Contest interactions and outcomes: relative body size and aggression independently predict contest status

In species with pronounced male armaments, body and weapon size often determine success in agonistic interactions. However, the behavioural components of interactions also play a significant role in determining outcomes and individuals that win agonistic interactions may not be the ones that start contests. In this study, we used dyadic, intrasexual assays to characterize agonistic behaviours and determine whether body size and/or the propensity to start interactions influences contest outcome. We characterized agonistic interactions from start to end in the sexually dimorphic forked fungus beetle, Bolitotherus cornutus. Males of the species are known to use their thoracic horns in intrasexual combat, but other aspects of male-male behaviour are undescribed. We created an ethogram that described transitions between behaviours and categorized actions as aggressive, nonaggressive and mounting. Individual aggressive and nonaggressive behaviours were highly repeatable between trials while mounting behaviours were not repeatable. The initiation of nonaggressive and mounting behaviours was not predictive of contest outcome. Relative body size and absolute aggression independently predicted contest outcome. Our results indicate that traits important for establishing contest outcome are not always correlated. Considering either aggression or body size alone may be misleading when determining competitor abilities. Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.