Abstract
Gaze following and the ability to understand that another individual sees something different from oneself are considered important components of human and animal social cognition. In animals, gaze following has been documented in various species, however, the underlying cognitive mechanisms and the evolution of this skill remain to be investigated.
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Therefore, in this thesis I addressed the cognitive mechanisms in a non-human primate and moreover I extended the range of investigated species to one that is phylogenetically more remote from humans, namely fish, in order to study the evolutionary history of this capacity. First, I demonstrated that long-tailed macaques follow the looking direction of a human experimenter. Subjects also ‘check-looked’, i.e. looked back and forth between the experimenter’s face and their gaze direction, which has been proposed to indicate that a subject understands that another individual is looking at a specific stimulus. Moreover, I found that gaze-shifts accompanied by a socially meaningful facial expression (mimicking a social event) elicited stronger gaze-following responses than neutral gaze-shifts. This modulation of gaze following may be due to a specific arousal- or attention-based mechanism or may be based on the subject understanding that the demonstrator is attending to something the subject cannot see. To further address these possibilities I investigated whether monkeys follow a conspecific’s gaze to a hidden location which would indicate a capacity to extrapolate the other’s line of sight to locations outside the field of vision. When a conspecific demonstrator gazed at a mirror hidden behind a barrier, subjects relocated to a position where they could see the mirror location and directed significantly more focussed looks behind the barrier than in a situation where there was no conspicuous gaze cue by the demonstrator. Taken together, these studies suggest that monkeys’ gaze following behaviour is a more flexible and accurate ability than has previously been shown. This capacity may thus be based on an appreciation of the fact that another’s gaze and attention can be directed at something specific. These results resemble findings in chimpanzees that have been associated with the ability to attribute perceptual states to others, which may be generically labelled ‘Theory of Mind’. Addressing the evolution of this ability, I extended the study of gaze following and a related capacity, visual perspective taking, to guppy fish. Research on social cognition has tended to focus on a restricted range of species, principally primates and, more recently, birds and some domesticated animals. However, remarkable similarities on a behavioural level have been documented for non-human primates and fish, behaviours that in primates have been used to assess advanced cognitive capacities. I found some evidence that guppies responded to the body orientation of others (potentially equivalent to gaze direction), but guppies failed to pass a visual perspective taking task. Further studies are required to confirm the present results and to address potential underlying mechanisms in this species. Altogether, this thesis is an example how evolutionary and mechanistic approaches to behaviour can be integrated and can shed new light on non-human animals’ cognitive capacities.
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