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Two mutually-excluding hypotheses explain the intensity of aggression between neighbors and their non-neighbors in territorial animals. On one hand, the 'dear enemy' hypothesis predicts that territorial animals should be more tolerant towards their neighbors than towards non-neighbors. Conversely, the 'nasty neighbor' hypothesis predicts increased aggression towards neighbors than non-neighbors. These different situations depend on who is more likely to be a real competitor, either a neighbor or a non-neighbor, and the intensity of resource competition. Male damselfly of Hetaerina vulnerata defend riverine, mating territories that is the main way to have access to females. These territories are not fixed and so males continuously defend them especially against neighbors. Given this, we tested whether the nasty neighbor principle operates in this species. We monitored a population during an entire mating season, and recorded duration of aggressive behaviors. As a key prediction of the nasty neighbor hypothesis, we expected that such behaviors last for longer when encountering a conspecific neighbor than a conspecific non-neighbor. We also predicted that such duration should be date-dependent as territorial competition could increase in the middle of the season when male density is at its highest. Our results corroborated that aggressive behaviors lasted longer against a neighbor male than a non-neighbor male but there was not effect of date. Thus, neighbors may pose a greater risk and may be a strong selective force than non-neighbors in terms of resource competition. Since neighbor males are continuously trying to widen their territory boundaries (a situation that has been also found in other study systems), an owner male may not even need to recognize his neighbors to fight back. [ABSTRACT FROM AUTHOR]