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Title: Western gorilla social structure and inter-group dynamics
Author: Morrison, Robin
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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The study of western gorilla social behaviour has primarily focused on family groups, with research on inter-group interactions usually limited to the interactions of a small number of habituated groups or those taking place in a single location. Key reasons for this are the high investment of time and money required to habituate and monitor many groups simultaneously, and the difficulties of making observations on inter-group social interaction in dense tropical rainforest. However, gorilla groups are known to have extensively overlapping home ranges, show affiliative inter-group interactions and often aggregate at resource hotspots. There is also genetic evidence of kin-biased behaviour between dispersed kin. This is all suggestive of a complex society in which inter-group interactions may follow an underlying multi-level social structure where affiliations are influenced by kinship, social exposure, ranging patterns, territoriality or foraging decisions. This thesis investigates the large scale society of western lowland gorillas, using novel technologies and analytical methods to overcome the considerable difficulties in studying large numbers of gorillas simultaneously. I use biases in movement patterns to investigate the cognitive rules used, and decisions made by this intelligent, social species, to navigate the limited space and resources they share with their neighbours. Using observational data from two forest clearings in the Republic of Congo, I quantify community structure by network modularity analysis and hierarchical clustering, demonstrating the presence of kin-based multi-level social structure in western lowland gorilla. The sizes of these gorilla social units follow a hierarchical scaling pattern similar to that observed in other mammalian multi-level societies including humans. The social structure detected at these forest clearings is consistent with a super-spreader structure, suggesting that clearings may act as important transmission hubs for disease, novel ideas, behaviour or culture. This demonstrates that intervention strategies targeting gorillas with home ranges near to forest clearings, particularly solitary males, may be highly effective for limiting the transmission of certain diseases. Modelling the movement patterns of a gorilla population across their ranges using camera trap data demonstrates that gorilla groups appear to actively avoid one another, both through avoidance of other groups at resource hotspots, and avoidance of areas regularly used by other groups. Gorilla groups visit sites less often the closer they are to another group's home range centre, with groups avoiding larger, more dominant group's home range centres to a greater extent. This, along with the increased avoidance of visiting a location on the same day as another group when close to their home range centre, is highly suggestive of the presence of territorial defence in western gorillas. The findings in this thesis demonstrate the presence of a kin-based multi-level social structure in western gorillas, with considerable similarities to that present in humans, suggesting that a key component of human social complexity may have evolved far earlier than previously asserted. They suggest that the social brain enhancements observed within the hominin lineage were not necessary to enable human multi-level social structure. I show that western gorillas demonstrate biases in their movement patterns consistent with the presence of some broader elements of territoriality, with regions of priority or even exclusive use, close to their home range centres. My findings strongly emphasise the importance of gorillas as a model system for human social evolution. This is due to both the common underlying multi-level social structure and the considerable similarities in inter-group territorial dynamics. In contrast to previous assumptions that interactions between gorilla groups are primarily random or due to aggressive mate competition, I find that these interactions appear to be based around a complex social structure influenced by kinship, territoriality and dominance.
Supervisor: Dunn, Jacob ; Walsh, Peter ; Bermejo, Magdalena Sponsor: Nacey Maggioncalda Foundation ; International Society for Human Ethology ; Sabine Plattner Africa Charities ; University of Cambridge
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Social structure ; multi-level ; hierarchical ; gorillas ; society ; western lowland gorilla ; hominid ; social evolution ; kinship ; disease transmission ; territoriality ; camera trap ; ranging ; movement dynamics