Bystander fallow deer engage in third-party behaviour based on similarities in contestant resource-holding potential

Competitive dyadic interactions frequently attract the attention of bystanders that follow and disrupt the ongoing contest. The motivation for bystanders to engage in such third-party behaviour is considered by several different theoretical approaches; one such model argues that bystanders intervene to prevent subordinates from gaining a winner effect, thereby becoming a threat to the individual's dominant status within the group. This study investigates the role of contestant morphology as a motivating factor in initiating third-party behaviour. We investigated whether two correlates of resource-holding potential, prerut body weight and antler weight, were associated with third-party behaviour in rutting male fallow deer, Dama dama. In addition, we examined whether a third factor, reproductive effort (weight loss during the rut) was associated with third-party behaviour. There was a positive relationship between antler weight and the number of times individual males experienced third-party behaviour during contests. This relationship was not evident for either prerut weight or weight loss. An analysis of third-party behaviour showed that as the difference in both prerut and antler weight between the competing males declined, there was an increase in the probability that the dyad would experience third-party behaviour. However, there was no such association between the difference in weight loss between contestants and third-party behaviour. Thus, dyads of males closely matched in terms of individual quality were likely to experience third-party behaviour. We show for the first time that morphological traits associated with resource-holding potential are important cues used by rivals as a basis for engaging in third-party behaviour.