Influences on resource-holding potential during dangerous group contests between wood ants

The fighting ability components of ‘resource-holding potential’ (RHP) may vary with body size, weapon size and energetic status. Theory predicts that agonistic signals should vary with RHP and any decision to ‘give up’ should be made by the opponent with lower RHP. In some species, an encounter is only resolved when one opponent is killed or critically injured. We investigated fatal fighting between groups of red wood ants, Formica rufa, and determined whether the probability that a focal group inflicts the first fatality in an encounter is influenced by the same correlates of RHP demonstrated to affect noninjurious fight outcome. In addition, we manipulated group size, an additional factor which is likely to be linked to RHP during intergroup conflicts, to determine whether members of larger groups concentrate attacks on their less numerous opponents. We found that, during most encounters, focal groups that inflicted the first fatality contained individuals of larger mean body mass than groups that received the first fatality. There was no evidence that energetic status directly influenced outcome, although groups that received the first fatality experienced a significant reduction in total body sugar compared to nonfighting controls. Finally, we found no evidence to suggest F. rufa concentrate their attacks in a way that leads to enhanced group RHP.