Date of Award


Degree Name

Doctor of Philosophy



First Advisor

Welch, Paul

Second Advisor

Pruetz, Jill


Cognitive research is the study of mental processes that guide behaviors and decision making in animals and humans. By studying the cognitive attributes of species within a taxon, as in the case of the order Primates, one can identify behavioral and cognitive components characteristic of certain taxa, such as families, species, or perhaps individuals. This allows us to estimate when and where these cognitive abilities arose in evolutionary history and to hypothesize about their cause and evolutionary origins. Episodic memory or episodic cognition gives an organism the ability to recall declarative forms of memories, which allows one to recall past experiences/histories and can then be used to teach, learn, or even promote socialization in many species (Dunbar, 2011). Additionally, it involves how that organism applies such memories to the past and future, thus allowing the organism to project “oneself” into time and space (Clayton, 2017). This is known as mental time travel or future-oriented memory. Scholars maintain that Homo sapiens (e.g. humans) have the most advanced degree of episodic memory (Suddendorf & Corballis, 2007; Tulving, 1985). However, when and how episodic and future-oriented memory evolved in the primate lineage, and if nonhuman and human episodic memory are comparable, is still unknown. Studying the foundation of this complex form of cognitive memory in a basal group of extant hominoids (apes), allows us to hypothesize about the phylogenetic, social, and perhaps objective mechanisms that were necessary for this complex cognitive system to evolve and persist throughout the primate lineage. Our current understanding of episodic memory is ambiguous, concerning what parts of the brain are truly utilized in this system and to what degree of episodic/future-oriented memory abilities are found throughout the animal kingdom. The current understanding is that very few taxa genuinely exhibit episodic memory capabilities (Allen & Fortin, 2013; Crystal & Suddendorf, 2019). The purpose of this research was to examine episodic/future-oriented memory in nonhuman primates, specifically in hylobatids (Family Hylobatidae), also known as small Asian apes, the gibbons, and siamangs. Hylobatids are small bodied, arboreal apes native to the tropical forests of Southeast Asia (Fleagle, 1976). Although apes, hylobatids have very different social, physical, and ecological distinctions from their larger-bodied cousins, the great apes (Reichard et al., 2016). These differences have often been used as a justification as to why hylobatid behavior and cognition have been postulated by some as “inferior” to the larger bodied great apes (Russon, 2004b; Tomasello & Call, 1997). However, without scientific evidence to back up this claim, we are still very unaware of the actual cognitive capabilities of these very cryptic small apes. In total, 12 hylobatid subjects were included in this study. Hylobates, Hoolock, Nomascus, and Symphalangus were used as representatives to investigate the episodic/future-oriented memory system of hylobatids by participating in a series of noninvasive, interactive, digital cognitive tasks presented on touchscreen technology and digital testing software. Subjects varied in age, sex, and life-histories and consisted of five different species, Symphalangus syndactylus, Hylobates pileatus, Hylobates mooloch, Hoolock leuconedys, and Nomascus leucongenys, within the four genera that make up the family Hylobatidae. To date, this is one of the largest and extensive cognitive studies done with hylobatids, and to my knowledge the only study investigating components of their episodic/future-oriented memory. The cognitive tasks employed in this study were a digitally based Paired Associates Learning Task (PAL) and Future-Oriented Memory Task (FOM). The PAL task was created by Cambridge Cognition, designed to assess acute visual episodic memory, and was employed using their Monkey CANTAB software (Cambridge Cognition). The FOM task was chosen to evaluate multiple aspects of episodic/future-oriented memory, including the “what, when, and where” paradigm, and assessment of executive function and anticipatory behavior of future-oriented memory associated with the episodic memory system. The digital portion of the FOM task was commissioned and designed by an independent software developer and was created using Python and Tkinter tool kit. Only one of the subjects (Jagger) chose to participate and complete the training procedure of the PAL task but did not meet the testing threshold to move on to the data collection portion of the task. All the other subjects attempted to participate in the PAL task but lost interest and failed to participate (not interacting with the touchscreen when PAL task was presented) after approximately ~<4 training sessions. Conversely, all subjects chose to participate in the FOM task in which three of the twelve participants performed above chance (>50%) in the task. With three subjects performing well above the predetermined threshold of >50% in the FOM task, which utilizes higher cognitive control including components of episodic/future-oriented memory, it can be established that hylobatids most likely possess the same cognitive hardware regarding complex cognitive memory as those of other extant apes. Thus, this study provides further evidence that complex cognition, including advanced episode-like/future-oriented memory capabilities, were present in the last common ancestor of all apes. Furthermore, subjects’ performances were statistically analyzed to see if age, sex, species, and or life history (LH) had an impact on task performance. There was no statistical significance between age, sex, species and/or LH for the individuals in this study, but this is most likely due to the smaller sample size and/or “uneven” number of individuals occupying each category. This is common in nonhuman primate cognition studies with ape subjects due to their endangered species status and scarcity in captive settings supporting cognitive research. The knowledge obtained by this research is significant to many different biological realms. In one aspect, this information strengthens our understanding of the cognitive traits the basal ancestor of all apes may have exhibited, therefore contributing to our understating of human cognitive origin. Furthermore, by understanding the cognitive attributes of a different species, we can further investigate their behavior and “species-specific” intelligence. For example, in-situ animal rehabilitation and reintroduction programs can use the results of this research to help maintain semi-captive populations while promoting educational programs, which can prevent human-wildlife conflicts. Additionally, both in-situ and ex-situ organizations/programs such as zoos, sanctuaries, laboratories, and rehabilitation centers can use the same information to design educational programs to promote animal welfare and conservation to the public, while simultaneously establishing new husbandry and enrichment protocols, which will promote progressive animal welfare (Cronin et al., 2017; Whitehouse et al., 2013). Note: The term hylobatid(s), gibbon(s), siamang(s), and small ape(s) are used in unanimity throughout this dissertation. All usage of such terms refers to an animal(s) that belongs to primate the family Hylobatidae.




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