The effects of decentralisation on foraging and organisation in social insect colonies Zoe Cook PhD University of York Department of Biology November 2013 Abstract Social insect colonies demonstrate some of the most striking social behaviours that are seen in nature. Social interaction in these species defines their behaviour and has a large impact on their success. A number of ant species organise their colony over several socially interacting, but spatially separated nests. This behaviour is known as polydomy. I used simulation modelling to investigate the costs and benefits of this behaviour. The results showed that colony organisation interacts with both foraging strategy and resource distribution to determine the effect of polydomy on foraging success. Importantly, I show that there are previously undiscussed costs to polydomy. I also addressed questions of how the social connections between the nests in a colony are organised. Complex interacting systems such as the trail system formed between nests in a polydomous colony can be represented as a network. I review the use of this representation in studies of social insects and provide suggestions for future studies on how social insect systems may provide insight into the construction and use of effective network systems in general. By representing the trail systems found between nests as networks I show that the systems formed by ants are highly efficient for the transportation of resources but balance this efficiency with the cost of producing trails. A number of simple mechanisms have been proposed for the construction of these networks. I test these theories and show that the simplest mechanisms suggested may not be sufficient to replicate the natural systems. This thesis contributes to the understanding of the behaviour of ants in polydomous colonies and to the understanding of distributed biological systems more generally. The work also provides a basis for future research on how efficient systems can be constructed using simple rules. 2 Contents Abstract ....................................................................................................................... 2 Contents ...................................................................................................................... 3 List of Figures ............................................................................................................. 6 List of Tables .............................................................................................................. 8 List of Accompanying Material .............................................................................. 10 Acknowledgements ................................................................................................... 11 Declaration ................................................................................................................ 12 1 General Introduction .......................................................................................... 13 1.1 Social insects as a model system for behaviour ....................................... 13 1.2 Polydomy ................................................................................................. 15 1.3 Foraging ................................................................................................... 20 1.4 Polydomy and foraging ............................................................................ 25 1.5 Organisation within polydomous colonies .............................................. 26 1.6 Modelling framework .............................................................................. 29 1.7 Thesis structure ........................................................................................ 34 2 Exploration versus exploitation in polydomous ant colonies .......................... 36 2.1 Abstract .................................................................................................... 36 2.2 Introduction .............................................................................................. 36 2.3 Model description .................................................................................... 39 2.4 Results ...................................................................................................... 47 2.5 Discussion ................................................................................................ 50 2.6 Acknowledgements .................................................................................. 54 3 The use of network analysis in the study of social insect colonies .................. 55 3.1 Abstract .................................................................................................... 55 3.2 Introduction .............................................................................................. 55 3.3 Types of networks within social insect colonies ..................................... 57 3 3.4 Constructing network representations ..................................................... 60 3.5 Answering questions with networks ........................................................ 65 3.6 Spatial effects ........................................................................................... 72 3.7 Network Dynamics .................................................................................. 74 3.8 Discussion ................................................................................................ 78 4 Robustness and efficiency in ant transportation networks ............................. 82 4.1 Abstract .................................................................................................... 82 4.2 Introduction .............................................................................................. 82 4.3 Methods ................................................................................................... 85 4.4 Results ...................................................................................................... 90 4.5 Discussion ................................................................................................ 93 4.6 Acknowledgements .................................................................................. 98 5 Mechanisms for the construction of transportation networks in polydomous ant colonies ........................................................................................................... 99 5.1 Abstract .................................................................................................... 99 5.2 Introduction .............................................................................................. 99 5.3 Model description .................................................................................. 101 5.4 Analysis ................................................................................................. 106 5.5 Results .................................................................................................... 110 5.6 Discussion .............................................................................................. 118 6 Discussion ........................................................................................................... 124 6.1 Summary of Chapters ............................................................................ 124 6.2 General comments ................................................................................. 127 6.3 Questions remaining and future work .................................................... 129 7 Appendix A ........................................................................................................ 135 8 Appendix B ......................................................................................................... 136 8.1 Colony Size ............................................................................................ 136 4 8.2 Pheromone decay rate ............................................................................ 136 8.3 Energy of agents .................................................................................... 137 8.4 Individual memory ................................................................................. 138 9 Appendix C ........................................................................................................ 140 10 Appendix D ........................................................................................................ 142 10.1 Efficiency ........................................................................................... 142 10.2 Meshedness ........................................................................................ 142 11 Appendix E ......................................................................................................... 143 11.1 Time and duration of sampling .......................................................... 143 11.2 Edge thresholding............................................................................... 144 12 Appendix F ......................................................................................................... 145 12.1 Mean number of edges within simulations ........................................ 145 12.2 Results of post-hoc analysis ............................................................... 147 Bibliography ........................................................................................................... 150 5 List of Figures Figure 1-1 Photograph of polydomous Formica lugubris colony.