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