Decision making by third-party rivals is associated with antler structure in the fallow deer

Bystanders often monitor opponents engaged in dyadic contests and use the information gathered to guide their future behaviour towards the combatants. On occasion, bystanders intervene in contests before they have been resolved; however, the reasons why bystanders engage in this form of third-party behaviour have received relatively little attention. Theory suggests that bystanders disrupt ongoing fights to prevent potential rivals from becoming a threat to their dominance status via a winner-effect process. Thus, third-party bystanders should act against fighting dyads in relation to the higher-quality contestant. Various structural features of the primary weapons (antlers) carried by fallow deer, Dama dama , are correlated with dominance; therefore, whether developmental (fluctuating) asymmetry, antler size and antler damage were associated with the likelihood and number of occasions that the bearer experienced third-party behaviour were investigated in this study. Results indicated that individuals with damaged antlers likely experience third-party behaviour, while antler symmetry and size are correlated with third-party behaviour if the bearer also has damaged antlers. Thus, the structural properties of antlers are associated with third-party behaviour consistent with theory that third-party rivals act in relation to the higher-quality member of the fighting dyad. • Third parties often disrupt fights to prevent rivals advancing in the hierarchy. • We tested if antler symmetry, size, and damage predicted third-party intervention. • Third-party behaviour was associated with antler damage. • Damage reflects the fighting style, but third parties respond to vigour. • This finding guides decisions to engage in third-party behaviour.