Bornean orangutans (Pongo pygmaeus) show an attentional bias toward a male secondary sexual trait

Visual attention mechanisms help organisms prioritize evolutionarily relevant stimuli, like threats and mating opportunities. Individuals may, therefore, attend to specific facial features. In humans, it has consistently been shown that secondary sexual traits and attractive faces capture and hold attention. By contrast, evidence for such biases in nonhuman primates, especially great apes, remains scarce. To address this gap, we conducted two eye-tracking experiments with four zoo-housed Bornean orangutans (Pongo pygmaeus), a species characterized by extreme sexual dimorphism. In both experiments, we found that orangutans exhibited an attentional bias toward fully flanged males, a sexually dimorphic trait of some adult males. They not only looked longer at flanged males but were also more likely to immediately fixate on them. This suggests that great ape cognition has been shaped by sexual selection in a similar fashion to humans, where attentional biases toward masculine and attractive faces are well-documented. At the same time, we cannot rule out the possibility that individuals attended more to flanged males due to their potential threat to both sexes. Nevertheless, by demonstrating attentional attunement to a secondary sexual trait, our findings contribute to the growing understanding of how sexually selected features influence cognition in nonhuman primates.