Abstract
Intelligence is one of the main features proposed to differ substantially between humans and other animals. Therefore, crucial for understanding human evolution is knowledge of the evolution of human intelligence. Complex social interactions may have selected for advanced cognitive capacities in primates and humans. One hallmark of human intelligence is
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Theory of Mind, the capacity to understand the mental states of others. Although non-human primates may not exhibit full-blown Theory of Mind, they may possess capacities required for or underlying Theory of Mind. One important capacity is Visual Perspective Taking, the ability to understand what other individuals can see. Although this capacity is found in apes, its presence in monkeys is much debated. In this thesis I study whether VPT or its prerequisites are present in monkey species and whether these capacities are employed in their natural behavioural strategies. When addressing primate capacities, I find with a new paradigm evidence for understanding the target of attention, a VPT prerequisite, in long-tailed macaques (Macaca fascicularis). Male macaques showed elevated interest in pictured social scenes that show dominance interactions inconsistent with the actual dominance hierarchy in their group than in those showing the actual hierarchy. Since the only difference between these pictures is the looking direction, indicating the object of attention, we can conclude that male long-tailed macaques understand the object of attention of conspecifics. An experimental test of VPT in long-tailed macaques shows that they understand what a conspecific can see in a competitive situation. Not only do they prefer the food item that is invisible for their competitor when they expect to obtain only one of two food items, they also adopt an alternative strategy of approaching the visible food first when they expect to obtain both food items before their competitor does. Therefore, long-tailed macaques, a monkey species, possess not only a precursor, but also real VPT. The social use of VPT is addressed in sexual behaviour, since monkeys may tactically hide sexual behaviour from group members. Indeed, rhesus (Macaca mulatta) and long-tailed macaques preferentially copulate in absence of bystanders. Next to the alpha male, also other male and female bystanders are avoided. However, there is no indication that the separation of a male and female from potentially disruptive bystanders is coordinated. Moreover, macaques do not hide their sexual behaviour behind opaque screens. Instead, they exploit the peripheral positions of non-alpha males by enlarging their distance from potentially disruptive bystanders before they engage in sexual behaviour. Although macaques tactically adjust their sexual behaviour to the presence of others employing relatively simple cognitive capacities, they do not use VPT to achieve this. In conclusion, monkeys, similar to apes, posses a precursor to Theory of Mind, namely VPT. I show that VPT is used in a relatively simple competitive situation, but not when more individuals are involved during sexual behaviour. Therefore, I find no support for the hypothesis that social complexity enhances the use of this capacity.
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