Investigating components of prospective cognition in gibbons (Hylobatidae)

  • Heather Taylor

Student thesis: Doctoral Thesis


Complex prospection, requiring projection of one’s mental self into a subjective future time, has been proposed to be uniquely human, with ongoing debate as to the criteria necessary for assessment of this ability. Focus on this dichotomous framework of future directed thinking, assessing whether non-human primates possess levels of prospection comparable to that displayed by humans, has detracted from assessment of the range of abilities and species specific adaptations found across primates. Current evidence for advanced prospection within non-human primates remains contested; however, the large apes have displayed greater proficiency at tasks requiring prospective cognition than monkeys. This thesis investigates components of prospective cognition within the small apes (Hylobatidae). Gibbons represent the only surviving divergence between large apes and monkeys. As such, assessment of basic prospection and component processes within gibbons allows for a clearer overview of the emergence of prospective abilities across the primate order.

Here, 31 gibbons (Hoolock leuconedys, n = 9, Hylobates moloch, n = 9, Hylobates pileatus, n = 5, Nomascus leucogenys, n = 6, Symphalangus syndactylus, n = 2) were first assessed on their ability to attend to the functionally relevant features of two rakes. One functional and one non-functional rake were presented during a raking in task, requiring selection of a functional rake in order to draw in an out of reach food reward. Pilot testing provided little support for this ability; however, given further testing with rake sets presenting more distinctive perceptual differences, gibbons were found to reliably distinguish between functional and non-functional rakes. Some evidence was found for subjects transferring knowledge across different rake sets, with subjects reaching criterion level performance faster during later experiments.

Once gibbons had learned the necessary skills to select a functional rake for reward retrieval, a series of experiments assessed their capacity for basic prospection. Subjects (H. leuconedys, n = 5, N. leucogenys, n = 3, H. pileatus, n = 3, S. syndactylus, n = 1) were again required to select between one functional and one non-functional rake; however, these rakes were now un-baited. This allowed for assessment of whether gibbons would select functional rakes for future use, with the selected rake being baited at a separate location following a time delay. Subjects reliably selected the functional rake when delays of up to five minutes were imposed between rake selection and reward retrieval. The increasing time delay did not greatly affect subjects’ performance, suggesting gibbons can relate temporally and spatially distinct events, displaying basic prospection.

Finally, a preliminary investigation of self-control capacities within gibbons (H. leuconedys, n = 3, H. pileatus, n = 3, N. leucogenys, n = 2) was conducted. Self-control is an integral feature of much prospective behaviour without which an individual cannot inhibit current desires in favour of future ones. Individual differences were found; however, three gibbons refrained from selecting an immediately attainable small reward, instead selecting a rake functional for retrieval of a larger reward at a second location. Taken together, the current findings provide initial evidence of both basic prospection and self-control within the small apes
Date of Award2019
Original languageEnglish
Awarding Institution
  • Abertay University
SupervisorClare Cunningham (Supervisor) & Scott Hardie (Supervisor)

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