The Thinking Skills of Two Gibbon Species: Representation, Reasoning & Reflection in Hoolock leuconedys and Nomascus leucogenys

Abstract

To reconstruct the evolutionary origins of our thinking mind, we can look to our closest living relatives: the primates. By virtue of the comparative method, primates can help us explore when, why and how our thinking skills evolved. As a multidimensional phenomenon, thinking involves three major components: representation, reasoning and reflection. In investigating these components in primates, many researchers have noted that, relative to monkeys, great ape skills seem particularly advanced. Yet, whether stronger thinking skills are only shared by great apes or are in fact found within all apes, is currently unknown due to our severely limited knowledge of gibbon cognition. This thesis aimed to increase this knowledge by examining the cognitive implications of the gibbon context, and by investigating five specific sub-components of their representation, reasoning and reflection abilities: object permanence, inferential reasoning, inhibitory control, behavioural flexibility and metacognition.

In testing the representational abilities of gibbons, Piagetian object displacement tasks were presented in a vertical format. The gibbons solved single visible, single invisible and double invisible displacements, and passed both drop-first and drop-last controls, suggesting their object permanence abilities extend to stage 6b, like those of great apes.

To investigate their reasoning abilities, the cups task was used. When shown visual information about where a reward was not hidden, the gibbons were able to infer from the reward's absence in one location to its presence in another. This suggests that gibbons can reason by exclusion. They could not, however, use auditory information to find a hidden reward, unlike great apes.

The gibbons' capacity for behavioural self-reflection was examined using four established paradigms from the comparative literature for testing response inhibition. In general, the gibbons performed towards the middle-to-low end of the primate range, suggesting that they may struggle with regulating their reaching responses. Yet, on a repeated innovation task, the gibbons were able to flexibly adapt their behaviour to changed task demands. Indeed, many subjects solved all four puzzles, with persistence, motivation, and efficiency significant predictors of problem-solving success.

Finally, the gibbons' capacity for cognitive reflection was assessed using a metacognitive information-seeking task. The gibbons adaptively sought information when they were unaware of the location of hidden food, but not when they had already witnessed its location. Importantly, this differential information-seeking pattern could not easily be explained by a hedonic response, response competition, or a generalised search strategy. There was also some evidence that the gibbons could use knowledge they had derived inferentially (not perceptually) to moderate their looking behaviour -- an ability that has been demonstrated in great apes but no other primates.

When integrated with the existing literature, the findings from this research suggest gibbons may share some enhanced thinking skills with great apes, but exhibit all traits to a lesser extent. Based on this interpretation, a preliminary reconstruction of the evolutionary origins of ape thinking skills was forwarded.

Keywords: comparative cognition, object permanence, inferential reasoning, inhibitory control, behavioural flexibility, metacognition, gibbons Show more