4The social environment is a pervasive influence on the ecological and evolutionary dynamics 5 of animal populations. Recently, social network analysis has provided an increasingly 6 powerful and diverse toolset to enable animal behaviour researchers to quantify the social 7 environment of animals and the impact that it has on ecological and evolutionary processes. 8 However, there is considerable scope for improving these methods further. We outline an 9 approach specifically designed to model the formation of network links, exponential random 10 graph models (ERGMs), which have great potential for modelling animal social structure.
11ERGMs are generative models that treat network topology as a response variable. This 12 makes them ideal for answering questions related directly to how and why social 13 associations or interactions occur, from the modelling of population-level transmission, 14 through within-group behavioural dynamics to social evolutionary processes. We discuss 15 how ERGMs have been used to study animal behaviour previously, and how recent 16 developments in the ERGM framework can increase the scope of their use further. We also 17 highlight the strengths and weaknesses of this approach relative to more conventional 18 methods, and provide some guidance on the situations and research areas in which they can 19 be used appropriately. ERGMs have the potential to be an important part of an animal 20 behaviour researcher's toolkit and fully integrating them into the field should enhance our 21 ability to understand what shapes animal social interactions, and identify the underlying 22 processes that lead to the social structure of animal populations. Randomisation-based analyses have many strengths, especially in animal social 64 network studies in which complex sampling issues often have to be controlled for (Farine & 65 Whitehead, 2015). However, using this approach controls for, rather than models, the 66 biological processes, such as site use, that generate network structure. Often these 67 processes can be directly of interest, yet treating them as a nuisance factor prevents us 68 from more fully understanding the role they play in shaping animal social systems. In this article we review the use of one of the more highly developed and flexible of 86 these statistical network approaches, exponential random graph models (ERGMs) (Lusher et 87 al., 2013;Robins et al., 2007). We start by providing a basic verbal description of the 88 modelling approach, illustrating some of the key aspects of model fitting with a toy 89 example. We then describe the previous uses of these models in the study of animal social 90 behaviour, before going on to discuss its strengths and weaknesses as a method to model 91 animal social networks and how these models can be extended to understand more 92 complex network datasets that are increasingly used to study animal behaviour (temporally 93 dynamic, bipartite and multilayer networks). Finally, we set an agenda for future research: