Agonistic interactions in the dragonfly Micrathyria ungulata: does male fighting investment come from an innate ability or an indomitable will?

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In fights between males for access to mating territories, the winner may be the rival with the highest fighting capacity or that places the highest value on the disputed site. However, it is also possible that both factors simultaneously affect dispute settlement. Therefore, a better understanding of contest resolution rules may be achieved by simultaneously investigating how winning chances depend on individual motivation linked to resource value and on individual traits linked to fighting capacity. In this study, we used males of the dragonfly Micrathyria ungulata to evaluate whether individual fighting ability, individual motivation, or the interaction between both factors determine the individual investment in territorial contests. For this, we simulated the approach of an intruder to trigger a territorial defense response by either resident (marked males present in the territory for at least 1 day before the experiment) or substitute males (marked males that occupied the territories after we removed the residents). Resident males fought longer against the potential intruder than their substitute counterparts. However, traits related to fighting ability had a marginal effect on the time invested in fights. These results indicate that resident males are more willing to invest in fights regardless of their physical or physiological attributes. Distinct motivational levels among individuals should increase the variation in mean contest duration among rival pairs. If this effect is widespread, it may have important implications for studies that investigate contest rules by relating fighting duration and individual traits. In many species, the winner of territorial fights is the individual with higher fighting ability or the one that values more the disputed resource and consequently is more motivated to fight. However, because these two factors may interact to determine the winner, investigations should benefit by simultaneously evaluating the effect of fighting ability and motivation on contest resolution. Here, we induced fights between focal males against artificially introduced rivals to test if male investment in fights is affected by an interaction between traits linked to fighting ability and motivational status. We showed that the prior occupation of a territory was the main determinant of the time spent fighting. We conclude that the previous occupation of a territory may increase individual motivation to fight due to higher valuation of the resource and eventually may surpass the effect of fighting ability to determine the winner.