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This is the fifth story in a series of articles on frugivory and seed dispersal, leading up to the FSD 2020 symposium in March next year.

How animals and humans use space and navigate their environment has always been an intriguing topic of research for behavioural ecologists. Memory is an essential cognitive process which allows an organism to store information about its environment for later retrieval, such as finding its way back to a fruiting tree in the middle of a dense forest. This ability to store and retrieve information is critical; survival is dependant on one’s ability to find the best routes when escaping from a predator or the memory of where your mate is. 

Brain evolution, and thus memory, in birds and mammals, particularly primates, has been attributed to the requirement of its members to adjust to coordinated, cohesive social groups which require novel social behaviours and specialised cognitive skills. That is, identifying one’s friend or foe in a large network of individuals requires a big large brain. This idea has been coined as the social brain hypothesis. However, other studies have posed that the evolution of brain size and cognitive skills in primates have been facilitated not only by sociality but also by foraging in a complex environment, e.g. searching for fruits that are often patchily distributed. This so-called ecological brain hypothesis proposes that the necessity of fine navigational efficiency through uncertain environments is what has forced large brains.

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A juvenile Lar Gibbon feeding on the fruits of Ficus altissima. The males and females are not sexually dimorphic in colouration. Their coat colour varies from black or dark brown to pale yellow. Their hands and feet are covered in white fur. Photograph: Kulpat Saralamba

Lar Gibbons or White-handed Gibbons (Hylobates lar) are near-threatened primates found in tropical forests of southeast Asia. They live in small family groups, mostly comprising of adult pairs and their offspring. Asensio  and his colleagues studied foraging in 11 troops of free-ranging Lar Gibbons to examine whether these primates possess a cognitive map of locations of fruiting trees. Each group was followed for five consecutive days, and the authors mapped all the trees and woody climbers in which these primates stopped to eat. Simultaneously, all the trees and climbers that were not visited but were within the home range of the gibbons were also mapped. Likewise, all large fig trees, a favourite of the gibbons, with or without fruits, were mapped for the study. They used 'change-point' tests to determine whether preferred trees and other landmarks resulted in any change in directionality of gibbon movement.

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An adult female Lar Gibbon, with an infant, on a Ficus pubilimba. Also called White-handed Gibbons, they occur in the tropical forests of southeast Asia. Photograph: Kulpat Saralamba

The gibbons were confirmed to be highly frugivorous, they fed on fruits about 90% of the time. Other minor items included flowers and leaves that were eaten in small quantities. About 94% of the time, the changes in movement direction were associated with feeding and most of the travel change-points were associated with preferred food trees and woody climbers that were out of sight. Other changes in travel direction were associated with intergroup encounters, other minor feeding sources or potential feeding locations. Thus, foraging behaviour determined the gibbon movement rather than social stimuli. This study supports that gibbons use a cognitive map of their home range that allows them to reach out-of-sight fruiting trees efficiently.

An adult female Lar Gibbon feeding on the fruits of Garcinia benthamii. Video: Kulpat Saralamba

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A Lar Gibbon 'brachiating' through the canopy of a fruiting tree. Brachiation is a form of locomotion, used predominantly by species of primates, like gibbons. Typically, these primates use their long arms to swing from one branch to another. Photograph: Kulpat Saralamba

Although other factors such as sensory cues (visual, auditory or olfactory cues) were not taken into account for the study, the authors explain that these factors were unlikely to have influenced travel decisions, given the long distances between consecutive change-point locations. The authors suggest that the observed pattern of travel indicates a mental representation of resource locations in large-scale spaces. This travel efficiency between preferred food trees suggests that the gibbons plan their daily travel routes based on some prior knowledge of resource availability over a large area. The gibbons also strategised their travel to monitor the fruiting of their preferred food trees. The authors suggested this based on the fact that the travel change-points were sometimes preferred food trees or large fig trees which were not fruiting at the time.

So, have you wondered how one fine day, out of the blue, a troop of monkeys found their way to the mango or guava tree in your backyard?

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An adult female Pileated Gibbon (Hylobates pileatus) feeding on the fruits of Ficus variegata. Also known as Capped or Crowned Gibbons, they occur in the tropical forests of southeast Asia. In a part of their range, their distribution overlaps with that of Lar Gibbons e.g. in Khao Yai National Park, Thailand. Photograph: Kulpat Saralamba

References:

Asensio, N., Brockelman, W. Y., Malaivijitnond, S., & Reichard, U. H. (2011). Gibbon travel paths are goal oriented. Animal Cognition, 14(3), 395-405. 

DeCasien, A. R., Williams, S. A., & Higham, J. P. (2017). Primate brain size is predicted by diet but not sociality. Nature ecology & evolution, 1(5), 0112.

Dunbar, R. I. M., & Shultz, S. (2017). Why are there so many explanations for primate brain evolution?. Philosophical Transactions of the Royal Society B: Biological Sciences, 372(1727), 20160244.