Aggressive interactions and demographic parameters in Libellula fulva (Odonata, Libellulidae)

Abstract

Male aggressive interactions and demographic parameters (sex ratio, survival, abundance and life span) of the dragonfly Libellula fulva (Odonata, Libellulidae) were studied for four years at a marsh in northwestern Italy by monitoring marked individuals. Perching males attacked every dragonfly passing near the perch. However, the mean homospeciflc attack distance was significantly longer than the heterospecific one and this suggests that males were able to discriminate among species, at least partially. In some instances, aggressive males succeeded in catching and copulating with females. Release‐recapture analyses indicate that the sex ratio was very biased towards males, even though male and female capture probabilities were equal. Females had a slightly greater mortality rate than males, but this difference in adult survival is not sufficient to explain why adult females were rather rare at our study site. Life span estimates are in keeping with field observations, indicating that most males stayed alive for less than 10 days. All these results suggest a connection between interactions and demographic parameters. Male aggressive behaviour can in fact be viewed as an adaptation to a sexual environment where the time for reproduction is very short and the probability of meeting a partner is very low. Aggressions may in fact enhance males’ probabilities to catch (and copulate with) females which are flying through the site. Weather conditions influenced males’ behaviour, attack distance being significantly and positively related with light intensity. Vice‐versa, weather conditions did not influence survival, maybe because of mild temperatures and scanty rain. The aggressive behaviour of L. fulva males might be classed as territoriality. However, the classical “defence of resource approach”; seems to be scarcely appropriate here, since no resource located inside the hypothetical territory was defended, at least at the perch site.