional bias towards flanged males. In the first experiment, we presented unique paired portraits of one flanged male and one unflanged male. If flanges were indeed a source of information for orang-utans, we expected the participating orang-utans to (i) gaze immediately at the portrait of the flanged male and (ii) spend relatively more time looking at the flanged male within each trial. In our second experiment, we extended our first experiment by presenting naturalistic stimuli (i.e., different perspectives and natural poses, including other parts of the body in addition to the face) of four different male orang-utans (two flanged, two unflanged) in all possible combinations. This allowed us to investigate whether the results from Experiment 1, with more controlled stimuli, would hold for more naturalistic stimuli. Furthermore, it allowed us to explore how orang-utans divide their attention when presented with two stimuli of males of the same morph and whether they have looking biases for specific individuals over others. Tom Roth.indd 157 08-01-2024 10:41