Western Gorilla Social Structure and Inter-Group Dynamics

Western Gorilla Social Structure and Inter-Group Dynamics Robin Morrison Darwin College This dissertation is submitted for the degree of Doctor of Philosophy March 2019 Declaration of Originality This dissertation is the result of my own work and includes nothing which is the outcome of work done in collaboration except as declared in the Preface and specified in the text. It is not substantially the same as any that I have submitted, or, is being concurrently submitted for a degree or diploma or other qualification at the University of Cambridge or any other University or similar institution except as declared in the Preface and specified in the text. I further state that no substantial part of my dissertation has already been submitted, or, is being concurrently submitted for any such degree, diploma or other qualification at the University of Cambridge or any other University or similar institution except as declared in the Preface and specified in the text. Statement of Length The word count of this dissertation is 44,718 words excluding appendices and references. It does not exceed the prescribed word limit for the Archaeology and Anthropology Degree Committee. II Western Gorilla Social Structure and Inter-Group Dynamics Robin Morrison 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 III 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. IV Acknowledgements Firstly, thank you to Peter Walsh, who took me on when I had little more than an undergraduate degree and a whole lot of enthusiasm and guided me into the world of gorilla research. Thank you for your honesty, pushing me to think bigger, write better and always question the status quo, and for your constant support and advice, even after it was no longer your job. Your surprise singing and comedy moments are greatly missed around the halls of Cambridge BioAnth. And of course, thank you for keeping me calm after my dramatic scorpion incident and only tweeting about it later. I also have Magdalena Bermejo greatly to thank for my initiation into gorilla research. You taught me so much about gorilla behaviour, habituation and life in the field. The experiences I had working as your research assistant have undoubtedly shaped so much of my research and will go on to do so. The opportunity to work together again throughout my PhD was a real privilege. To me, Ngaga Camp will always be my home in Congo and Pluton group remain the finest gorilla group I will ever meet. A huge thank you must go to Jacob Dunn who adopted me as his supervisee and was a constant source of sage advice and reliable support. Your encouragement and positivity, especially over the later stages of my PhD has been indispensible. Robert Attenborough also deserves a great deal of thanks for his support throughout my PhD. Helping find the funding that enabled me to take up my place on the programme, providing advice on literature to read and always knowing who to call to find the answers. And of course, for all the sandwiches. So many people within BioAnth at Cambridge have helped throughout my PhD. In particular the wonderful Jo Osborn for helping with all of my gorilla poo related problems, my many risk assessments and advice on lab work. Also for checking I was still alive every week when on fieldwork. Another big thank you goes to Sylvia Hogg, custodian of the Henry Wellcome Building who not only looks after the building but also all of us working in it. Thank you for your kind words and constant supply of biscuits. And thank you to Fabio Lahr for helping with my many IT related needs and last minute laptop emergencies. Huge thanks go to all the V brilliant BioAnth PhD students I’ve shared my time here with, for your friendship, moral support and general hilarity. I am extremely grateful to the many wonderful people at Ngaga Research Site that worked with me during my fieldwork there. To Magdalena Bermejo and Germán Illera, that incorporated me into their team and taught me so much about camera trapping and gorillas. I will now forever be highly skilled with an electric screwdriver after my extensive lessons on correct camera trap installation from Germán. To the incredibly talented trackers Lepale Grace, Okoko Zepherin, and Okele Gabin who allowed me to accompany them around the forest and collected so much of the data vital to this PhD, and to all those others involved in this data collection. Finally, a big thank you to the guides of Ngaga, particularly Alon Cassidy, Teske Erasmus and Andreas Boussery who provided such great entertainment and friendship after a hard day in the field. I am hugely indebted to Terry Brncic and Eric Arnhem for enabling me to briefly join the research team at Nouabalé-Ndoki National Park, and experience the magnificent Mbeli Bai. A massive thank you to Milou Groenenberg for teaching me the ways of Mbeli research, from gorilla IDing tips, to the best recipe for macaroni cheese on a camping stove without access to any actual cheese. The few weeks we spent together camping on the platform may have been the most I have ever learnt in such a short space of time. Thank you for so patiently explaining all of your data collection protocols to me, enabling me to incorporate the Mbeli data within my PhD. A massive thank you goes to the brilliant research assistants Jess Bahamboula and Aristide Mouele with whom I shared so much during my time at Mbeli, for dealing with my terrible French and always remaining so positive and motivated. Also for remembering to bring the dinner when we had to run away from poachers. And to Claudia Stephan for hitting the ground running on her arrival, whipping us all into shape, and Mbeli camp with it. I am excited to see the amazing research produced from Mbeli under your leadership. A big thank you to the Mbeli Bai guiding and camp team: Issa Dogo, Makanya Bernard, Lamba Jean, Ndombi Jean, Loya, Jean-Marie and Nestor Massimbo.

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