LAUSR

11 12 Territorial contests often occur in the presence of conspecifics not directly involved in the 13 interaction. Actors may alter their behaviour in the presence of this audience, an ‘audience 14 effect’, and audiences themselves may alter their behaviour as a result of observing an 15 interaction, a ‘bystander effect’. Previous work has documented these effects by looking at 16 each in isolation, but to our knowledge, none has investigated their interaction; something 17 that is more likely to represent a realistic scenario for species where individuals aggregate 18 spatially. We therefore have a somewhat limited understanding of the extent and direction of 19 these potentially complex indirect social effects on behaviour. Here we examined how 20 audience and bystander effects work in tandem to modify resident male aggressive behaviour 21 towards intruders in European fiddler crabs, Afruca tangeri. We found that male crabs with 22 an audience showed greater aggressive behaviour towards an intruder compared to males 23 without an audience, but only if they had acted as a bystander to an aggressive signalling 24 interaction prior to the intrusion. Indeed, bystanding during aggressive interactions elevated 25