Variation in Gorilla Behaviour - and Culture?
Culture is a large part of what makes us human. However, we are not the only species that exhibits culture. Culture in non-human animals, defined as "group-typical behavioural patterns shared by community members that to some degree are reliant on socially learned and transmitted information" (Laland & Hoppitt 2003, Laland & Janik 2006), has sparked much interest among scientists, particularly because of the implications for understanding the origins of culture in humans (Laland & Janik 2006, Boesch 2003, Dean et al. 2014). Cultural traits in animals span the domains of diet, foraging techniques, tool use, and social interactions. Cultural social interactions may include "social conventions", which are defined as dyadic social behaviours or communicative behaviours which are unique to particular groups or cliques (Perry et al. 2006, Leca et al. 2010, Nakamura et al. 2000).
One can argue that a trait is cultural a) if it is customary (performed by most individuals of a particular age/sex class) or habitual (performed by several individuals of a particular age/sex class) in at least one site but absent in at least one site, b) if ecological and genetic explanations can be excluded as the explanation and c) if innovation and/or social learning can be inferred.
Among the great apes, the least amount of evidence for culture and social learning is available for gorillas (Byrne 2007, Whiten 2011). To consider if gorillas have cultural traits we started by looking at the behaviour of gorillas in different locations in the wild. We listed potential cultural traits in wild gorillas from five sites by examining variation in the occurrence of behavioural traits that could potentially be influenced by social learning and are not due to ecological or genetic variation.
Three groups of western gorillas (Gorilla gorilla gorilla) were observed at Bai Hokou, Dzanga-Ndoki National Park, southwestern Central African Republic, three groups were observed at the Mondika Research Center which straddles the border of the Central African Republic and the Republic of Congo, and one group was observed in Moukalaba-Doudou National Park, Gabon. Regarding eastern gorillas (Gorilla beringei beringei), three groups of mountain gorillas at the Karisoke Research Center, in the Volcanoes National Park, Rwanda were observed and one group in Bwindi Impenetrable National Park, Uganda.
Of the 41 behaviours considered, 23 met the criteria of potential cultural traits, of which one was foraging related, nine were environment related, seven involved social interactions, five were gestures, and one was communication related. The remaining 18 traits could not be considered as potential cultural traits, largely because they occurred to some level at all sites.
We observed variation in occurrence of behaviours among gorillas at the five field sites. The strong correlation between the behavioural dissimilarity and geographic distance indicates that a genetic influence cannot be ruled out as affecting the occurrence of the behavioural traits among the populations, particularly between the mountain gorillas and western gorillas, but it does not exclude the possibility of social learning. However the low similarity score between two western gorilla sites that are far from each other (Bai Hokou and Moukalaba), indicating that they have high similarity in the occurrence of traits, in comparison to Bai Hokou and Mondika, which are only 60 km apart from each other, would argue against genetic influence.
We observed variation in the occurrence of half of the potential cultural traits between the two mountain gorilla sites, among the three western gorilla sites, and among all five sites. Half of the behavioural variants reflected differences between western gorillas and mountain gorillas, which are different species. The other half reflect differences within the mountain gorilla subspecies and the western gorilla subspecies. Despite the difficulty of removing the possibility of genetic influences on the occurrence of traits (Langergraber et al. 2011, Krützen et al. 2011), our results are consistent with evidence of potential cultural traits in both species of gorillas.
Several gestures and social traits were observed in one population of mountain gorillas, but not in the other (nor in the three western gorilla populations). This offers some of the strongest support for behavioural variants being cultural since these traits are by nature social (less likely to be environmentally influenced) and these two populations have been isolated from one another only relatively recently. Genty et al. (2009) suggest that a large majority of gorilla gestures are part of a species typical repertoire, but that their use may be based on contextual learning because they are used in a highly flexible manner; this could include the lack of using some gestures in some locations. This interpretation of gestural communication does not preclude the possibility that their use can be socially learned and transmitted and therefore be considered cultural rather than ecologically or genetically driven.
To further examine culture in gorillas, future studies should systematically record the occurrence of particular behaviours and search for variation among groups and possible routes of social transmission.
Summary of an article by Martha M. Robbins and co-authors
Original article: Robbins, M. M., Ando, C., Fawcett, K. A., Grueter, C. C., Hedwig, D., Iwata, Y. et al. (2016) Behavioral Variation in Gorillas: Evidence of Potential Cultural Traits. PLoS ONE 11(9): e0160483
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