Do male swordtails assess opponents or rely on intrinsic factors during agonistic contests?

Agonistic encounters are costly in terms of time, energetic resources expended and the risk of injury. Contestants are expected to reduce these costs by assessing their chances of winning and altering their behaviour accordingly. Until recently, the prevailing view of mutual (or opponent) assessment in contests, whereby rivals assess their own resource holding potential (RHP) relative to their opponent, remained unchallenged. The alternative of self- (or own-size) assessment, where individuals rely soley on assessment of their own fighting ability, was overlooked but recently gained some empirical support. We tested the predictions of these alternative modes of assessment behaviour in staged territorial encounters between pairs of male swordtails, Xiphophorus helleri. Males were isolated from social interaction prior to being released simultaneously into a test aquarium. Contests consisted of a combination of displays, includung the species typical S-threat behaviour, and direct fighting, followed by role reinforcement through chasing. We used relationships between measures of individual RHP and the cost of contest behaviour to distinguish between hypotheses and to determine the mechanism underlying contest decisions in swordtails. Initial results suggest opponent assessment occurs during contest between male swordtails.