Exaggerated rostra as weapons and the competitive assessment strategy of male giraffe weevils

During agonistic contests, males may assess the relative differences between their own and their opponent’s resource-holding potential or simply persist in a contest according to an internal threshold. Game theory models such as the sequential assessment model, energetic war of attrition, and the cumulative assessment model were developed to place assessment strategy in an evolutionary framework. Distinguishing between these models requires one to document contest structure and test predictions of the relationship between the size of the contestants and contest duration and probability of escalation. This study focused on the escalating fighting behavior and competitive assessment strategy of the New Zealand giraffe weevil (Lasiorhynchus barbicornis). Aggressive fighting occurred frequently between males of all sizes, and body length (which included rostrum length) was an important predictor of contest outcome. Contest duration was negatively related to body length difference and the size of the winning male and showed a nonsignificant positive relationship with loser size. The probability of fight escalation showed a significant negative relationship with body length difference, a positive relationship with loser size, and a nonsignificant negative relationship with winner size. Contests were also found to escalate, but not de-escalate, and behavioral matching did not occur during all phases. Male behavior during contests was therefore most consistent with mutual assessment under the sequential assessment model.